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Increasing power for the VW TDI: basic performance upgrades

It is easy to get more power out of a turbo diesel engine for two reasons.  First, a turbo engine can increase power and response by reducing restriction in the exhaust system.  For more details on turbocharging systems and exhausts for turbos, please read: 1000q: turbocharging.  Second, the diesel engine can run well in a wider range of air/fuel ratio (AF).  This means that unlike a gasoline engine where the AF must stay in a small range to run well, a diesel engine can safely get more power by adding more fuel.  Just give the engine more fuel to the point of diminishing returns and you will get more power.  However, give it way too much fuel and you will get high exhaust gas temperatures!  This is the opposite of a gasoline engine where running rich, to a point, will give you less power but lower exhaust gas temperatures.

Another downside of too much fuel is excess smoke caused by unburned fuel.  A little extra fuel from a "chip" won't kill all the mosquitoes in the neighborhood, but a "chip" and fuel injectors that are too large might create a smoke screen when you are heavily boosting the turbo.  How much smoke is acceptable is highly subjective from owner to owner and even varies from car to car!  Excess smoke associated with unburned fuel is also a sign that fuel is being wasted.  Way too much fuel will raise the exhaust gas temperatures and stress the engine and turbo.  For a longer answer, please read 1000q: turbocharging.  The short answer is that upgrading the fuel system or chipping the car will only produce gains up to a certain point.  Past that, you must upgrade other parts of the engine to be able to efficiently burn more fuel, like the intercooler, turbo, and piping, and computer programming.  Upgrading your car's power to extreme levels will slightly shorten the engine's lifespan and could produce such high stresses that it even might damage the engine if you are not careful.

These basic upgrades mentioned below will not significantly reduce engine reliability or lifespan.  Also see 1000q: advanced and extreme performance upgrades for advanced performance upgrades such as intercoolers and upgraded turbos.  Also know that some performance upgrades may cause the clutch to slip.

Chip and software upgrades for the VW TDI diesel 

The first recommended modification would be a chip/software change.  This is a modification to the stock programs on the car's computer.  The 4th generation (mk4) VW TDIs require soldering for some chips, so it is recommended to ship it to a professional chipping company if you want a chip.  Other programs don't require soldering, you just "reflash" the data on the car's computer.  3rd generation (mk3) ECU are socketed so you just pop out a chip and install a new one by hand.  (there are some early passats which use soldered chips).  For a daily driver, this is an excellent first step since it preserves and improves stock drivability and won't significantly reduce fuel economy as long as you don't have a lead foot!  I do not recommend chips for nonturbo or nonsupercharged or non diesel cars because you would have to pay hundreds of dollars for a chip that only increases power by small amounts, often only noticeable at high rpm near redline.  With a turbo or supercharged car, you could gain 20-50 peak horsepower (depending on the car) with a much better power curve from chipping due to the properties of turbocharging and diesels.  Some chip vendors for the TDI will also reflash/upgrade their chip software for the original owner for free or at a discount, so that as your modifications change you dont have to worry about the cost of adjusting the software.  

TDI chip vendors include KermaTDI, Alligator, Rocketchip, Wetterauer, Upsolute, Revo Technik, Oettinger.  I do not prefer or endorse any specific chip but KermaTDI and Alligator have kept their programs updated and are easy to contact, especially if you use larger than stock turbos, nozzles, etc.  Rocketchip sells a good product but there have been many complaints about not returning emails or calls, this could be an issue if you need support.  The other brands will also work fine for a basic chipping but have not updated their programs in a while.  The risk of clutch slip is increased when you increase the power.

Why you can't just raise the boost

If you're new to turbo car tuning, you may have heard that just turning up the boost turns up the power.  More boost only equals more power if fuel is also increased.  In some gasoline cars, the fuel maps or injectors, etc., may not support increased boost.  You must do your research on each gasoline car or each diesel car before modifying since turning up the boost can cause engine damage.  In the 1996-2006 VW TDI, the car's computer controlled fueling will not increase fueling in response to increased boost.  Unlike most gasoline turbo car's open loop/closed loop control, a TDI diesel is closed loop only.  There is no o2 sensor to meter fueling and after the injection pump in earlier cars or pumpe duse injectors in later cars, there is no metering of fuel by the car's computer.  With a diesel, you should be thinking about adding more fuel and adding enough air to control smoke, keep EGT down, etc.

