Disclaimer: While considerable effort has been made to make the information provided in this section as complete and accurate as possible, it does not and cannot cover all possible situations. The authors cannot accept any responsibility for any damages which may occur from the use or mis-use of this document, nor can the authors accept any responsibility for any damages which may result from personal injury or property damage which allegedly may be caused by the use or mis-use of this document. No responsibility is accepted for missing or incorrect information. Those who intend to modify their vehicle shall accept all responsibility for performing the work which may be described below. If you have any comments or suggestions for additions or revisions, please contact the site administrator.

Any mention of specific brand names, manufacturers, vendors, etc. shall not be considered an endorsement, neither in favour nor against. In cases where particular products or services are available from a limited number of sources, we have listed some of these sources purely to enable the reader to be able to find the products or services in question, not as an endorsement, and this doesn't mean that such products or services aren't available elsewhere. In cases where products or services are widely available, we have generally NOT listed any specific sources.

The following sections of this document are organized as follows. First, a general introduction to the operation of a diesel engine is provided so that the concepts behind various modifications can be understood. Next, various modifications are discussed one at a time, and within each section where appropriate, information is provided concerning the benefits, disadvantages, cost, any necessary cautions that may be associated with it which are specific to that particular modification, suggested modifications to go along with this modification to enhance its effectiveness, and finally the "bang for the buck" factor. The final sections discuss issues other than engine-related performance.

Throughout this document, you will see references to VAG-COM. Many diagnostic and calibration procedures cannot be performed without a VW specific diagnostic scan tool. VAG-COM is a reasonably priced software and cable that allow any Windows based computer to be used to communicate with the vehicle's on-board diagnostic system. Generic OBD-II code scanners (such as those which may be found at independent garages that do not specialize in VW/Audi) will normally NOT be sufficient - you need a VW specific scan tool. For more information about VAG-COM and to purchase a copy for yourself, see https://www.ross-tech.com/ - highly recommended.

This thread identifies VAG-COM users who may be willing to help: https://forums.tdiclub.com/showflat.php?Board=UBB17&Number=309894

Here's a database of VAG-COM users who might be able to help you: https://www20.brinkster.com/beowulf9/tdi/vagcom/

Know which model you have. If you have a '96 or '97 Passat TDI in North America, you have a B4 chassis with what is known in this document as an A3-style engine, and you have a Garrett GT15 turbocharger. If you have a '97 through early '99 Jetta TDI in North America, you have an A3-style engine and you may have the Garrett GT15 turbocharger but more likely you have a KKK K03-006 turbocharger. If you have a New Beetle TDI, or a '99.5 or later Jetta or Golf TDI in North America, you have an A4-chassis vehicle, and you have a Garrett VNT-15 turbocharger.

The engine code number can be found on the build sheet which is typically near the spare tire on the floor of the trunk. In North America, engine code 1Z means 90hp A3-style engine with Garrett GT15 turbocharger and Pierburg MAF (mass air flow sensor). Engine code AHU means 90hp A3-style engine with KKK K03-006 turbocharger and Pierburg MAF. ALH means 90hp A4-style engine with Garrett VNT-15 turbocharger and Bosch MAF. European models don't necessarily correspond in specifications, engine codes, and model years (you're on your own, as far as figuring out what type of turbo and MAF sensor you have).

Finally, if you are reading this section because of a feeling that your vehicle is down on power, make sure it isn't because there is something wrong with it! Refer to the "Troubleshooting" section of this TDIFAQ.

1. Een algemene inleiding in de prestaties van diesel motoren met in een historisch perspectief
   The diesel engine operates by drawing air into the cylinders without any fuel (unlike a gasoline engine). The air is compressed much more than is the case with a gasoline engine, then near the point where the piston reaches the end of its compression stroke, fuel begins to be injected at high pressure. Within about a millisecond after fuel injection begins, the fuel ignites in the hot, highly compressed air. Combustion continues as fuel continues to be injected until some time after the piston begins moving down again, then the hot gases push the piston down the power stroke. Then the exhaust valve opens and pushes the spent gases out of the cylinder. This is the basic operating principle behind the 48hp 1.5 litre diesel engine used in the 1976 VW Rabbit ... hardly a performance vehicle.
   
   The more fuel you inject, the hotter the gases are during the expansion stroke (up to a point ...), the more power you get. But in order to burn fuel and transform it into heat, you also need air. If you inject too much fuel ... rather than more power, you get black smoke. Continue adding fuel beyond that point (which actually occurs over a range of air/fuel ratio, it's not a sharply defined point), and the smoke just gets blacker and blacker.
   
   Enter the turbocharger, the device which has transformed the modern diesel engine into something capable of being much more interesting than in the old days, by cramming more air into the cylinders and allowing more fuel to be burned before reaching the limits imposed by black exhaust smoke. In the mid eighties, VW fitted a turbocharger to their diesel engine (which now displaced 1.6 litres), bumped up the fuel delivery a bit, and increased power output to 68hp. There was an "ecodiesel" version sold in the USA, which had the turbo but not the increased fuel delivery, in the interest of less smoke from the exhaust ...
   
   How to improve on the turbocharged diesel engine? Enter the intercooler. Turbochargers increase the pressure of air, but in the process of compression, the air gets hot. The intercooler removes some of that heat, thus increasing the density of the intake air even if the pressure is kept the same. Then the fuel delivery can be bumped up a bit more. There was a version of the earlier turbo diesel engine sold in Europe with an intercooler and increased fuel delivery, with a useful performance boost.
   
   The saying goes that "there is no replacement for displacement", so in 1993, the displacement of the VW 4 cylinder turbo diesel engine was increased to 1.9 litres, and the standard turbocharged but non-intercooled version developed 75hp.
   
   But around the same time, a revolution in thermal efficiency and exhaust emissions was about to take place: the electronically controlled direct-injection VW diesel engine, the TDI. Something not mentioned in the first paragraph, is that for a wide variety of reasons, automotive diesel engines have historically been "indirect-injection" ... the fuel is injected near the top of compression stroke into a small chamber adjacent to the main cylinder and the burning mixture then expands into the main chamber, a process which controls the noise level but has a certain amount of heat loss. VW adapted direct injection to their diesel engines, and controlled the noise level and exhaust emissions using sophisticated electronic controls. The turbulence necessary to mix fuel and air within scant milliseconds is achieved using a bowl-shaped chamber in the top of each piston (the bottom of the cylinder head is completely flat). The extreme heat generated by combustion - capable of cracking or melting pistons if not controlled - is dealt with partly by careful design of the injector nozzles, the pistons, and the electronic controls; partly by reducing overall cycle temperature and pressure through the use of the intercooler; partly by oil-spray jets aimed at the bottom of each piston.
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2. Beperkende factoren voor het vermogen van diesel motoren
   The same limiting factors apply to hot-rodders as to original manufacturers, although they may differ in importance for each application.
   
   1. Exhaust smoke. Too much fuel without enough air, especially fuel that is injected long after the piston starts going down ... and you get excessive black exhaust smoke. The standard tuning of a 90hp TDI engine is extremely under-fueled ... there is a rather large safety factor against black exhaust smoke. But it's easily possible to reach the smoke limit by going too far with modifications that add fuel. The amount of exhaust smoke is affected not only by the air/fuel ratio, but also by the timing of the END of the fuel injection period. Fuel which is injected early, and especially that which comes in within a few crank degrees after the end of the compression stroke, has more time to mix with air and burn properly than fuel which is injected late.
   
   2. Exhaust temperature. The more fuel you add, and the longer that fuel injection continues after the piston starts moving down, the higher the exhaust temperature gets. (This response is NOT the same as for a gasoline engine.) Too high, and the exhaust turbine of the turbocharger suffers a meltdown. (Design limits for steady state "turbine inlet temperature" are 750 C / 1382 F for the GT15 and KO3 turbochargers, and 850 C / 1562 F for the VNT15. The author suggests a healthy margin below that.) The deductive reader has already observed that proper injection timing is critical for both smoke and exhaust temperature.
   
   3. Peak cylinder pressure. It's tempting to crank up the boost pressure and advance the injection timing to get rid of smoke, and to a certain extent, this approach is possible. But the design limit for cylinder pressure is within reach, and both of these factors increase the peak cylinder pressure. Too much cylinder pressure, and you'll get blown head gaskets, bent or broken connecting rods, cracked pistons, and who knows what else. Peak cylinder pressure is affected by the boost pressure (more correctly, by the absolute intake manifold pressure), the compression ratio, and the timing of the start of fuel injection. Every 2 degrees of timing advance results in 15 bar (about 10% compared to stock) more cylinder pressure. So you want to run no more boost pressure than necessary to control smoke and exhaust temperature, and no more timing advance under load than necessary for optimum efficiency and to control smoke. At sea level, be very cautious about using more than 18 psi boost pressure and/or more than 12 degrees BTDC timing advance at full load and 3000 rpm ...
   
   4. Piston temperature. Adding more fuel increases piston temperature. Advancing the injection timing under load increases piston temperature ... more reason for using no more timing advance than necessary. Lowering intake temperature drops it a bit. Increasing boost pressure drops the temperature (because the extra air is diluting the heat due to combustion). Lowering the oil temperature drops the piston temperature a bit (remember those piston-cooling oil-spray jets).
   
   5. Turbo compressor "surge" and "choke" limitations. This is a very complex topic, but to put it into simplified terms, each blade of a turbo compressor acts like a miniature airplane wing. What happens if you try to fly an airplane at a speed which is too low? The wing "stalls", and if uncontrolled, the airplane falls out of the sky. What happens if you ask for too much boost pressure while the amount of flow through the compressor is too low (i.e. too high *turbo* speed but too low *engine* speed)? The compressor wheel goes into "surge" which has harmful effects ... it may be a factor in breaking the shaft of the turbocharger (which has happened on VNT15 turbochargers! - this risk is not zero). The bottom line is that you don't want to reach full boost pressure (18 psi at sea level) until about 2000 rpm, and you want no more than about 6 psi boost pressure at 1500 rpm, to avoid compressor surge. The "choke" limitation of turbochargers limits the flow rate at the TOP end of the operating range, but it won't be an issue in this application unless you are asking for WAY more boost pressure than the turbo and engine can cope with.
   
   6. Mechanical limitations. The device inside the injector pump which controls the amount of fuel being delivered can only go to a certain maximum position before a fault will be detected and the ECU will ruin your day by going into a safe "limp mode" condition.
   
   7. Drivability limitations. The engine control module contains a program for maintaining a steady engine speed at idle and during part-load constant-speed conditions. If the response characteristics of the system are changed too much (e.g. by increasing fuel delivery), unstable operation can occur, which the driver will feel as an extremely annoying shuddering. Aftermarket fuel-adding devices that have an "on/off" characteristic (as opposed to a smooth response to increasing engine load) WILL result in extremely annoying abrupt response.
   
   BOTTOM LINE ... When selecting the engine's operating parameters, whoever is responsible for doing so must have some basic understanding of these limitations! Some popular modifications are "cookie cutter" and these limitations have been dealt with to the extent possible at the design stage. In other cases, YOU are on your own and are responsible for proper calibration of the engine's operating parameters. Where possible, the sections below contain at least a little bit of information to get you started, if this is the case.
   
   The complete engine, turbocharger, intake system, fuel system, emission control system, sensors, and electronic controls must be thought of as a complete integrated system. Changes to one area may have side-effects on other areas which require compensation to make the complete system operate correctly ... and sometimes compensation and/or moderation are required in order to prevent either gradual or catastrophic failure.
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3. Voor je gaat veranderen
   Wees er zeker van dat je auto 100% in orde is!
   
