Once you have done the basic performance modifications, more extreme and more expensive modifications can give you more power. While these modifications will increase the power, they may also slightly decrease the reliability of your car. The first few "advanced" modifications can be purchased as off the shelf components or by vendors requiring little modification or customization by the installer. At the bottom are "extreme" level modifications which would be completely custom jobs and would require a higher level of customization and fabrication by the owner (or mechanic). Also remember that one modification by itself may not give the full "advertised" power increase, adding supporting modifications creates a synergy which increases the effectiveness of the other upgrades. In other words, 2 modifications that would each give 10 hp when installed separately would give 25 hp when both installed.
With all of these modifications except the larger intercooler or clutch kit, there is a risk of blowing head gaskets from excess pressure or melting pistons or damaged bearings if you are lucky. If you are unlucky, a cracked rod will shoot through the engine block and blow a hole in the side of the engine. None of these modifications except the intercooler or clutch kit are recommended without basic knowledge of how it works and what you expect the result will be! Also note that additional power is pointless if you don't have traction, so make sure you choose a tire that can put the power to the ground!
Also keep in mind that when doing these modifications to a 4 cylinder VW engine, depending on your personal preferences, you may be reaching the point of diminishing returns where a diesel truck or gasoline sports car would be more cost efficient in terms of speed vs money spent. Of course, none of these will give the same gas mileage as a small 4 cylinder diesel will but this article's focus is on power upgrades, not fuel economy! For example, a Dodge Ram Cummins with a 5.9L can get 400hp and 700 torque with just basic performance upgrades using off the shelf parts and a lot less money and effort than taking a VW to 240 hp and 300 lb-ft tq! In the end, the whole point of modifying your car is to suit your taste and preferences, so read on!
These upgrades are difficulty level 2-5+, depending on your experience and access to fabrication.
Every car that runs higher than stock boost levels could benefit from this. The reason why I listed this as an advanced performance modification instead of a basic performance modification is because some intercoolers advertised as direct bolt on still require up to 8 hours of test fitting and modification. Other year TDIs have no off the shelf aftermarket intercooler upgrade at all, so they would be custom jobs. Installing a chip to raise the power of the diesel is only a difficulty level of 1. Installing an aftermarket intercooler would be rated at a difficulty of 2 to 3, depending on access to tools and fabrication experience. A completely custom intercooler would be rated at difficultly level 4. Basically, the intercooler would have to fit inside the space of the old intercooler, have the correct piping and piping adapters, most likely silicone adapters, and have the proper mounting holes to bolt up to the old spot. All of this requires detailed measurement and parts sourcing.
There are aftermarket side mount intercoolers available for VW Mk4 cars but the bottom pokes out beyond the bottom of the front fascia, leaving it more vulnerable to parking or speed bump damage than the stock intercooler. If this is acceptable to you, it is your best choice. There are also front mount kits available for VW Mk5 cars to replace the stock front mount intercooler, but they require test fitting and cutting to install. With all other makes of turbocharged diesels, your best bet is to talk to a local auto performance shop that specializes in turbo and fabrication. Here are some pictures of a VW Mk4 car with aftermarket intercooler, click the thumbnails for a larger view.
In choosing components, remember that bar-plate intercoolers, while more expensive, are efficient air-air intercooler designs. For more details on intercooler design, please read 1000q: turbocharging- intercoolers. Also try to fit quality silicone couplers and t-bolt clamps since they tend to leak less.
This is a chip that is custom to your car. Since each car is slightly different, using an off the shelf aftermarket chip that is tailored to your specific modifications will still leave untapped power and economy. A custom chip may give the car slightly better power and economy, each car is different. In addition, your tolerance for smoke, having a stronger clutch, etc., are all different. Dieselinside and KermaTDI's chip tuning takes logs of your setup and adjusts it as necessary. It is done by email and requires your feedback on setup. Rocketchip also is known for custom tuning on-site tuning.
