TDI FAQ

Common questions and answers before proceeding further -

Q. Is it going to affect my warranty?

A. ANY modification that you make to the engine or its control systems could provide grounds for a manufacturer or aftermarket warranty supplier to deny a warranty claim.

Q. Is my engine going to blow up?

A. Whatever modification you choose, if it is done in moderation and properly set up, the likelihood of serious problems is very low (but not zero). In the later parts of this section that discuss individual modifications, wherever possible, we have provided some information to get you started on proper set-up of whatever it is that you are doing.

Q. Is it going to use more fuel?

A. Typically, properly-done modifications to a diesel engine DO NOT significantly increase fuel consumption unless the "extra" power is being requested all the time ... normally not possible in North American driving conditions. It should remain within a few percent one way or the other. The reason is that under part-load conditions that account for the vast majority of normal operation, most properly-done modifications do not significantly change the way the engine operates.

Q. Can I get <generally some ridiculously large number> HP from this engine?

A. The standard response for anyone involved in motor racing is this: "Speed costs money. How much do you have?" And it's not a linear relationship, either ... more like exponential. Keep in mind the limiting factors discussed in an earlier section. READ IT AGAIN. A conservative approach to engine tuning, using proven methods and components, will create little or no side effects if done properly. A more aggressive approach requires a certain amount of re-engineering of the vehicle and perhaps accepting some loss in long-term durability. Before setting a lofty horsepower goal, ask some questions. What are you going to be using the vehicle for? What are the essential minimum requirements for it to fulfill that role? What potential benefit is there to going well beyond those requirements and can the resulting expense and potential loss of durability be accepted?

It doesn't take 180 horsepower to cruise at 100 km/h ... or even 180 km/h. A bone stock 90hp engine will do that without any worries. It doesn't take 180 horsepower to do zero to 100 km/h in 8 seconds, either. So set realistic goals. Don't just pick a horsepower number for the sake of having it. For a daily driver vehicle, the overall results of a reasonable and conservative approach will be more satisfactory and less costly to achieve.

Q. Do I need to change how I maintain the vehicle?

A. All of the big power adders (chip, box, injectors) add more fuel, and if you use the extra power frequently, there will be an increased amount of soot added to the engine oil. It is advisable to shorten the oil change interval in these situations, and due to the potential for more stress and more heat loading, don't even think about using anything other than synthetic oil of the proper viscosity grade and specification approvals for your vehicle. It's also advisable to keep the air intake system in optimum operating condition, considering all components including air intake screen (if present - consider removing it), air filter, intercooler, and intake manifold.

Q. Is it going to increase exhaust emissions? Will I fail my annual inspection?

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. The text in this document says I have to make other changes along with what I want to do. Do I REALLY have to do all that? Why can't I simply do what I want to do and nothing else?

A. In cases where other changes are recommended, if language such as "It will be necessary to ..." is used, then typically those changes really are necessary in order to prevent a bad side-effect, or at least to reduce the magnitude of the bad side-effect. In most cases, these recommendations are made based upon experience. You may find that if you try to skip steps, sooner or later you'll find out why those other changes were recommended ... hopefully not in the form of a shattered turbocharger or broken connecting-rod ...

Q. It says in this document that [some combination of modifications] is too much, or that [some setting] is too much. Why?

A. Typically - past experience, judgment, known limitations, known design documentation, calculations and extrapolations from similar but slightly different variations, and on and on ...

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 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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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 www.dieselgeek.com (USA) or www.autobraun.com (Canada). Injector nozzel replacements are also available at places like 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 ... 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 ( 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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4a. 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:
http://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 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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4b. 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.
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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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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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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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 vacuum 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 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. Bigger Turbo

It's tempting to think that installing a bigger turbo will result in more boost pressure and hence more power. Problem is ... it doesn't work that way! The turbocharger boost pressure in these vehicles is electronically controlled. The engine gets the boost pressure it wants, as long as everything is operating as it should. Changing the turbo doesn't change the electronic setpoint, so unless the turbo is installed along with suitable reprogramming of the electronics, the benefit will be ZERO.