   Voor je een investering doet in een serieuze zaak als tuning, kontroleer alle onderhoudspunten of je ervan te verzekering dat je auto in tip-top conditie is. Het heeft weinig zin veranderingen te doen aan een auto welke een onderhoudsachterstand van jaren heeft. Geef speciale aandacht aan de luchtinlaat en de intercooler, omdat deze twee een significant negatief effect op de prestatie kunnen hebben als ze niet in orde zijn. Wees er zeker van dat alle regeleenheden met de juiste afstelling werken. (De LMM en de N75 zijn daarvoor beruchte onderdelen). Zie de Problemen sektie van deze TDI-FAQ.)
   
   Als je auto meer dan een paar jaar oud is dat zou het wel eens voldoende kunnen zijn om hem in nieuw-staat terug te brengen. Als je dan toch nog meer vermogen wilt, gebruik dat alleen de beste olie die er is, houdt je strikt aan het onderhoudsschema en zorg dat de luchtinlaat en de intercooler in perfecte conditie zijn want die snel een negatief effect op de houdbaarheid van de auto. Zorg dat je de prestaties krijgt waarvoor je hebt betaald.
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4. Gebruikelijke vragen en antwoorden voordat je verder leest ...
   
   Q.  Heeft het invloed op mijn garantie?
   A.  ELKE verandering die je aan de auto aanbrengt (waar/wat dan ook) kan een reden voor de fabrikant (of een andere garantie verstrekker) zijn om een garantie claim af te wijzen.
   
   Q.  Blaas ik mijn motor hiermee op?
   A.  Welke modificatie je ook kiest, als het met mate wordt toegepast en op de juiste manier, dan is de kans op serieuze problemen erg klein (maar niet nul). In de delen hierna, welke over individuele veranderingen gaan (waar mogelijk), staat informatie over hoe je op logische en verantwoorde manier moet beginnen met datgene waar jij mee bezig bent.
   
   Q.  Gaat het meer brandstof verbruiken?
   A.  Modificaties die op de juiste manier zijn uitgevoerd leiden NIET tot een hoger brandstof verbruik, tenzij natuurlijk dat extra vermogen regelmatig gebruikt wordt. Want bij huis-tuin-en-keuken-gebruik van de auto, verreweg het grootste deel van de tijd zal dat het geval zijn, veranderen de goed uitgevoerde modificaties de werking van de motor NIET.
   
   Q.  Kan ik <meestal een idioot aantal> pk's uit deze motor halen?
   A.  Het standaard antwoord voor iedereen die zich met racen bezig houdt is dit: 'snelheid kost geld, hoeveel heb je?' En geld-snelheid is geen lineair verband, eerder ... exponentieel (de eerste 10 pk extra kost 100 euro, de volgende 10 pk extra kost 400 euro). Denk aan de 'Beperkende factoren voor het vermogen van diesel motoren' welke hierboven besproken zijn. LEES DIE OPNIEUW. Een voorzichtige tuning, welke zich-bewezen manieren en onderdelen gebruikt, zal (bijna) geen neven effecten hebben, als het tenminste op de juiste manier is toegepast. Een meer agressieve benadering heeft een bepaalde re-engineering van de motor nodig en gaat op termijn misschien wel ten koste van de duurzaamheid. Voor je jezelf een hoog doel (veel pk's) stelt, denk aan een paar dingen: waar gebruik je de auto voor? wat zijn de belangrijkste minimum eisen daarvoor? waarom zou je meer willen dan die belangrijkste minimum eisen, en zijn die kosten en het mogelijk verlies aan duurzaamheid het wel waard?
   Je hebt geen 180 pk nodig voor een kruissnelheid van 100 kmh ... of zelfs 180 kmh, een standaard 90 pk motor doet dat zonder problemen voor je. En je hebt geen 180 pk nodig om van nul naar 100 kmh te accelereren in 8 sekonden. Zet jezelf daarom een realistisch doel. Kies geen aantal pk's uit, alleen voor de 'heb'. Ben je een 'dagelijkse' rijder dan zal het resultaat van een redelijke en conservatieve tuning meer dan tevredenstellend zijn en met relatief weinig kosten te bereiken.
   
   Q.  Moet ik het reguliere onderhoud van de auto dan veranderen?
   A.  Alles wat vermogen toevoegt (chip, box, injectoren) spuiten meer brandstof in en als je dat extra vermogen regelmatig gebruikt zal er meer dan gebruikelijk roet in de olie terecht komen. Dan is het aan te bevelen het interval voor het verversen van de olie te verkorten, en vanwege de toegenomen motorbelasting en hitteontwikkeling mag je er niet eens aan denken andere olie te gebruiken dan wat precies is voorgeschreven. Het is ook aan te bevelen om het lucht inlaat systeem in optimale conditie te houden, en denk daarbij aan alle componenten inclusief het lucht inlaat scherm (overweeg het te verwijderen), luchtfilter, intercooler, lmm.
   
   Q.  Gaat het de uitlaat emissies verhogen? Kom ik wel door de jaarlijkse APK?
   A.  It may or may not affect exhaust emissions, but if properly set up, it is unlikely that the result will be a failed annual inspection. In most locations in North America, light duty diesel vehicles are either not subject to periodic emissions inspection at all, or are only subject to an exhaust opacity ("blackness") test. There may be a visual check that emission control components are in place and appear to be functioning. It takes a very poorly-running TDI engine to fail an exhaust opacity test, regardless of modifications (but see next paragraph). The possible need to pass a visual inspection should be considered before making modifications that are highly visible under the hood, and the procedures used for your local inspection should be considered prior to committing to a large expenditure.
   Exhaust opacity tests at idle (most locations), or 2500 rpm no load, or at any constant cruising speed, are easy to pass. If your local jurisdiction does a dynamometer test at full load (most don't) that's a bit tougher, but even so, if your vehicle is properly set up it's not normally a problem. In the sections below, we have attempted to warn about configurations that might be prone to excessive exhaust smoke under load.
   
   Q.  In dit document wordt gezegd dat ik, naast wat ik van plan ben, nog meer veranderingen moet doen. Moet dat echt? Waarom kan ik niet alleen doen wat ik wil en verder niks?
   A.  Als er, in die gevallen waar andere veranderingen worden aanbevolen, teksten worden gebruikt als 'het is nodig om...', dan is het echt nodig om die extra verandering(en) WEL te doen om ongewenste neven effecten te vermijden, of op zijn minst die ongewenste neven effecten tot een minimum te beperken. In de meeste gevallen zijn die aanbevelingen gebaseerd op ervaring. Je mag die aanbevelingen natuurlijk negeren, maar dan kom je er vroeger of later toch achter (hopelijk niet door opgeblazen turbo's of koppakkingen) waarom die extra veranderingen toch aanbevolen worden...
   
   Q.  In dit document wordt gezegd dat sommige combinaties van tuning teveel van het goede is, of dat sommige instellingen teveel van het goede zijn. Waarom?
   A.  Vanwege - het verleden, beoordeling, ervaring, gemaakte schade, bekende beperkingen, bekende ontwerp beschrijvingen, berekeningen en extrapolaties van vergelijkbare zaken, en ga zo maar door ...
   
   Q.  Why not just import a European model that already has more power?
   A1. For Canada - This CANNOT be done legally. You CANNOT register a vehicle in Canada which was not originally built to North American standards unless it is more than 15 years old. If the vehicle was not originally built to North American standards, Transport Canada WILL NOT allow the vehicle to be registered for the road. You CANNOT modify a vehicle to meet North American standards to satisfy Transport Canada - it has to be built to North American standards originally when it rolled down the original manufacturer's production line. And they don't make anything other than a 90hp TDI to North American standards (as of 2003 model year). Refer to https://www.riv.ca/ and link to the import regulations.
   A2. For USA - There are authorized vehicle modification facilities that are capable of modifying certain vehicles (not all) to meet North American safety standards, but if it is fitted with an engine that hasn't got EPA's approval, that's going to be a big problem. For all reasonable and practical intents and purposes, it is impossible to import a vehicle into the USA and legally register it for the road if that vehicle was not originally built to North American standards by the original manufacturer (or it is more than 15 years old).
   A3. General - Most vehicle registration systems are only concerned with the VIN (Vehicle Identification Number) - the chassis number. They typically don't pay attention to the engine number. There is a theoretical possibility that a drivetrain could be imported somehow (complete with ALL sensors, wiring, electronics, and accessories - typically these aren't the same as North American models) and put into a North American vehicle. No doubt this process isn't legal due to the use of an engine which has not gone through the EPA / Transport Canada approval process, but suppose somehow one finds a way around that. Now you have a vehicle in North America that has parts on it - including some scheduled maintenance parts, like the timing belt - that cannot be obtained through normal channels. You've now spent loads of money, and nobody will know how to fix it, and nobody will be able to get parts for it. And what happens a few years down the road when something better finally comes along? What little resale value is left owing to the difficulty of servicing goes right out the window. FORGET IT.
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5. Discussies over populaire veranderingen aan de motor
   Het volgende gedeelte gaat over populaire modificaties. Het begint met de meest gebruikelijke modificaties die altijd succes hebben. Verder in het stuk gaat het over modificaties die minder gebruikelijk zijn en soms alleen maar effect sorteren als ook andere veranderingen worden doorgevoerd. Aan het eind staan enkele zaken die of nutteloos zijn, of erg riskant, of extreem duur voor wat het oplevert, of die een zeer grote kennis van zaken vereisen om het voor elkaar te krijgen.
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1. De chip (chippen, chiptuning)
   Popular manufacturers available in North America are Upsolute (https://www.upsolute.com/) and Wetterauer (https://www.chip-tuning.com/). Since all operating parameters of these engines are electronically controlled and their operation is governed by a memory chip in the engine ECU (electronic control unit), this modification consists of installing a different program on one or more "chips" contained within the engine ECU to alter the engine's operating parameters. Typically, these chips will slightly increase the maximum turbo boost pressure, considerably increase the maximum quantity of fuel delivered, and may slightly advance the start-of-injection timing under load, but with little or no effect on calibration at part-load conditions.
   
   Installation: On A3 and B4 vehicles, the affected chips are "socketed" to the main board, so it is possible for an aftermarket chip to be purchased and installed by a moderately competent do-it-yourself mechanic following the manufacturer's instructions. On later models (New Beetle, '99.5-on New Golf and Jetta) the affected chips are soldered to the circuit board and can only be serviced using highly specialized tools, generally only by the vendor of the chip. It is recommended that this task be left to the vendor of the chip in question, in all cases.
   
   Advantages: Easy, since you don't have to do it yourself. No home calibration or adjustment is necessary (although the fuel injection timing must be properly set to avoid excessive exhaust smoke). Suitable for someone who wants more power but knows very little technically about how the vehicle operates. Completely invisible to the eye, and to electronic scan tools unless the user of the scan tool knows specifically what to look for and is specifically testing for it. There is a large power gain, instantly noticeable when driving the vehicle. All vehicle diagnostic and sensor-checking functions remain intact.
   
   Disadvantages: For later models, it is difficult (but possible) to reverse the modification should the need arise. For earlier models, reversing the modification is straightforward due to the socketed chips. It's hardly an issue ... you'll never want to go back to stock anyway. You are your own warranty.
   