With more torque comes the need for more clamping force at the clutch. Since turbo diesel engines produce more torque than horsepower, this can quickly create a slipping clutch if you go beyond the basic performance modifications. TDIs also tend to produce boost spikes which create a power spike, creating clutch slip where there otherwise shouldn't be. Clutch kits are rated for torque, so choose a clutch that will exceed the projected level of power you want. I also suggest choosing a street, full faced clutch disk. Puck clutches, clutches that are star shaped instead of a round disc, are best for drag racing because they engage quickly and harshly. Puck clutches are either engaged or not and this will cause harsh drivability in normal street driving. Some people say that puck clutches are fine for street use but I suggest that you not get one unless you have already driven a similar puck clutch because drivability is highly subjective. Unless you plan on drag strip use, I do not suggest a puck clutch.
Please read 1000q: clutch FAQ for a lot more details, pictures, and basic VW TDI clutch details. See 1000q: mk3 remove transmission and 1000q: mk3 remove clutch for more details on mk3 1996-1999 jetta/passat. The common upgrade for the TDI is a vr6 pressure plate and clutch. The TDI clutch is not the same as a vr6 clutch, it has a different part number and is slightly different. The vr6 clutch kit will be a direct bolt on to the mk3 flywheel, mk4 cars require either a new flywheel or a mk4 specific clutch kit because they use a dual mass flywheel.
The stock flywheel for diesel cars is sufficient to reuse if you want to keep the same size. Just make sure to resurface it whenever you install a new clutch disk and wipe the flywheel and pressure plate down with rubbing alcohol or brake cleaner before installation for best results. Also note that for drag strip cars, a heavy flywheel is preferable to a light flywheel. This is because you can store more energy in a heavy spinning flywheel to get the best 60' time.
If you have a DSG transmission, the official limit is 258 lb-ft torque for the transmission. HPA motorsports did endurance testing on high powered VW's DSG transmission and found that they held up fine with early fluid changes. See more specifics about the DSG in 1000q: DSG FAQ.
An larger turbocharger will supply a greater flow of air more efficiently at
higher boost levels than a smaller turbo that has exceeded it's efficiency
range. Keep in mind that larger turbos trade higher rpm performance
for some lower rpm performance. In other words, you lose low end power and
gain high end power. If you have a 4 cylinder diesel and use it for
everyday driving, I wouldn't choose a turbo that is too large since you lose
more low end power for daily driving than a 6 or 8 cylinder
diesel. If you have a VNT turbo, the loss of low end power will not
be as noticeable compared to a conventional turbo. If you have already made other performance
modifications, the level of low end power may still be higher than a stock car
and since drivability is highly subjective, it's best to test someone else's car
first. See 1000q: turbocharging
for more details on turbocharging theory.
See 1000q: Volkswagen TDI turbo upgrade chart for a comparison of available turbos. This chart is not all the possible turbos that could be used, just the ones that are sold by TDI vendors. It is not an endorsement of any specific turbo or vendor.
Many diesel parts vendors sell hybrid (a combination of stock and larger components) and larger off the shelf turbos that can be directly bolted onto the stock turbo location. Unless you have experience in choosing turbo components, get plenty of advice before changing the turbocharger. Don't rely on one person's advice because their preferences for drivability and performance goals may be different than yours and is highly subjective!
The section of exhaust piping coming out of the turbo is called a downpipe, due to the configuration of the cars. A few cars have the turbo exhaust exiting up in which case the pipe is called an up-pipe. The section of piping immediately downstream of the turbo is the most flow sensitive part of the whole exhaust. There are vendors who sell these parts as direct-bolt on modifications for the mk3 and mk4 generation cars. Keep in mind that the bottleneck downstream resulting from a stock exhaust will reduce the efficiency of this part, but every little bit helps. To get the most value and most performance from a downpipe, use it with a full custom exhaust. No vendor sells an off-the-shelf full turbo back exhaust. See 1000q: turbocharging: downpipe for pictures and more detailed explanations.
The article 1000q: turbocharging has a lot more information and pictures on choosing exhaust components and identifying quality parts. Building a full custom exhaust yourself requires some fitment and experience welding, so I have included it in as the most difficult advanced level performance modifications. Of course, a cat back exhaust can be made at any muffler shop so most people will not be doing the welding themselves since it would probably be much easier and faster to go to a muffler shop. There are no vendors that sell off-the-shelf turbo back performance exhaust systems for the TDI.