Fuel injectors or nozzles

The VW TDI responds well to larger fuel injectors or nozzles as an alternate basic fuel upgrade (instead of a chip).  This delivers more fuel to the engine without changing the car's computer.  Keep in mind that gasoline cars require a narrow AF ratio, this can cause the engine to run too rich and run poorly unless you have a "chip" for that size fuel injector, within limits.  Diesel engines are variable ratio engines and running rich will produce more power as long as you can supply enough air to burn all that fuel.  One advantage of changing fuel injectors vs. a chip is that it also increases the amount of fuel to the engine and maintains stock drivability.  Another advantage is that all fuel injector nozzles will wear out over time so upgrading the fuel injectors/nozzles may prove to be a good value alternative to buying new nozzles in the same size.  For detailed fuel injector information and technical details, see 1000q: fuel injector and nozzle TDI FAQ.  For the updated detailed installation procedures, see 1000q: fuel injector / nozzle install.

The below information is specific to 1996-2006 VW diesels.  With these diesels, you can adjust this by using a VAG-COM, a VW Auto Group Communication cable, to change the ECU "adaptation" and fine tune the fueling to help reduce smoke and make very small changes to fuel consumption.  Note: the automatic and manual transmission VW diesels have different sized fuel injectors and injection pumps!  The automatic transmission uses smaller nozzles but a larger injection pump.  Putting a larger injection pump on a manual transmission car is like putting larger nozzles on an automatic transmission car because they both increase fueling.  More information is in the fuel injector FAQ linked in the above paragraph.

.184 injectors: stock on 90hp North American manual transmission diesels. This would be a mild upgrade for automatic transmission cars, although recalibrating with a VAG-COM is recommended for automatic cars because they have a larger injection pump, this will adjust smoke.

.205 injectors:  stock on 110hp European manual diesels.  Keep in mind that they also had a larger turbo and different ECU to supply more air to burn more fuel.  Note that power gains from switching from .184 to .205 will be less than .184 injectors with a chip.  For most car setups, this is a good choice.

.216 injectors:  stock on the 150hp 5 cylinder VW t4 diesel vans.  It is not recommended since a chip with .184 or .205 injectors would be more efficient at raising power safely, although it can be done.  When combined with a chip, you should be adding larger turbos, intercooler, exhaust, and fueling supporting mods.  See Extreme power modifications for more details.

All pumpe duse injectors:  the replacement is more complicated than earlier cars.  If you have a pumpe duse, I would recommend getting a chip and seeing how you like it first before swapping injectors.  If you still want injectors, kermatdi at http://www.kermatdi.com/ sells larger pumpe duse injectors.  If you don't know if you have a pumpe duse, all VW diesels in North America model years 2004-2006 are pumpe duse.  For more details, see 1000q: model year differences - pumpe duse.  Also note that since pumpe duse engines came with more power than earlier cars, getting a chip will give you more power than earlier cars that only have a mild chip and nozzles.

The picture shows the location of the fuel injectors on a VW diesel (non pumpe duse) circled in blue.  Cars with pumpe duse have the injectors located inside of the valve cover so it is harder to swap them out.

Chip + fuel injectors

Using both an aftermarket chip and fuel injectors will also produce more power than one modification alone.  However, since the ECU doesn't know if you have larger injectors or not, make sure you get a chip that is written for larger injectors.  Otherwise, you will have higher levels of smoke and lower economy which a chip will fine tune.  Also note that because of the higher power levels, your clutch could start to slip due to the higher torque.  Note that turning off the EGR with VAG-COM is not recommended since it could set off a check engine light and the ECU may adapt and result in lower fuel economy.

If you have an automatic, keep in mind that they have a larger injection pump than manual cars so if you are using larger nozzles with a chip, the suggested upgrade is to use injectors that are 1 size larger: .184.  Any larger than .184 with an automatic transmission and you could get smoke. .205 is the suggested maximum size you can use with a chip in manual transmission with no other supporting modifications.  Again, each car, each software tune, each driver, and their tolerance for smoke is slightly different.  Other mods may increase or decrease the amount of smoke and power you see.  Clutch slip is more likely with chip + nozzles since using them together makes more power and torque.  As a conservative recommendation, the mk4 TDI should use smaller nozzles and a light tune or only a chip or only nozzles to prevent clutch slip. Mk3 and Mk5 cars seem to be have stronger clutches.

Fueling limitations

Keep in mind that delivering extra fuel at the engine must be supported by extra fuel flow in the whole system.  The fuel filter must be unclogged, the injection pump must be able to supply a sufficient amount of fuel.  The automatic transmission VW diesels had a 11mm injection pump, the manual transmission had a 10mm injection pump.  Basically, the maximum amount of fuel that the smaller manual injection pump made was offset by using larger fuel injectors.  The diesel long block, the main engine itself, is the same in auto and manual transmission cars, but the auxiliary parts such as the flywheel, the injection pump, etc., are different.  As a result, if you require a lot more fuel than what the injection pump can mechanically deliver, the ECU will sense a fault and can go into a safety mode.  