   Cost factor: US$300 - US$500 range
   
   Cautions specific to this modification: Injecting more fuel using the stock injectors inherently extends the injection period later in the power stroke, so exhaust temperature will go up during prolonged operation at greater than the stock power output. Due to richer air/fuel and extended injection period, the tendency is towards more smoke in the exhaust, but normally the chip manufacturers stay within limits of exhaust smoke that are acceptable to most people and within limits of acceptable exhaust temperature. The vehicle will be more sensitive to fuel quality and proper setup of the injection timing for exhaust smoke. Set injection timing within specifications per shop manual but close to the "advanced" limit in order to ensure that the pump has enough capability to provide sufficient timing advance under load, otherwise exhaust will be smokier than necessary and the exhaust temperature will be higher than necessary. *DO NOT* use the "adaptation" settings in VAG-COM to provide more timing advance than the chip already specifies.
   
   Suggested modifications in parallel: Turbo boost gauge; exhaust gas temperature gauge if vehicle is used for trailer towing or for prolonged operation near maximum speed and load; larger oil cooler. De-restricted intake and exhaust systems won't hurt.
   
   "Bang for the Buck": If in doubt, DO IT!
   
   What if I want more? A chip can be made to work with the next size bigger injectors, although it may take some effort (see later section). If any problems arise because of this, you are on your own to solve them. Clutch slippage, turbo damage, and high exhaust temperature are potential issues if modifications go far beyond that of a chip. Some people have run tuning boxes together with a chip, but this defeats the safety margins that the chip and box programmers have each individually built in, and there is a high probability that the maximum fuel delivery of the injector pump will be reached, causing all sorts of drivability and "check engine" problems. Dyno testing has found limited benefit from chip and box together ... typically there is more clutch-destroying torque, but little more peak power.
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2. Tuning Boxes
   There are too many different suppliers of "tuning boxes" to list here. In some cases, the same device is sold under different names. They vary from simplistic passive electrical circuits to microprocessor-controlled devices. Some have multiple adjustments to allow calibration to different operating conditions and some are "plug and play" with no adjustment possible. All tuning boxes operate by altering the communication between the fuel injection pump and the engine ECU so as to increase fuel delivery under certain conditions. Typically, tuning boxes will considerably increase the maximum amount of fuel delivered at full load, will not change the steady-state boost pressure, and may or may not have some effect on injection timing.
   
   Installation: Plug-and-play types are easy for the do-it-yourself mechanic following the manufacturer's instructions. Generally, unplug the large electrical plug at the injector pump, plug the box in so that it is now "in between" the two parts of the injector pump's electrical plug. Most boxes require one wire to be spliced into the wiring harness of the vehicle, either at the MAF sensor, the accelerator pedal position sensor, or the manifold air pressure sensor. Soldering may be required to do this, or the box manufacturer may provide suitable plugs to allow it to be done without disrupting stock wiring harnesses. Adjustable types may require experimentation following installation in order to get the settings right ... if you have a non-standard configuration, expect this calibration to take a lot longer than actually installing the device. PLEASE ASK QUESTIONS RELATED TO A SPECIFIC UNIT TO THE VENDOR OR SEARCH EXISTING THREADS ON THE FORUMS to avoid creating unnecessary forum traffic.
   
   Advantages: Relatively easy installation, possible for the do-it-yourself mechanic. Some types have fine-tuning adjustments which can either be left alone or customized to a given application. Plug-and-play types are suitable for someone who wants more power but knows very little technically about how the vehicle operates. The modification is readily visible but easily removed should the need arise. Electronic scan tools may not display correct sensor values due to the way the box modifies some stock signals in order to perform its job. There is a large power gain, instantly noticeable when driving the vehicle, but typically gains are smaller than achieved by a chip. All vehicle diagnostic and self-checking functions remain intact but the accuracy of some sensor displayed values may be affected - easily solved by temporarily removing the box from the system.
   
   Disadvantages: The modification is readily visible but easily removed should the need arise. On boxes that require a spliced or soldered wiring harness connection, that connection (even if unplugged) is going to be more or less visible (depending on where you put it) and not easily removed. Some tuning boxes have been known to cause drivability problems and "check engine" lights under certain conditions. Sometimes the adjustable boxes can be adjusted to eliminate these faults, but not always. You are your own warranty. Boxes with multiple adjustments allow fine-tuning to arrive at an optimum setting, but having the ability to adjust something inherently means that there are far more possible ways to get it wrong rather than right. But boxes with no adjustments cannot be adjusted if they don't work in your particular application. PLEASE ASK QUESTIONS RELATED TO A SPECIFIC UNIT TO THE VENDOR OR SEARCH EXISTING THREADS ON THE FORUMS to avoid creating unnecessary forum traffic.
   
   Cost factor: US$200 - US$400 range
   
   Cautions specific to this modification: Injecting more fuel using the stock injectors inherently extends the injection period later in the power stroke, so exhaust temperature will go up during prolonged operation at greater than the stock power output. Due to richer air/fuel and extended injection period, the tendency is towards more smoke in the exhaust, but normally the box manufacturers stay within limits of exhaust smoke that are acceptable to most people and within limits of acceptable exhaust temperature. The vehicle will be more sensitive to fuel quality and proper setup of the injection timing for exhaust smoke. Set injection timing within specifications per shop manual but close to the "advanced" limit in order to ensure that the pump has enough capability to provide sufficient timing advance under load, otherwise the exhaust will be smokier and exhaust temperature will be elevated. *DO NOT* use the "adaptation" settings in VAG-COM to provide more timing advance than the box already specifies.
   
   Suggested modifications in parallel: Exhaust gas temperature gauge, if vehicle is used for trailer towing or for prolonged operation near maximum speed and load; larger oilcooler. De-restricted intake and exhaust systems won't hurt.
   
   "Bang for the Buck": Generally very good, but beware of potential problems, especially if you are trying to use the box together with another major power-adding modification. Power gains are typically going to be less than that of a chip, due to the inability of currently available tuning boxes to do anything about the boost pressure.
   
   What if I want more? Some people have run tuning boxes together with a chip, but this defeats the safety margins that the chip and box programmers have built in, and there is a high probability that the maximum fuel delivery of the injector pump will be reached, causing all sorts of drivability and "check engine" problems. If you see yourself wanting more power than the box provides on its own, then this may not be the best way to go, because if you want to burn more fuel then you need more air, and boxes available to this date do not affect the boost pressure. Using a tuning box together with bigger injectors may or may not work, but it typically isn't the best way to go unless the box is specifically set up for the bigger injectors, because neither box nor injectors specify more intake air than standard, and you'll need more air if you want more power rather than black smoke.
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3. Bigger Injectors
   If you have a vehicle with a 90hp TDI and 5-speed manual transmission, you have injectors with 0.184mm orifices. If you have a 90hp TDI with automatic transmission in North America, you have injectors with 0.158mm orifices and a higher-pressure injector pump so as to push the same amount of fuel through these smaller injectors (for emissions reasons, at a cost of increased stress on the components and increased engine noise). The European 110hp model has what are known as 0.205mm injectors, and the European "Multivan" 150hp 5-cylinder TDI has what are known as 0.216mm injectors. All of these injectors are dimensionally interchangeable, so it is possible to install injectors one or even two sizes larger in place of the original ones. This will increase fuel delivery without the engine ECU knowing a thing about it, and it will do it without pushing the limits of the injector pump, nor will it extend the duration of the injection period - and not extending the duration is better for smoke, power, exhaust temperature, and efficiency.
   
   Larger injectors are available in North America from https://www.dieselgeek.com/ (USA) or https://www.autobraun.com/ (Canada). Injector nozzel replacements are also available at places like https://www.kermatdi.com/. Refer to the "Allowable combinations" table below before committing yourself to anything. You cannot order these parts from a North American VW dealer.
   
   For the 90hp 4-cyl 5 speed injectors (known as 0.184 injectors) here are the part numbers you'll need (upgrade for automatics in North America only):
   (Qty 3) - 028 130 202 P (there may be a suffix on the end indicating that it is reconditioned)
   (Qty 1) - 028 130 202 Q (there may be a suffix on the end indicating that it is reconditioned)
   
   For the 110hp 4-cyl injectors (known as 0.205 injectors) here are the part numbers you'll need:
   (Qty 3) - 028 130 201 T (there may be a suffix on the end indicating that it is reconditioned)
   (Qty 1) - 028 130 201 S (there may be a suffix on the end indicating that it is reconditioned)
   
   For the 150hp 5-cyl T4 Multivan injectors (known as 0.216 injectors - biggest commercially available) here are the part numbers you'll need:
   (Qty 3) - 074 130 201 K (possibly with a V on the end indicating reconditioned rather than new)
   (Qty 1) - 074 130 202 R (possibly with V on the end)
   
   Don't worry about reconditioned versus new. Reconditioned injectors contain all new working parts inside a re-used housing, and are quite a bit less expensive. Also don't worry about the opening pressure of the injectors, it affects the idle settings a bit but not much else.
   
   Installation: This modification is not recommended for people who don't have good vehicle wrenching ability and troubleshooting skills. Professional installation might be possible, but if you're not comfortable doing this installation yourself, then you probably don't have the required skills and knowledge to troubleshoot and correct any problems or side-effects that may occur. The mechanical part of the installation is fairly straightforward, but you will need VAG-COM to recalibrate some functions in the ECU (easy if you have it ... https://www.ross-tech.com/ ), and some engine/injector/ECU combinations will require physical modifications to the turbo boost control system in order to get more air into the engine without blowing something to smithereens. To install the injectors, remove the clamps that tie pairs of injector lines together, loosen lines 1 and 4 at the pump end and remove them at the injector end. (Note the relatively low torque, not counting the initial "crack" to break the fitting loose.) Remove the clamps that hold injectors 1 and 4 down. Remove the drain-back hoses from injectors 1 and 4. Remove injectors 1 and 4 by applying a twisting action with a 15mm wrench while pulling up on the injector. Clean soot out of the seating surface in the hole in the cylinder head. Stick a new copper sealing washer to the new injectors with a dab of grease on the washer, slip them in, and torque the clamps to 20 lb.ft. Re-install all injector lines and tighten the lines at the pump. (Do not overtighten these fittings!) Start the engine (it will run on two cylinders at first) - run it for 5 seconds after the idle smooths out. Then repeat this process for injectors 2 and 3, remembering to deal with the electrical plug for #3, and transfer the drain-back fitting from the old #3 injector to the new one. Following completion of this, connect VAG-COM, and re-calibrate the EGR per section 7.j of this TDIFAQ, and re-calibrate the fuel delivery at idle to about 3 mg/stroke if you have a stock ECU and you only went up one injector size, or to the leanest possible setting (i.e. biggest reported number that can be achieved within specs) if you went up two injector sizes with a stock ECU or if you went up one injector size with a chipped ECU. On vehicles equipped with a VNT15 turbo, you should back off the initial response of the turbo to avoid dangerous operating conditions at low engine RPM and high engine load - see item 4b on boost control system modifications later in this document.
   
   Advantages: Larger injectors are unique among the major power adders in that they inject a larger amount of fuel within the same amount of crankshaft rotation, and thus, for a given power delivery, they keep smoke and exhaust temperature to the minimum while also avoiding the increase in peak cylinder pressure caused by over-advanced start-of-injection timing. Given that the 110hp VW TDI engine is almost identical to the 90hp except for larger injectors, and that the 190hp "race TDI" developed by VW Motorsport uses even larger injectors but roughly the same injection timing and duration as stock, this method would appear to be the one favored by the engineers who develop the VW TDI. All injectors other than 0.216 look the same other than the part number, so the modification is invisible to the eye, and in the case of the 0.216 injectors, it takes a knowledgeable eye to spot a minor difference. This modification is transparent to electronic scan tools. Power gain will range from mild to wild depending on how big you go, but will be easily noticeable when driving the vehicle. All vehicle diagnostic and sensor-checking functions remain intact although the reported fuel delivery will no longer be accurate since the ECU has no way of knowing about the extra fuel being injected.
   