Keep in mind that while few shops can make a custom downpipe since it requires more fitment testing and removal of more components than most shops want to work on (or that you want to pay), almost all muffler shops can make a custom downpipe-back exhaust because it is easily accessed. Downpipes are also exposed to much more heat than cat-back exhausts so the need for high quality workmanship is less. Avoid adding a straight pipe without a muffler because states that require emissions inspections require a muffler and it will result in an obvious failure during visual inspection. A well designed muffler will also not reduce the amount of power you make and will make the exhaust much quieter. Lastly, adding straight pipes may cause an annoying droning resonance at lower rpms unless you add resonators or venturi restrictors inside the exhaust.
By removing restrictions in the exhaust system after the turbo, you create the greatest energy ratio before and after the turbo, making the turbo more efficient. This means making the exhaust piping as straight as possible, using only mandrel bends, making sure the downpipe flow is smooth, and removing emissions equipment (such as catalytic converters, for off road use only!)
Everything here is a difficulty level of 5 or 5+...unless you pay someone else to do it. Even then you will have a hard time finding some who knows the car, knows what to do, and lives nearby. Be prepared to send parts out to an expert.
Tubular exhaust manifolds
Well designed equal length tubular manifolds are always better in performance than cast iron manifolds but are less durable, cost more, and are difficult to fabricate. They enhance turbo exhaust flow by making sure that the exhaust pulses do not interfere with each other. Since the exhaust pulses come out of the cylinder head at different times, making the exhaust runners the same length helps the exhaust pulses arrive at the turbo collector area without colliding with the other exhaust pulses due to timing. This enhances flow and turbo spool up. Please read 1000q: turbocharging for a more detail explanation of turbo theory. The stock turbo may also be part of the exhaust manifold so changing just the manifold may not be possible without more modification. If you are in the market for a tubular manifold then you should have no problem with the necessary modifications since it will all be a custom project...or be prepared to pay someone else to do it!
The major problem with tubular manifolds that they are difficult to fabricate and fit in the limited engine bay space. Since they are also welded instead of cast iron, it is only a matter of time before the welds crack and leak out exhaust gases. Every welded manifold will crack, the only difference is that good materials and good welds will last much longer than poor materials and poor welds. I am not referring to time intervals beyond the life of your car, a mediocre tubular manifold could fail in 1000 miles. The problem is that the constant heat cycling, the weight of the turbo and pipes hanging off the exhaust manifold, and greater engine vibration, especially in a diesel engine, will weaken the metal until it cracks. A good weld can actually outlast the metal around it but in this case, the metal around the weld will just crack. A cast iron manifold is cast in 1 piece so they are much less susceptible to cracking during the normal lifespan of your car. However, the exhaust manifold can be removed without too much trouble in the TDI, so it's up to you. For cars that require lots of effort to remove the exhaust manifold, I would rather get a cast manifold.
Since this article was written, vendors have come out with aftermarket manifolds for the TDI. Unless you have access to a high quality fabricator nearby, I would contact the vendors directly to see what options they have. Here are direct links to compare cast manifolds from from JSperformance, from Kermatdi. Each appears slightly different in coatings or other modifications so contact the vendor directly to see what they offer.
Here is another example of a tubular exhaust on a TDI head with a custom
intake manifold welded onto the stock manifold. Fabrication by
This is the most extreme level of engine modification since it requires a new or rebuilt engine and a high level of testing and knowledge. If the car already has modified engine supporting modifications such as a full custom exhaust, a larger fuel system and turbocharger, a larger intercooler, but you still want more power, you need a custom engine. Some goals of a custom engine would be boring to make the cylinders larger in diameter, stroking to make the up and down stoke of the pistons longer (not desirable in diesels), or lowered compression ratios by dishing out the cylinder face or bowl. Dishing out the cylinder face or deepening the bowl in the piston lowers the compression ratio. By lowering the compression ratio, you lose low end power but can use higher turbo boost and can gain high rpm power. Keep in mind that the amount of money required after all these modifications will exceed the price of buying a used sports car that will still be faster and handle better than what you will end up with. I personally feel that this level of modification is not cost effective, but so is buying new cars and modifying your car, so it's up to you! Since labor is about the same, larger engines give a better bang for the buck.
Basically, if you have arrived at this level of modification of 5+
difficulty, you know more than me and should
already be experienced with custom modifications and know all the details of
what you want already. Here is an example of an
engine with a reduced compression ratio (achieved by boring out the piston bowl)
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