1996-2003 TDI did not have any electric fuel pump, the injection pump was the fuel pump.  Adding a "lift pump" from a 2004+ TDI, a low pressure in-tank electric fuel pump, will boost fuel pressure to the injection pump and support higher power levels at higher rpms.  In the above picture, the injection pump is the rectangular metal pump below the left two fuel injectors.

Air limitations

Remember, adding more fuel must be accompanied by more air, otherwise you are just over fueling the engine, creating smoke, and unnecessarily raising the exhaust gas temperatures.  Everything else being equal, you can't turn the boost up beyond safe limits without raising temperatures and stressing the turbo.  See 1000q: turbocharging to understand more about turbocharging, and read 1000q: advanced performance modifications for how to efficiently add more air.  Unlike many other cars, an aftermarket oiled high flow air filter is NOT recommended for the TDI due to the mass air flow sensor (MAF), especially the mk4 generation.  There is a strong link between aftermarket high flow oiled cotton air filters and MAF failure when used on a TDI.  In addition, to save parts and inventory costs, VW used the same air filter on most mk4 TDI and the 240hp R32 Golf, and there is more than enough air filter capacity for all generations of TDI.  See 1000q: MAF faq for more details on the MAF sensor.

Advanced timing

The way advanced timing will increase power is by injecting fuel later as the engine piston is moving up and down.  By adding fuel later in the stroke, you create greater pressure and energy.  Note that this also increases stress and heat on the engine by a small amount.  Fuel that is injected early has more time to burn and produce energy than fuel that is injected late.  As long as you stay within the normal operating range of engine timing as found in your service manual, advancing the timing may result in more power, easier starting, and better fuel economy but hotter temperatures and engine stress.  Note that there is an optimum point for timing.  Anything other than this maximum point will decrease power and mileage, but this is impossible to know since there are too many variables involved.  As long as you stay within the normal operating range of engine timing, you are safe.  Slightly advanced timing will also result in easier cold starting.

Note that biodiesel also slightly advances the timing.  It does not change the mechanical timing because that is set by the injection pump and ECU.  Due to the properties of biodiesel and higher cetane, biodiesel will burn faster when injected, effectively advancing the timing slightly.

Also note that there are two different ways to advance timing: through physically adjusting the injection pump pulley, or through VAG-COM adaptation.  If you have an aftermarket chip, the chip maker has already adjusted the timing as necessary!

To adjust the injection pump timing, adjust the injection pump as shown in (for mk3 cars: 1000q: timing belt part 2) , OR (for mk4 ALH cars: 1000q: timing belt part 2).  Use a VAG-COM to make sure that you are staying within the normal operating boundaries and adjust the pump as necessary.  mk4 pumpe duse cars (2004+) do not have mechanically adjustable injection timing.

More advanced power upgrades

For most people, this is where they stop.  You can still buy off the shelf performance upgrades to further increase your power, but past this point, the value of dollar/horsepower in the upgrades is reduced and you may also experience more reductions in fuel economy.  The most common advanced performance upgrade is a larger intercooler.  However, only the VW Mk4 cars have off the shelf replacement intercoolers, so I have included this modification in advanced performance modifications.  Also note that beyond this point, the likelihood of clutch slipping is greatly increased.  Mk4 cars may get clutch slip with just a chip, so remember that if you want more power you need more clutch clamping force!

Also note that performance tires are also recommended to get the most power to the ground.  Sticky tires usually wear out faster and give less fuel economy than harder non performance tires, so take this into account when thinking about what your goals are.

Other advanced performance upgrades would include custom exhaust components, upgraded turbo systems, or custom made tuning chips.  Keep in mind that one of the biggest advantages of 4 cylinder diesels is that they have enough power to serve as daily drivers and deliver nearly unbeatable fuel economy.  The most common diesel passenger cars, VWs, are not very good sports cars when compared to true sports cars.  They are light but the suspension setup is not that great, front wheel drive isn't good for a sports car, and you are limited by tire width (because suspension clearance of tires).  None of these traits contribute to making a very good sports car out of the box.  A stock used corvette will always be a better sports car than a brand new TDI and for less money.  A corvette doesn't have rear seats, but this article's focus is on power upgrades, not rear seating or fuel economy.

Other cars, such as the mercedes benz or domestic 4wd trucks would be a better candidate for very high power they have at least 6 cylinders, start with much more power than a 4 cylinder, and have the potential for more streetable power.  In the end, the whole point of modifying your car is to suit your taste and preferences, so if you want to upgrade your existing car read on to 1000q: advanced performance upgrades

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