   Disadvantages: Beyond the start we've given you in the "installation" section and the "Allowable Combinations" section below, you're on your own for ironing out any hiccups that may occur and not breaking something expensive in the process. You are your own warranty. You might need a different setting for reported fuel quantity at idle if you run into shudder problems (leaner - i.e. larger reported quantity - tends to reduce shuddering). Some ECU and chip and injector combinations may have shudder problems that cannot be resolved without going to a different chip, and not all combinations have been tested by someone. For example, the Upsolute chip for the '96 Passat "BK" ECU is not at all compatible with 0.216 injectors and (to my knowledge) has not been tested with 0.205 injectors, but the stock chip for the same ECU is OK with 0.216 injectors if AND ONLY IF reported idle fuel quantity is set at 6.4 mg/stroke. For the '97 Passat and Jetta, the Upsolute chip is compatible with the 0.205 injectors, although you'll need more boost pressure than the chip specifies. For the later models, no compatibility issues have been reported as far as drivability is concerned - but see the "Allowable combinations" table below.
   
   Cost factor: US$550 - US$700 range, and you had better have VAG-COM ( https://www.ross-tech.com/ - about US$200).
   
   Cautions specific to this modification: Since the ECU doesn't know about the extra fuel, it's up to you to either find settings that control exhaust smoke and exhaust gas temperature (if possible), or drive in a way that doesn't cause issues (not a good solution to require this to be done). READ THE "ALLOWABLE COMBINATIONS" SECTION CAREFULLY FOR YOUR PROPOSED SETUP. Most of that is based upon experience, and if you skip steps, sooner or later you're likely to find out why those other modifications are recommended. On A4-chassis 5-speed vehicles, going 2 injector sizes up with stock ECU or 1 size up with chipped ECU is likely to cause clutch slippage ... figure on an upgraded clutch. On automatic transmission vehicles, beware of exceeding the transmission torque limits, which are not known at this writing, but whoever finds out is in for an expensive lesson. On vehicles with VNT15 turbo and manual transmission, beware of high-load operation at low engine revs (see turbo discussion in "Limiting Factors" above, and item 4b - boost control system modifications - below).
   
   Suggested modifications in parallel: Better clutch on A4-chassis manual transmission, if modifications more aggressive than 1 size bigger injectors with stock chip are done. Better final drive and differential (Quaiffe or Peloquin with bolted ring gear - only possible on manual transmission). Refer to the "Allowable Combinations" section below, for additional changes that may be necessary for specific installations. Refer to item 4b (boost control system modifications) as well. A larger oilcooler may also be a good idea
   
   "Bang for the Buck": Pretty good, if you're capable of sorting out the ECU issues with VAG-COM and sensor tweaks, and you don't skip steps, and you don't try to do something that cannot be done. Lousy, if you bit off more than you can chew, or if you have no clue about sorting out those issues, and end up going back to stock or buying more parts in order to get a more conservative setup that's more suitable for normal use. The difficulty of sorting out these issues will depend on how wild you choose to go ... the wilder you go, the tougher the issues are going to be to deal with. See "Allowable combinations" discussion below.

   What if I want more? Install a better intercooler and max out the fuel delivery, provided that your ECU will allow it without causing unacceptable shudder. Custom chip programming. Bigger turbo (which won't do anything without custom chip programming). Lower compression ratio (which you had better do if you go much beyond 18 psi boost pressure). Quaiffe or Peloquin final drive. Better bring lots of money ...
   
   Allowable combinations of injectors and other modifications
   
   This section is formatted as "If you are starting with ... then you can install ... provided that you also do ... subject to the following risks ...". See elsewhere in this FAQ for the "also recommended" modifications. ECU recalibration information is in the Maintenance Procedures section of this FAQ. Boost bleed for A3 and B4 vehicles is discussed in a later section (can't do it that way on an A4). Mechanical boost controllers are discussed in the Troubleshooting section of this FAQ under the topic "Fluctuation of boost pressure". It is highly recommended that a boost gauge be installed in the vehicle, for any situation where the corrective measures listed involve changing anything related to turbo boost control.
   
   On A4-chassis vehicles, it is possible to fit an automatic's 11mm injector pump to a vehicle with a manual transmission. This document doesn't explicitly address this possibility, largely because it is an expensive modification and has only been done on a very small number of vehicles. If you propose to do this, then for purposes of interpreting the following data, pretend that fitting an 11mm pump in place of a 10mm pump is like installing injectors that are one size larger. (11mm pump with 0.184 injectors is like a 10mm pump with 0.205 injectors, etc.) If you have an A3 or B4 vehicle, the A4-chassis injector pump's wiring harness is not compatible, although we've heard that it is possible to make it work given enough effort. The 11mm pump is only available in A4-chassis form and can therefore only be fitted to an A3 or B4 with considerable difficulty due to the wiring differences.
   
   Fuel system and engine control components from "pump-duese" engines are not at all compatible with the distributor-pump engines. This discussion pertains only to the 90hp distributor-pump engines as sold in North American specification through model year 2003. The 110hp distributor-pump engine already has 0.205 injectors as standard.
   
   If you are starting with an A3 or B4 with stock chip and no tuning box, you can install 0.205 injectors provided that you recalibrate the EGR system so that maximum possible intake air is specified, and if you experience rough idling or shuddering at light load, recalibrate reported injection quantity at warm idle to around 3.0 mg/stroke. There shouldn't be any difficult issues to resolve. Power gains will be moderate ... probably a bit less than a chip with stock injectors, but this solution more closely resembles VW's official solution for the 110hp model (see "Advantages" above). Only install a tuning box if it is specifically suitable for use with this chip and injector combination.
   
   If you are starting with an A3 or B4 with stock chip and no tuning box, you can install 0.216 injectors provided that you recalibrate the EGR system so that maximum possible intake air is specified, and recalibrate reported injection quantity at warm idle to around 5 to 7 mg/stroke (shuddering and drivability problems will probably be limiting factors - larger reported quantity helps both issues), and you install a bleed on the pressure sensing line that goes to the ECU so that the resistance upstream (to sensing line) and downstream (to atmosphere) of the ECU sensing port is about equal, and you install a mechanical boost controller in parallel with the standard N75 system calibrated to 17 psi maximum boost pressure which experience has been found is sufficient to adequately control exhaust smoke - fine-tune up or down if required, without exceeding 18 psi boost pressure. Tuning boxes will probably not work in this application - if you're having trouble controlling shuddering and/or exhaust smoke, adding more fuel is the last thing you want to do. There shouldn't be any issues beyond those discussed here. This configuration has been tested (author's vehicle), and has proven to work very well after the initial hiccups were ironed out, with acceleration tests showing power slightly greater than that of a chip with stock injectors in the same vehicle, *BUT* you need to be capable of dealing with the side effects!
   
   If you are starting with an A3 or B4 with Upsolute or Wetterauer chip and no tuning box, you can install 0.205 injectors provided that you recalibrate the EGR system so that maximum possible intake air is specified, and recalibrate reported injection quantity at warm idle to around 5 to 7 mg/stroke (fine tune for shudder and/or exhaust smoke), and you install a bleed on the pressure sensing line that goes to the ECU so that the resistance upstream (to sensing line) and downstream (to atmosphere) of the ECU sensing port is about equal, and you install a mechanical boost controller in parallel with the standard N75 system calibrated to 18 psi maximum boost pressure which is the maximum allowable. You'll be fighting hard to control exhaust smoke with this configuration, and it may not be possible to completely solve the problem. A bigger intercooler, although costly, is really what is required. Although similar configurations have been tested on several vehicles, long term durability under high load conditions is not proven, since the power output will considerably exceed standard and is well beyond that of a chipped engine with stock injectors. Do not use any type of tuning box in this application - you've already got too much fuel. Clutch problems are likely ... see elsewhere in this TDIFAQ. Drag-racers should upgrade the final drive and differential. Remember, YOU ARE YOUR OWN WARRANTY.
   
   If you are starting with an A3 or B4 with Upsolute or Wetterauer chip, don't even think about 0.216 injectors unless you're prepared for internal engine work to lower compression, a slightly larger turbo (but not too much!) such as a VNT17, custom intercooler, upgraded clutch, upgraded final drive, extra oil cooling, probably custom ECU programming to deal with all the issues that will occur. This will cost thousands of dollars to get right. Long term durability under high load conditions is not likely to be very good. We know of one engine with a similar configuration, having about 150hp to the wheels (roughly double the stock figure!), which suffered cracked pistons after about 80,000 km of admittedly hard driving.
   
   If you are starting with an A4 5-speed manual with stock chip and no tuning box, you can install 0.205 injectors provided that you recalibrate the EGR system so that maximum possible intake air is specified, and if you experience rough idling or shuddering at light load, recalibrate reported injection quantity at warm idle to around 3.0 mg/stroke. There shouldn't be any difficult issues to resolve ... you're basically changing a 90hp into a 110hp. Power gains will be moderate, probably a bit less than using a chip ... but see "Advantages", above. Only install a tuning box if it is specifically intended for use with this chip and injector combination. Since you are your own warranty, it's worth checking whether you are getting boost pressure too close to the "surge" regime and making adjustments if required ... see separate section 4b below.
   
   If you are starting with an A4 5-speed manual with stock chip and no tuning box, it seems likely that you can install 0.216 injectors provided that you recalibrate the EGR system so that maximum possible intake air is specified, and recalibrate reported injection quantity to a value that has yet to be determined but within the specification range of 2.2 to 9.0 mg/stroke (probably higher in the range) to control exhaust smoke. This combination hasn't been tested to this author's knowledge, but based on the A4's stock boost pressure being quite close to the final configuration of a B4 test vehicle with these injectors, it should work out with a bit of fine tuning. Exhaust smoke will likely be marginal at full load, though. It is strongly advised that the initial response of the turbocharger be de-tuned to avoid "surge" operation, per section 4b below, to preserve the turbocharger. Clutch problems are possible with this arrangement ... see elsewhere in this TDIFAQ. Tuning boxes will probably not work in this application - the smoke situation will be marginal, so adding more fuel won't help.
   
   If you are starting with an A4 5-speed manual with Upsolute or Wetterauer chip and no tuning box, you can install 0.205 injectors provided that you recalibrate the EGR system so that maximum possible intake air is specified, and recalibrate reported injection quantity at warm idle to around 4 mg/stroke (fine tune for exhaust smoke), and make the turbo negative VNT adjustment indicated in section 4b below - DO NOT skip this step, it is vital to the life of your turbo. Although similar configurations have been tested on several vehicles, long term durability under high load conditions is not proven, since the power output will considerably exceed standard. Do not use any type of tuning box in this application - you've already got too much fuel. Clutch problems are almost a certainty ... see elsewhere in this TDIFAQ. Drag-racers should upgrade the final drive and differential. Remember, YOU ARE YOUR OWN WARRANTY.
   
   If you are starting with an A4 5-speed manual with Upsolute or Wetterauer chip, don't even think about 0.216 injectors unless you're prepared for internal engine work to lower compression, a slightly larger turbo (but not too much!) such as a VNT17, custom intercooler, upgraded clutch, upgraded final drive, better oil cooling, custom ECU programming to deal with all the issues that will occur. This will cost thousands of dollars to get right. Long term durability under high load conditions is not likely to be very good.
   