Also, turbochargers which are too large for the engine will result in weaker low-end and mid-range response. So if a bigger turbo is installed without doing anything else, the result will be poorer bottom end response and no difference in top end response!

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. $ $ $ $ !!!

Cost factor: To do properly ... EXPENSIVE!!!

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):
http://forums.tdiclub.com/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=4&t=002533&p=

The famous fuel-delivery wiring harness modification:
http://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:
http://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 ...)
http://forums.tdiclub.com/cgi-bin/ultimatebb.cgi?ubb=get_topic&f=4&t=004059&p= (... but this one does, and contains appropriate warnings)
http://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:
http://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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f. Trailer Towing Issues

Torque versus Power - Power is the rate at which the torque is produced (pound-feet per second or Newton meters per second). If two objects produce the same torque, the faster rotating object has more power. Towing a trailer typically involves getting a heavy trailer moving from a low speed, and pulling up steep hills with minimal slowing down, and for these applications, you need torque.

The TDI, like all diesels, is better suited for producing torque than producing power. The force driving the engine's pistons downward is the expansion of the air/fuel mixture within the cylinders. A greater expansion means more force on the piston. The extra compression ratio available allows a larger percentage of expansion than gasoline does. This greater expansion produces more force to push the piston downward (torque).

Diesel fuel does not burn as rapidly as the more volatile gasoline. The relatively slower expansion limits the maximum piston speed and this results in a lower maximum engine speed (rpm). The lower engine speed limits the power (torque times rpm).

The TDi engine produces far more torque than a comparable gasoline fueled engine, but only up to the engine speed that approaches the air/ Diesel fuel expansion rate limit (about 3800 to 4000 rpm). The similarly sized gasoline fueled engine can rotate faster than this Diesel limit, and as a result has a higher power rating above this speed.

Towing capacity of TDi powered Volkswagens is not described consistently in owners manuals. Some owners manuals indicate that the towing capacity has not been tested, others indicate that trailer towing is not possible for technical reasons, others give a small towing rating. Consider that some - possibly all - European 90hp models are given a towing rating of 600 kg (1320 lbs) without trailer brakes, or as much as 1200 kg (2640 lbs) if the trailer has brakes and the maximum slope does not exceed a certain (fairly generous) amount, and provided that certain cooling-system modifications are done (which are already present, on vehicles equipped with air conditioning).

Forum members have towed trailers weighing as much as 1500 lbs without problems, even in summer and without any cooling system changes. Obviously the performance envelope is reduced in every way, and be prepared for a big penalty in fuel consumption. For highway driving, the frontal area of a trailer is a major concern. Obviously it's sensible to make sure that the car is in good shape considering engine, tires, and brakes. Take it easy while driving with a trailer behind - this is prudent for both safety and longevity of the tow vehicle. Don't mash the pedal to the floor and leave it there.

The greater torque of the TDi produces a superior tow vehicle engine when compared to the gasoline powered equivalent. This does not mean that greater weights may be towed, only that the same weight can be towed with more ease. The guidelines for towing with a gasoline powered Volkswagen can be used as a reference for towing with the TDi. The additional weight of the trailer will mean more usage of the turbocharger and hotter turbocharger temperatures. Be sure to allow additional time for the turbocharger to cool before shutting off the engine.

Vehicles whose performance has been increased (see previous sections) will on the one hand be considerably more pleasant to drive while towing, but on the other hand, caution is necessary so as not to exceed heat-related limits on the turbocharger, engine oil, and the engine itself. It's best to use either a boost gauge or an exhaust temperature gauge so as to remain within the limits of the engine's standard power output most of the time, and only use the extra power for short periods of acceleration. Be particularly cautious on long uphills; it's tempting to use the extra power to maintain full highway speed, but this might not be healthy for your turbocharger or your engine oil. An auxiliary engine oil cooler may be advisable, or perhaps simply temporarily removing the plastic pan beneath the engine to permit more air circulation.

Due to the much greater likelihood of running close to full load for extended periods of time, trailer towing vehicles should always be conservatively tuned ... not tuned right to the edge of blowing to bits.
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g. 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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h. 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 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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i. 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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j. 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 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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