   If you are starting with an A4 automatic with stock chip and no tuning box, you can install 0.184 injectors provided that you recalibrate the EGR system so that maximum possible intake air is specified, and if you experience rough idling or shuddering at light load, recalibrate reported injection quantity at warm idle to around 3.0 mg/stroke. Only install a tuning box if it is specifically intended for use with this chip and injector combination. There shouldn't be any difficult issues to resolve.
   
   If you are starting with an A4 automatic with stock chip and no tuning box, you should be able to install 0.205 injectors provided that you recalibrate the EGR system so that maximum possible intake air is specified, and recalibrate reported injection quantity at warm idle to around 3.0 mg/stroke, possibly a larger number if exhaust smoke proves to be an issue. This particular combination, at the time of this writing and to the knowledge of this author, has not been tested. Do not use any type of tuning box in this application, it will be marginal on exhaust smoke without adding still more fuel.
   
   If you are starting with an A4 automatic with stock chip, don't even think about the 0.216 injectors, it will smoke too much.
   
   If you are starting with an A4 automatic with Upsolute or Wetterauer chip and no tuning box, you can install 0.184 injectors provided that you recalibrate the EGR system so that maximum possible intake air is specified, and recalibrate reported injection quantity at warm idle to around 3.0 mg/stroke, or possibly slightly greater if exhaust smoke proves to be an issue. Do not use any type of tuning box in this application, it will be marginal on exhaust smoke without adding still more fuel. This configuration has been tested on quite a few vehicles with no known issues at this writing. Long term durability of the automatic transmission in this application is not known.
   
   If you are starting with an A4 automatic with Upsolute or Wetterauer chip, don't even think about 0.205 or 0.216 injectors unless you have a transmission rebuild in the budget ... in addition to all the other things you'll need to control the exhaust smoke.
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4. Boost Control System Modifications
1. Boost Control System Modifications - A3 and B4 cars with GT15 or KO3 turbochargers - 
   This discussion must be separated into two parts due to the completely different operating characteristics of the wastegate-type turbochargers from the VNT-type turbochargers. This first subsection focuses on the wastegate-type turbochargers.
   
   The purpose of these modifications on the wastegate-type turbochargers is to increase the steady-state boost pressure while greatly reducing the overshoots and fluctuations of boost pressure that are characteristic of the stock boost control system. This modification is only useful if other modifications (e.g. injectors) have been made that result in unacceptable exhaust smoke with the stock boost pressure. It is not to be done unless there is smoke from the exhaust and other causes of smoke (wrong injection timing, bad injectors, bad MAF sensor, EGR calibration problems) have been ruled out.
   
   We can take no responsibility for what you do to your own car. Be aware that if you choose to do this, you are on your own. If you blow something up, you did it, not us. This modification can be made very cheaply but BE CAREFUL! It's very easy to blow something up by using too much boost pressure. Don't even think about messing with this unless you first install a boost gauge so you can watch what's happening. On the A3 models, the red hose in the engine compartment is boost pressure straight from the compressor discharge - easy to tap a pressure gauge into. To avoid false readings, the gauge connection should be made adjacent to the turbo or manifold before any signal-modifying bleed devices or mechanical boost controllers.
   
   The boost pressure is controlled using a sensor which is inside the ECU. There is a small black hose which attaches to the intake manifold just where the intake manifold turns down towards the engine. (Make sure you're looking at the elbow that turns down towards the engine itself, not the one that turns down and goes into the turbocharger!) If you trick the engine computer into seeing less boost than there really is, then the engine computer will control it to a higher level. This is only useful if you have done other things that increase fuel delivery, and you need more air to control smoke and exhaust temperature as a result.
   
   For a detailed discussion on this matter, follow this link:
   https://forums.tdiclub.com/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=4&t=000529
   
   It has been found advisable to use a mechanical boost controller in parallel with the stock boost control system if you do this. Refer to the Troubleshooting section of this FAQ, under the symptom "Fluctuation of boost pressure".
   
   Do not operate the standard turbocharger at more than 18 psi boost pressure at sea level. (This must be de-rated in proportion to barometric pressure at higher altitudes.) Do not operate an engine with a stock compression ratio at higher than 18 psi boost pressure under any circumstances.
   
   Installation: Straightforward for the knowledgeable do-it-yourself mechanic. Professional installation isn't really feasible, because most mechanics won't have a clue about how this is going to work and won't want to chance it.
   
   Advantages: For vehicles that have higher fuel delivery than stock, this provides a method of controlling the smoke, and the mechanical boost controller provides control of the boost spikes and fluctuation that are characteristic of the stock boost control system.
   
   Comments: Only worth doing on a highly tuned vehicle.
   
   Disadvantages: A boost gauge is MANDATORY if modifications are done to the stock boost control system, and the driver is well advised to pay attention to that gauge, since these modifications remove the built-in safety features of the electronic control system. You are your own warranty. Remember ... no more than 18 psi boost pressure, under any circumstances!
   
   Cost factor: Next to nothing for the bleed device, $35 for the mechanical controller from https://www.dawesdevices.com/ - and figure on another $100 or so for a boost gauge and a place to put it.
   
   Cautions specific to this modification: YOU are responsible for making sure that you don't do something destructive to your vehicle.
   
   Suggested modifications in parallel: This is only appropriate for a highly tuned vehicle with chip and injectors, that needs close to the limit of boost pressure (18 psi) in order to control exhaust smoke. A larger oilcooler may also be a good idea
   
   "Bang for the Buck": Good.
   
   What if I want more? Since this modification is only appropriate for a vehicle that already has more and is smoking badly as a result ... you already have more..
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2. Boost Control System Modifications - A4 cars with VNT turbochargers
   This discussion must be separated into two parts due to the completely different operating characteristics of the wastegate-type turbochargers from the VNT-type turbochargers. This second subsection focuses on the VNT-type turbochargers.
   1. 4b.1. Foreword and Disclaimer
      
      This section discusses modifications intended to *prevent* turbo damage, so NOT doing anything about it may actually be the more dangerous scenario. Nevertheless, we can take no responsibility for what you do to your own car. Be aware that if you choose to do this, you are on your own. If you blow something up, you did it, not us. BE CAREFUL! It's very easy to blow something up by using too much boost pressure. Don't even think about messing with this unless you first install a boost gauge so you can watch what's happening.
      
      The purpose of these modifications is NOT to increase the steady-state boost pressure (which is already higher than the older vehicles use, and very close to the safe upper limit if the vehicle is "chipped") but rather to control the fluctuations of boost pressure characteristic of the stock boost control system, and to REDUCE the amount of boost pressure at low engine speeds in the interest of preserving the turbocharger by avoiding operation in the "surge" regime. This will actually *reduce* responsiveness at low engine speeds, but it is a necessary step to preserve the life of the turbo, if other modifications which increase fuel delivery at low engine speed have been done. These modifications are only useful if other modifications (e.g. injectors) have been made that result in either unacceptable exhaust smoke or power fluctuation due to fluctuations in boost pressure that the stock boost pressure control system cannot control, or unacceptably high boost pressure at engine speed below 2000 rpm. The VNT turbo must not reach full boost pressure until above 2000 engine rpm, and should preferably produce no more than about 6 psi boost pressure at 1500 rpm, to avoid operating the compressor in the highly damaging "surge" regime. Turbo manufacturers recommend staying at least 10% clear of operation in the surge regime.
      
      Since the A4 cars do not have any small-diameter hoses which carry boost pressure, it will be necessary to install a suitable fitting to access boost pressure. This fitting should preferably be located as close as possible to the turbo compressor outlet, such as on the lower intercooler pipe ... remove the pipe from the vehicle to install the fitting there. In order to facilitate future removal of this pipe for timing belt changes, use some sort of quick-connect detachable fitting to your new boost signal hose.
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   2. 4b.2. Test Procedure for Turbo Calibration - Low Speed Regime
      
      If you have an automatic transmission, you don't need to worry about operating the turbo in the "surge" regime. At full engine load, the transmission will either downshift to a lower gear or unlock the torque converter, and in either case, the engine speed will be above the critical zone. Nevertheless, it may be worth performing a test similar to what is described below, and confirm that the transmission downshifts if floored at 2000 rpm in top gear before the boost pressure reaches 18 psi, and downshifts if floored at 1500 rpm in top gear before boost pressure reaches 6 psi in response to smooth application of pedal.
      
      On manual transmission vehicles, to test whether you have a potential "compressor surge" issue, install a boost gauge, and then in 5th gear on level ground, preferably at an altitude of not more than 1000 metres, abruptly floor the accelerator at 1900 rpm and note the boost response. Ideally, it should smoothly rise to a peak which is not more than 18 psi somewhere near 2000 rpm. Try tramping the pedal to the floor at engine speeds between 1900 and 2100 rpm. It should never exceed 18 psi in this speed range. Not even for a split second. If it passes the tramp-the-pedal test, try accelerating through that speed range starting at slightly lower speeds and try it in different gears - confirm that initially after flooring the pedal, the boost pressure is lower than its eventual peak when below 2000 rpm, and that the peak is not reached until 2000 rpm. If it passes THAT test, reduce speed to 1500 rpm, and see if you can get more than 6 psi boost at 1500 rpm (don't floor it at this speed unless a few initial tests at part load suggest that you can floor it without exceeding that pressure). If it ever exceeds 6 psi at 1500 rpm or 18 psi at 2000 rpm, or if the peak boost pressure - whatever it is set to - is reached before 2000 rpm (never mind what happens at higher engine speeds), then you have a potential "compressor surge" issue, and you should consider performing the "negative VNT adjustment". Turbo manufacturers recommend staying 10% clear of the surge regime, and if you get it so that it passes these tests no matter what you try, you'll be around 10% clear of the surge line. (Don't be surprised if a bone-stock TDI fails this test procedure ... no chip no tuning box no injectors no nothing. Stock calibration is extremely close to the surge regime and normal production tolerances may put it into the surge regime.)
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   3. 4b.3. VNT Linkage Adjustment
      
      To slow down the rate of boost pressure rise at low engine speeds, using a 10mm wrench and suitable hand tools, release the locknut of the VNT linkage (located above the diaphragm housing on the turbo) and adjust it 1 turn in the direction of lengthening the linkage, then re-tighten the locknut. YOU ARE NOW YOUR OWN TURBO WARRANTY. (If your vehicle was hot-rodded such that this modification was needed, you were already your own turbo warranty anyway, but hopefully the turbo is less likely to blow apart now.)
      
      NEVER NEVER NEVER shorten the length of this linkage to less than it was stock. Attempting to increase the boost pressure at low engine speeds, or attempting to get the turbo to accelerate to full boost faster (which will cause a massive overshoot) is asking for a blown turbo. It has happened, so don't neglect this possibility!!
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   4. 4b.4. Test Procedure for Boost Control System Response
      
      After performing the above procedures, to test whether you have a potential "spike" issue, in high gear at 2500 rpm, abruptly floor the accelerator and note the boost response. In the ideal world, it'll go smartly to the boost setpoint (which is not more than 18 psi) and stay there, but in reality with the stock boost control system, you'll get some overshoot. If it overshoots and fluctuates wildly before settling down, you need to do something to control boost spikes. If it survived this test at 2500 rpm, try it at 3000 rpm. Remember that 18 psi is the safe limit for continuous operation at sea level and this must be de-rated in proportion to barometric pressure at higher altitudes. If you're scared by what you see (and if you drive like that!), you need to do something to control boost spikes!
      
      A mechanical boost controller in parallel with the stock boost control system can be installed as per the Troubleshooting section of this FAQ, under the symptom "Fluctuation of boost pressure". It is critical that any vent hole in this boost controller be soldered closed to prevent a vacuüm leak, and this boost controller will ONLY work in parallel with the stock boost control system so as to eliminate spikes. It cannot be used to raise the boost pressure beyond standard. Do not operate the standard turbocharger at more than 18 psi boost pressure at sea level. Do not operate an engine with a stock compression ratio at higher than 18 psi boost pressure.
      
      Installation: VNT adjustment is straightforward for the home mechanic but you're playing with something that costs several thousand dollars to replace - that's the scary part. Boost controller and boost gauge installation is more involved due to the need to partially disassemble things in order to install the fitting and gauge, and get the boost controller calibrated. Professional installation may be possible but ordinary garage mechanics probably won't want to touch it. If your vehicle has side curtain airbags and a "double DIN" radio, you're going to have a tough time finding a place for a permanent boost gauge.
      
      Advantages: For vehicles that have higher fuel delivery than stock, this provides a method of controlling the boost spikes and fluctuation that are characteristic of the stock boost control system, in order to avoid potentially dangerous operation of the turbo in the "surge" regime, and in order to avoid potential overspeed of the turbo or engine damage due to overboost conditions.
      
      Comments: A boost gauge is MANDATORY if modifications are done to the stock boost control system.
      
      Disadvantages: You are your own warranty.
      
      Cost factor: Nothing for the VNT adjustment, $100-ish for the gauge and fittings, $35 for the mechanical controller from https://www.dawesdevices.com/ - note that the installation will be non-standard per "Troubleshooting - Fluctuation of Boost Pressure" section of this TDIFAQ.
      
      Cautions specific to this modification: YOU are responsible for making sure that you don't do something destructive to your vehicle. But if you have done other modifications, be aware that NOT doing this may be more dangerous than doing it.
      
      Suggested modifications in parallel: None as a result of this, but this modification is recommended as a result of other changes.
      
      "Bang for the Buck": Good, if you need it to control issues caused by other modifications and it saves your turbo. Lousy, if you're doing it for the sake of doing it, or if you bugger something up and break something expensive.
      
      What if I want more? Since this modification is only appropriate for a vehicle that already has more ... you already have more.
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5. Air Intake System Modifications
   
   The stock air filter element can be replaced with an oiled-cotton (e.g. K&N) or oiled-foam (e.g. Pipercross, Amsoil) filter element in the standard filter box, or the entire airbox can be replaced with either a "cone" filter that draws air from within the engine compartment, or an enclosed-type high-flow filter that draws air from outside the engine compartment. The "snow screen" in the duct leading to the airbox (A4-chassis) or in the base of the airbox (A3, B4) can be removed at no cost and no side effects other than transferring responsibility for cleaning the air to the air filter which is responsible for doing that anyway. The snorkel on the intake side of a B4 airbox can also be removed with no side effects. On models where removing the snorkel would make a difference between drawing air from within or from outside the engine compartment, either leave the snorkel there so as to draw in cold air, or make other provisions for getting outside air into the area. If you alter the intake, MAKE SURE that potential water splash and rainwater issues are dealt with at least as effectively as the stock setup.
   
   Installation: "Panel" filters are no more difficult to install than a standard air filter. "Cone" filters and cold-air intake systems frequently require installation of support brackets which should be done according to manufacturer's instructions. However, all of these filters require specific methods of periodic cleaning and re-oiling, which if not done correctly, will result in reduced filtration efficiency (dirt getting past the filter and into the turbo and engine). On foam-type filters, always use a "sticky" filter oil (Amsoil has a good one) which must be worked into the full depth of the foam using fingers. You need a sparse coating of the filter oil but it must be completely dispersed throughout the entire filter media to be effective. IMPORTANT NOTE: Regardless of the type of air filter chosen, make 100% sure that it is sealed against leakage of unfiltered air!! A leaky gasket around the outside of an otherwise good air filter WILL let more dirt into the engine than the entire filter element itself, so be careful about installation - REGARDLESS of what type of air filter is used!
   
   Advantages: Not having to periodically replace the standard paper filter element (but you still have to periodically clean the replacement filter element). On some models where the standard airbox is difficult to deal with (B4 Passat!) the cone filters are easier to install in a way that ensures that there are no leaks of unfiltered air.
   
   Comments: Performance gains are fairly small at best ... some would argue that they are non-existent.
   
   Disadvantages: If not maintained correctly, dirt can get into the engine. The same is true of ANY air filter including the stock one, but aftermarket types are designed to be cleaned and re-oiled periodically, and this must not be ignored or done improperly. If installed in a way that has any gaps around the filter element, dirt can get into the engine, although this is equally true of the standard filter element. You are your own warranty.
   
   Cost factor: Nothing (snorkel / screen removal, to US$20ish (panel filter) to US$200 range for fancier systems.
   
   Cautions specific to this modification: Make absolutely sure that oiled-type filter elements are properly maintained. Check periodically for any evidence of contamination due to leakage of unfiltered air. This is good practice regardless of the type of filter element used.
   
   Suggested modifications in parallel: Doesn't really matter.
   
   "Bang for the Buck": Good due to no cost (for removal of restrictions in the stock system) to rather poor due to limited power gain and higher cost (cone filters and fancy air intake systems). If you don't properly maintain the washable-type filter elements, they can be bad news for your turbo and engine ... just as failure to periodically replace standard paper air filter elements.
   
   What if I want more? Do something that increases fuel delivery ...
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6. Exhaust Modifications
   
   It has been found that the noise level of a TDI engine is acceptable without any muffler at all. It is possible to cut out the standard mufflers (one or both of them) and replace them with either straight pipe sections or "resonators" or straight-thru low restriction performance-type mufflers. Alternatively, the entire system can be replaced aft of the catalytic converter. Some have experimented with the downpipe between turbo and catalyst, but with mixed results.
   
   Installation: Welding required - not normally for the do-it-yourself mechanic.
   
   Advantages: The less restriction you have downstream of a turbo, the lower the exhaust temperature will be, and the quicker the turbo will accelerate to deliver boost pressure. On Passat TDI's, which have a poorly-designed stock exhaust system, modifying the exhaust system can get rid of that annoying resonance at 1200 and 2400 rpm, with a bit of experimentation. An additional flex pipe downstream of the catalyst is reported to be successful.
   
   Comments: Performance gains are fairly small to non-existent. Leave the catalyst in place, it has minimal impact on power (no more than 1 horsepower) - this is worth saving the environment for.
   
   Disadvantages: Some authorities have legal requirements that a vehicle must have a muffler, so one may have to incorporate something that at least looks like a muffler even if it doesn't really do anything, to keep the authorities happy.
   
   Cost factor: US$40 for replacing mufflers with straight pipe, to US$600 for a complete stainless steel "cat-back" exhaust system.
   
   Cautions specific to this modification: It might take some experimentation to get rid of resonance at certain engine speeds which will vary depending on configuration.
   
   Suggested modifications in parallel: Doesn't really matter. De-restricting the exhaust is a good thing to do on highly-tuned vehicles that need all the help they can get to reduce the exhaust temperature.
   
   "Bang for the Buck": Rather lousy. In view of limited to non-existent horsepower gains, but theoretical improvement in exhaust temperature ... the less you spend on this, the better.
   
   What if I want more? Do something that increases fuel delivery ...
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7. Intercooler Modifications, Intercooler Duct Modifications
   
   Cooler air going into the engine means that other modifications that increase fuel delivery can be done, or if the exhaust is smoky at full load, the denser air will help turn some of that smoke into power. It's not worth doing if the vehicle is in an underfueled state of tune.
   
   Installation: Typically, extremely difficult custom fabrication - not normally for the do-it-yourself mechanic. It may be possible to find boost pipes available in kit form, but such a kit isn't likely to include the intercooler. The stock boost pipe from turbo to intercooler on an A4-chassis (located below and to the extreme right of the engine, below the front pulley) looks suspiciously restrictive.
   
   Advantages: Anything that reduces the air intake temperature of a turbo engine closer to ambient temperature is a good thing, especially on highly-tuned vehicles. Anything that enables normal boost pressure to be developed with less work required of the turbocharger is also a good thing, although the pressure drop caused by the intercooler and ductwork is insignificant so actual measurable difference will be very small to nonexistent.
   
   Comments: Only worth doing on a highly tuned vehicle. The standard intercooler is already 65% - 70% efficient which is fine for the stock state of tune, and even for quite a bit beyond that.
   
   Disadvantages: High cost and the likely need to have the vehicle out of service for some time during the installation. You are your own warranty.
   
   Cost factor: If you have to ask ... you probably can't afford it.
   
   Cautions specific to this modification: Make sure that the intercooler piping arrangement can absorb the normal movement of the engine and drivetrain, and make sure the intercooler piping arrangement doesn't complicate required maintenance access. Make sure the intake air temperature sensor is downstream of the intercooler.
   
   Suggested modifications in parallel: This is only appropriate for a highly tuned vehicle with chip and injectors, that needs close to the limit of boost pressure (18 psi) in order to control exhaust smoke.
   
   "Bang for the Buck": Poor, in view of high cost. Should note that some people have installed a vent in the fender liner behind the intercooler, which costs next to nothing but has questionable benefit. If you do this, make sure it has slotted vanes angled such that dirt and debris flying off the front wheel doesn't hit the backside of the intercooler.
   
   What if I want more? Custom intercooler work isn't appropriate for anything short of a highly tuned vehicle with chip and bigger injectors at a minimum, in which case, you've already got more.
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8. Advancing the Injection Timing
   
   There is a misunderstood impression that more timing advance can produce more power. The correct statement is that the *optimum* amount of timing advance will produce the most output, and anything more or less will have adverse effects. Every 2 degrees of timing advance beyond the so-called "minimum advance for best torque" raises peak cylinder pressure by about 15 bar (about 10% increase from stock) and raises piston operating temperature by an unknown amount. Both of those are headed towards holes in pistons if taken too far.
   
   There are two different "injection timing" settings. There is a mechanical adjustment procedure which involves setting the proper mechanical timing of the pump based upon information from VAG-COM's "group 0 basic settings" display, following the procedure in the shop manual. There is also an ECU adaptation channel which can be adjusted using VAG-COM. These procedures are NOT INTERCHANGEABLE and have completely different effects. The mechanical adjustment procedure following the shop manual (and preferably setting the timing within specs but in the upper range of the graph in the shop manual) is the right thing to be doing. Never touch the adaptation channel ... always leave this at stock specifications.
   
   Installation: Not really an "installation". The correct mechanical adjustment procedure for the pump requires VAG-COM, the shop manual (for specification chart and proper procedure) and a selection of wrenches and sockets. Don't adjust it if it's already within specs, and very small adjustments make a big difference. The incorrect VAG-COM adaptation method only requires VAG-COM, but you shouldn't be doing it anyway, so forget that the possibility exists.
   
   Advantages: Setting the injector pump mechanical adjustment so that it is close to the top of the specification range (but still within specs) ensures that under conditions when maximum timing advance is requested (e.g. full load), the pump can actually deliver the requested amount of timing advance. If it can't deliver maximum advance under those conditions, the result will be more smoke, higher exhaust temperature, and reduced efficiency. Proper mechanical timing also makes the engine easier to start under moderate temperature conditions, since under those conditions, the automatic timing adjustment mechanism is not operating. The (non-recommended) VAG-COM adaptation method increases requested timing advance under all speed and load conditions. Advance too much and run the engine too hard ... and the result will be severe engine damage, so don't do it!
   
   Disadvantages: There are no disadvantages to having the injector pump's mechanical timing exactly where it should be! But for the VAG-COM adaptation adjustment ... You are your own warranty, and this is something that is headed towards holes in pistons.
   
   Cost factor: Next to nothing, if you do it yourself and have VAG-COM. (Some models require the bolts at the injector pump sprocket to be replaced, which will cost a couple of bucks.)
   
   Cautions specific to this modification: To do the proper mechanical adjustment, you need to know which way to turn a wrench and have the required "mechanic's common sense". If you don't know which way to turn a wrench, then leave the work to someone else. You shouldn't be doing the VAG-COM adaptation method due to the possibility of severe engine damage.
   
   Suggested modifications in parallel: Irrelevant.
   
   "Bang for the Buck": Irrelevant, you should have the mechanical pump timing correct anyway. BAD NEWS, for the VAG-COM adjustment method, due to potential engine damage.
   
   What if I want more? Do something that actually accomplishes something ...
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9. Grotere turbo
   Het is verleidelijk om te denken dat een grotere turbo meer druk zal opleveren en dus meer vermogen. Het probleem is ... zo werkt dat niet! De turbodruk in de TDI's wordt elektronisch geregeld, dus de motor krijgt de turbodruk welke het moet krijgen volgens de ECU, zolang tenminste alles werkt zoals het moet werken. Een grotere turbo monteren verandert de gevraagde druk door de ECU echt niet, dus het resultaat van een grotere turbo is NUL (tenzij de turbo is geïnstalleerd met een juiste herprogrammering van de ECU).
   Bovendien, turbo's welke te groot zijn voor de motor resulteren in geringere opbrengst onderin en middenin het toerenbereik. Dus als je een grotere turbo installeert, zonder verder iets te veranderen, dan zal het resultaat een geringer vermogen onderin zijn en geen enkel verschil bovenin het toerenbereik!
   Installation: Typically, extremely difficult, unless in the form of a kit which is complete with the required different exhaust manifold, downpipe, intake connections, and lubrication connections, in which case it is simply difficult. If you are on your own for any of the auxiliary components, custom fabrication will be required. It should be noted that complete turbo assemblies (including the exhaust manifold) from other models of TDI engines will go in a lot more easily. Keep in mind that the manifold arrangement on a transverse-engine car isn't the same as that on a longitudinal-engine car, to the extent that the assemblies cannot be interchanged directly because they won't directly fit into a vehicle that has the engine sitting the other way.
   Advantages: Good thing to do on a highly tuned vehicle that needs more boost pressure than the stock turbo can safely deliver ... but ...
   Disadvantages: In order to properly take advantage of a bigger turbo, not only do you need reprogramming to make it work, but also the compression ratio of the engine needs to be reduced by some means in order to control peak cylinder pressure. And that's on top of the fuel delivery modifications that are the only reason for considering a turbo swap in the first place. $ $ $ $ !!!
   Kostenfactor: als je het goed doet ... DUUR !!!
   Cautions specific to this modification: You are SERIOUSLY your own warranty for this situation! Reliability of such a highly tuned engine isn't likely to be very good if the extra power is used frequently or for extended periods.
   Suggested modifications in parallel: Everything. Otherwise, you're wasting your money.
   "Bang for the Buck": Awful, if you do this and you didn't have to for other reasons. Still pretty lousy in view of high cost, if done properly.
   What if I want more? Are you crazy???
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10. Camshafts
    
    This is a largely unproven area. Given that diesel engines cannot have much of a valve overlap period (the stock cams have NO overlap), and that the low-revving characteristics normally warrant conservative camshafts, there isn't likely to be much overall benefit. Cams that "breathe" better at the higher engine speeds where horsepower measurements are made typically result in a loss of low-end and mid-range response, so consider your driving habits and be wary of claimed high gains in an engine speed range that you don't use. One thing is for sure: this isn't the place you want to START modifying a diesel engine. Start with something that increases fuel delivery, instead. And only consider camshafts if the exhaust smoke situation warrants better breathing at high revs and yet is OK down low, but you're at the limit of the turbo.
    
    Advantages: Unknown - unproven as of 9/02.
    
    Disadvantages: Unknown - unproven as of 9/02. Camshafts that have later exhaust valve closing or earlier intake valve opening compared to the stock cam will have less margin of error in the alignment between crankshaft and camshaft. In more severe cases - i.e. if there is more than even the slightest amount of overlap - valves will hit pistons. Camshafts that have higher lift than stock will put more stress on the valve springs - but it's unknown whether that's a significant issue or not with this engine, due to lack of experience. Camshafts that have much later intake valve closing will typically have a loss of low-end and mid-range power offsetting gains in top-end power. This also causes a lower effective compression ratio during starting, so don't go too far, or else ...
    
    Cost factor: Unknown - probably a few hundred dollars.
    
    Cautions specific to this modification: TDI camshafts that will work with the stock pistons must always have a specified exhaust valve closure several degrees BEFORE top dead centre (i.e. before the official end of the exhaust stroke) and the intake valve opening must not happen until several degrees AFTER top dead center (i.e. after the intake stroke has already begun) - based upon measuring the timing at 1mm lift. The stock intake valve timing is 16 degrees ATDC opening / 25 degrees ABDC closing for 189 degrees duration at 1mm lift. Beware of non-standard methods of specifying timing and duration ... often aftermarket cams will have the timing referenced to the fully closed position rather than the standard 1mm in order to "pump up the numbers" while in reality the cam is a lot more conservative than it first appears.
    
    Suggested modifications in parallel: Different cams are probably only appropriate for an engine that's underfueled (no smoke) in the low rpm range, and overfueled (smoke) at higher engine speeds. (This is actually a common scenario.) In other words, it shouldn't be the first thing to tackle. It should be one of the LAST things after doing all sorts of other things to increase fuel delivery. And if it's suspected that an engine is overfueled at high revs and needs more air, make sure the problem isn't a clogged intake manifold or wrongly set static injection timing ...
    
    "Bang for the Buck": Unknown.
    
    What if I want more? Unknown.
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11. Blow-off valves or BOV or similar
    
    Since people insist upon asking about these devices, they need to be discussed ... The purpose of a "blow-off valve" or "diverter valve" on a gasoline engine is to give boost pressure someplace to go when the throttle plate is abruptly closed, to prevent operating the compressor in the "surge" regime. Since a diesel engine doesn't have a throttle plate, a gadget like this serves absolutely no purpose and has no business anywhere on a diesel engine. "But I like the cool noise ..." Too bad, this discussion is focused on things that WORK. Even if you manage to find some way to make such a device work, the noise of people "in the know" laughing at you because they realize that it has absolutely no place on your engine will drown out the noise that the device itself makes.
    
    Installation: Impossible. Cannot be made to work even if you insist upon it.
    
    Advantages: None.
    
    Disadvantages: Won't work. People who actually know how a diesel engine works will laugh at you for even considering it. You are your own warranty.
    
    Cost factor: Too much, all things considered.
    
    Cautions specific to this modification: Won't accomplish anything.
    
    Suggested modifications in parallel: Irrelevant.
    
    "Bang for the Buck": One hundred percent total waste of time and money.
    
    What if I want more? Do something that actually accomplishes something ...
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12. Nitrous Oxide
    
    Not normally suitable for a diesel engine. Since diesel engines normally run lean (except when belching black smoke!) adding more oxidizer isn't going to accomplish anything. If the engine is over-fueled due to extensive other modifications, then this might help power output ... for a little while, until something breaks.
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13. Propane Injection
    
    Although it has been done (search forums), this subject won't be discussed here due to the high risks associated with propane injection. The risks are not only in terms of reliability and durability, but also drivability (most have simple "on/off" controls which cause very abrupt response - very unpleasant to live with on a day-to-day basis) and most importantly, the safety of yourself and others in the vehicle due to the compressed flammable gas storage and the fuel lines containing pressurized flammable gas.
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14. Sensor Tweaks and Various Home-grown Mods
    You are SERIOUSLY on your own if you try any of this. These links are provided because the information is out there and the questions get frequently asked, not necessarily because any or all of them are good or bad. Many of these, if done correctly, can be useful, while the same idea wrongly implemented will wreak all sorts of havoc. YOU ARE YOUR OWN WARRANTY. If you try one of these stunts but get it all wrong and bugger up your vehicle, don't come running to us, and don't go running to the dealer to get them to straighten you out.
    
    Water injection (poor man's intercooler):
    https://forums.tdiclub.com/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=4&t=002533&p=
    
    The famous fuel-delivery wiring harness modification:
    https://forums.tdiclub.com/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=4&t=001893&p=
    
    Faking out the manifold pressure sensor to raise boost on an A4-chassis that has a remote boost pressure sensor outside the ECU ... read ALL of the following before trying this stunt:
    https://forums.tdiclub.com/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=4&t=004077&p= (right idea, but doesn't tell you which wire to splice ...)
    https://forums.tdiclub.com/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=4&t=004059&p= (... but this one does, and contains appropriate warnings)
    https://forums.tdiclub.com/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=4&t=003627&p= (a few reasons for not bothering to try it ...)
    
    Faking out the manifold pressure sensor to raise boost on an A3 or B4 that has the boost pressure sensor inside the ECU and connected to the intake manifold with a rubber hose:
    https://forums.tdiclub.com/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=4&t=000529
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15. Turbo Timers
    
    Strictly speaking, these aren't performance items, but they are sometimes seen as aftermarket items on turbocharged gasoline engines. The purpose is to give the engine a prescribed period of idling so that the turbocharger can be cooled by the engine oil (and coolant, in many cases) that circulates through it. (It has NOTHING to do with waiting for the turbocharger to "spin down". That takes a quarter of a second.)
    
    Diesel engines have lower exhaust temperature than gasoline engines. At anything less than full load, the temperature is FAR lower. It is low enough that the turbochargers in these vehicles do not have a connection to the engine coolant system - only to the lubricating oil. Under most conditions, there is no need to wait for the turbocharger to cool down, and hence, no need for a turbo timer.
    
    The only potential exception is if you are towing a trailer or operating the vehicle at close to top speed (i.e. 160+ km/h) on the highway or climbing the side of a mountain pedal to the metal. But in most cases, you don't just come straight off such full load conditions and stop. You trickle through neighborhood streets, or you wander through a parking lot in search of a spot. Usually this is more than enough time even if you're driving the car hard. And if it DOES take less than a minute measured from the time you first take load off the engine (still coasting out on the road) until you get to a parking spot ... just let it finish that minute at idle.
    
    You don't need a turbo timer.
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6. Aanhanger trekken
   
   De verhouding trekkracht versus vermogen bepaalt de mate waarin die trekkracht wordt geproduceerd (pound-feet per sekonde of Newton meters per sekonde). Als twee voorwerpen dezelfde hoeveelheid trekkracht hebben, dan zal de sneller draaiende meer vermogen hebben. Bij het trekken van een aanhanger gaat het erom een zware aanhanger in gang te zetten vanaf een geringe snelheid, en om steile hellingen op te gaan met minimaal snelheidsverlies. Hiervoor heb je trekkracht nodig.
   
   De TDI is, zoals alle diesels, beter uitgerust om trekkracht te produceren dan vermogen. De kracht die de zuigers naar beneden duwt komt van het brandstof-lucht mengsel in de cilinders. Grotere uitzetting betekent meer kracht op de zuigers. De extra compressieverhouding die beschikbaar is hebben een grotere uitzetting tot gevolg, veel meer dan bij benzine motoren. Deze grotere uitzetting produceert meer kracht om de zuigers naar beneden te duwen (trekkracht).
   
   De diesel vloeistof brandt niet zo snel als de vluchtiger benzine. De relatief langzame uitzetting tijdens de verbranding begrenst de maximale zuigersnelheid en een geringere maximale topsnelheid is het gevolg (rpm). De lagere motorsnelheid begrenst het vermogen.
   
   De TDI motor produceert veel meer trekkracht dan een vergelijkbare benzine auto, maar slechts tot aan de motorsnelheid welke begrensd wordt door de limit van de uitzettingscoefficient van het lucht-diesel mengsel (ca. 3800 - 4000 rpm). Een benzine motor van dezelfde grootte kan sneller draaien dan de diesel, wat resulteert in een hoger vermogen bij hogere snelheid.
   
   De kwaliteit van een TDI om een aanhanger te trekken wordt door VW niet echt eenvormig omschreven in elke gebruikershandleiding. In sommige handleidingen staat dat de aanhanger-capaciteit niet getest is, andere schrijven dat het trekken van een aanhanger niet mogelijk is om technische redenen, nog andere vermelden dat slechts geringe aanhanger-capaciteit aanwezig is. Merk op dat sommige (waarschijnlijk alle) Europesche 90pk modellen een aanhanger-capaciteit hebben van 600 kg (1320 lbs) zonder dat de aanhanger geremd hoeft te zijn, of tot 1200 kg (2640 lbs) bij een geremde aanhanger. De maximum helling moet wel redelijk zijn, en extra koelvermogen is aan te bevelen (is aanwezig op een auto met airco).
   
   Forum leden hebben zonder problemen aanhangers getrokken tot 700 kg, zelfs in de zomer en zonder enige veranderingen aan het koelsysteem. Natuurlijk verminderen de prestaties, en wees voorbereid op een hoog brandstof verbruik. Op de autoweg is het voor-oppervlak van de aanhanger van groot belang. Vanzelfsprekend moet de auto in goede conditie zijn wat betreft motor, banden en remmen. Doe voorzichtig met een aanhanger er achter, dat komt de veiligheid in het verkeer en de duurzaamheid van de auto ten goede. Stamp het gaspedaal niet tot op de bodem en hou hem daar een tijd vast.
   
   Voor het trekken van een aanhanger maakt de grotere trekkracht van de TDI hem superieur t.o.v. benzine motoren. Dit betekent niet dat er een groter gewicht getrokken mag worden, alleen maar dat hetzelfde gewicht veel makkelijker getrokken wordt. De richtlijnen voor de aanhanger-capaciteit van een VW benzine auto kunnen ook gebruikt worden als leidraad voor de TDI. Door het het aangehangen gewicht zal de turbo harder moeten werken en wordt daardoor warmer. Zorg voor extra tijd waarin de turbo af kan koelen voordat je de motor uitschakelt.
   
   Auto's waarvan het vermogen door tuning is toegenomen (zie elders in dit stuk) zullen aan de ene kant nog prettiger met een aanhanger omgaan, maar aan de andere kant moet je nog meer zorg in acht nemen om de extra warmteontwikkeling van de turbo, de motorolie en de motor zelf binnen de perken te houden. Het is verstandig om een turbodrukmeter te hebben, en ook een thermometer voor de uitlaatgassen, om goed in de gaten te houden dat je de meeste tijd binnen de grenzen van de mogelijkheden blijft, en alleen maar de extra kracht gebruikt voor korte tijd. Wees bijzonder voorzichtig bij lange hellingen; het is verleidelijk om de extra kracht te gebruiken om de normale kruissnelheid aan te houden, maar dat is niet echt gezond voor de turbo en de motorolie. Een extra oliekoeler is dan aan te bevelen, of verwijder heel eenvoudig de plastic beschermkap van de motor aan de onderkant om meer luchtcirculatie mogelijk te maken.
   
   Omdat het zeer waarschijnlijk is dat je met een aanhanger langere tijd veel dichter tegen de maximum capaciteit van de motor aanzit, is het erg verstandig auto's die regelmatig aanhangers trekken, slechts conservatief te tunen.
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7. Fuel Consumption
   
   High fuel economy claims are often subject to ridicule and accusations of miscalculation, deceit, or violation of basic laws of physics. Low mileage lamentations are often accompanied by accusations of fraudulent misrepresentation by the manufacturer and questions regarding possible defective or misadjusted components. Both of these types of comments are prevalent among those who drive the TDi. These engines are so efficient with fuel that a small percentage change in mileage results in a large numerical change in miles per gallon.
   
   The TDi equipped Volkswagens are sold throughout the world. Different mileage calculations are used in other locations and may not convert to equivalent numbers. In North America, mileage is often specified as miles per gallon. The UK may also refer to miles per gallon. Be aware that the quantity of fuel contained in a "gallon" depends on the type of gallon, US or Imperial. The more common consumption rate used elsewhere in the world is liters per 100 kilometers. When comparing expenses remember that "dollar" is the name of currency in Australia, Canada, New Zealand, and the USA. The exchange rate fluctuates between these currencies. The conversion of one measurement to another gives additional opportunity for errors. When comparing someone else's set of figures to yours bear in mind that their driving style, wheel alignment, weight, acceleration techniques, braking habits, climate, preferred cabin temperature, tire pressure, etc., etc., are different than yours. These are some of the reasons for the disclaimer "your mileage may vary". Bear in mind that no matter what your TDi's mileage, it is better than it would be if you were using a gasoline engine in the same car under the same conditions.
   
   The automatic transmission used in these vehicles increases fuel consumption by about 20% compared to the manual transmission.
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8. Suspension
   
   The suspension components used on these vehicles are common to other 4 cylinder VW models which use the same chassis. A wide variety of aftermarket components are available for most VW vehicles, and a large number of aftermarket shops deal with these components. As a result, only general recommendations will be discussed here.
   
   People who wish to modify suspension fall into two groups: those who want to lower the vehicle so as to achieve a certain appearance, and those who wish to improve the handling performance of the vehicle. For various reasons, these objectives typically do not coincide with each other: modifications which lower the vehicle usually have an adverse effect on handling.
   
   Lowering a VW Golf-chassis-based vehicle is BAD, for the following reasons.
   
   The angle of the lower control arms on the front MacPherson strut suspension is such that if the vehicle is lowered, it will tend to send the camber of the front wheels in the wrong direction in response to body roll. These lower control arms should always have the pivot point at the outer ball joint closer to the ground than the pivots which attach to the chassis, when the vehicle is at static ride height. If this is not the case, the geometric roll center of the vehicle can be below ground level which will tend to INCREASE body roll, the camber of the front wheels will go in the wrong direction when body roll is encountered, and the steering geometry will be adversely affected.
   
   The reduced amount of suspension travel above static ride height before hitting the bump-stops necessitates the use of springs with a stiffer spring rate, with adverse effects on ride quality. This forces the use of dampers with more damping, again adversely affecting ride quality. You cannot have lowered suspension together with improved ride quality ... if an attempt is made to preserve original spring and damping rates while lowering the suspension, the ride quality will still be adversely affected due to the frequency with which the suspension hits the bump-stops ...
   
   There is marginal ground clearance below the oil pan, and on later models, that oil pan is cast aluminum and is prone to fracturing. If that happens ... you need a tow truck. And you'll have to worry much more about the bottom of the front bumper striking curbs.
   
   Now that this issue has been dispensed with, what can be done to improve handling?
   
   These vehicles are typically set up to understeer strongly. Better wheels and tires will help a lot (wide variety available, see https://www.tirerack.com/ or any of a large number of other sources), but what is really needed to reduce understeer is greater rear roll stiffness. The end with greater roll stiffness tends to slide first. To shift grip away from the rear and towards the front, while increasing overall roll stiffness to reduce body roll, add an additional rear antiroll bar. BEWARE, understeer is built into the vehicle for the benefit of drivers who don't know how to control oversteer. If you install a rear antiroll bar, and a situation occurs where the vehicle oversteers and the driver doesn't manage to control it ... don't say you weren't warned. You're the one who chose to do it. Also remember that even if YOU feel that you can control an oversteer situation, family members or other people driving the vehicle may not be able to.
   
   Better dampers, and springs with a slightly greater spring rate, will improve turn-in precision, at some cost in ride quality. Don't install stiffer springs on stock dampers. You need more damping to control motions resulting from the greater spring rate. Firmer dampers and stock springs is fine within reason.
   
   The author's vehicle has Bilstein HD struts and shocks on stock springs and no additional antiroll bar, with performance tires 10mm wider than standard on rims 1" larger diameter than standard, with a profile 10 points lower than standard (in other words, conventional "+1" sizing). This arrangement is well suited to this vehicle's daily-driver mission where the outer limits of adhesion aren't normally tested, but it gives very noticeable improvement in turn-in and steering feedback, and raises the ultimate amount of grip available without compromising the margin of stability associated with the built-in understeer of the vehicle.
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9. Brakes
   
   The brakes used on TDI equipped vehicles are standard VW components used in a wide variety of vehicles. There are a large number of aftermarket shops that handle VW performance parts, which can be a help in this area. It is normal to only upgrade the front brakes of front-wheel-drive vehicles, since the rear brakes only handle a relatively small proportion of the braking loads. The drum brakes used on the rear of A3 Jetta TDI models are not a limiting factor in brake performance.
   
   Retrofit of ABS to a vehicle which was not originally equipped with it, is for all rational intents and purposes, impossible. There are too many components which are different between a vehicle equipped with ABS and a vehicle not so equipped.
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10. Driveline

    1. Clutch
    
    For the A3 and B4 cars, if an upgraded clutch proves to be necessary as a result of engine upgrades, the VR6 clutch assembly is a direct replacement, and any aftermarket clutch that is suitable for a VR6 application is also suitable for a TDI application.
    
    For the A4 cars, a VR6 or VR6-compatible clutch assembly can be installed if the flywheel is replaced with a flywheel from a 4-cylinder G60 engine.
    
    Upgraded clutch parts are available in the USA from https://www.dieselgeek.com/ and elsewhere.
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    2. Final drive and differential
    
    The weakest link in the O2A (for A3 and B4 cars) and O2J (for A4 cars) 5-speed manual transmissions involves the final drive and differential assembly. Vehicles with performance engine modifications, which may experience hard drag-racing-style launches, wheelspin that results in wheel hop, and similar actions which place the greatest stress on the final drive assembly, should have the final drive and differential upgraded to reduce the chance of having problems in this area. Quaiffe or Peloquin final drive units employ a "torque biasing differential" that improves grip in slippery conditions while eliminating the spider-gear cross-shaft which is a weak point in the stock differential. These final drive units should be bolted to the final drive ring gear with high-strength bolts, usually available as part of the final drive kit. Installation of such components is best left to experts in manual transmissions.
    
    Be aware of the possibility that some earlier models may use a different style of half-shaft mounting arrangement compared to other models that use a similar transmission. There have been a few situations where apparently the half-shaft mounting arrangement of an O2J transmission was found on what is otherwise an O2A transmission. Consult with experts in VW manual transmissions, such as wherever you intend to purchase the unit from or whoever you are planning to get to install it.
    
    The CV or constant-velocity joints on the half-shafts are the next weakest link, but not much can be done. If you do hard launches from a standing start ... be prepared for shortened CV joint life.
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Deze pagina is voor het laatst gewijzigd op 22 januari 2006.