VAG Group Turbochargers

"All You Need To Know About VW Audi Group Turbochargers"

The best project engines and biggest gains tend to come from turbocharged engines and the VAG group has fitted a number of turbos to their cars over the years so we will now take a look at these turbos, look at the differences between them and your best upgrade options.

We also look at power limits for a turbo, you may well be able to push a turbo beyond the safe limits, but the lifespan of the turbo will be reduced. You will typically get a whining, or siren noise and then smoke before the turbo finally fails. These warnings are the point at which you should look to replace it.

A failed turbo can cause quite a bit of damage, especially if a compressor breaks up and gets sucked into the engine.

Comparison Between K03, K03s, and K04

The K03 turbo was offered between the years 1996 and 2000 after which the relatively higher performance K03s replaced it.

We have a video to go with this article, if you prefer to watch and listen rather than read so see our Guide to K03 K03s and K04 Turbos on our YouTube channel.

Engine Types

The K03 turbos are used with relatively simpler 1.8T AGU engines. Such engines have a MAF sensor and the traditional cable operated throttle with relatively smaller injectors.

The K03s turbos are used with the AUM engines that not only have MAF but also MAP sensors. Such engines use the improved drive by wire throttle which offers a faster response and improves fuel economy and performance.

Major manufacturers of K03, K03s and K04 turbos include: Stigan & BorgWarner

Differences between the KO3/KO3s

The most considerable difference between the different types of turbos is in the number of blades. The K03 has 11 whereas the K03s has 8 blades.

The turbos share similar footprints and dimensions with notable differences being the actuator that opens the turbine bypass valve typically a 65N or 85N although some models had a 2 ports actuator. The 180hp engines are generally given actuators of a higher quality and greater opening force which is logical as they are running higher boost.

Using a larger compressor on the intake side will generally speaking increase power and is the route many hybrid turbo makers take.

K03 and K04 turbos generally speaking, require a different manifold and have a few plumbing differences. This is especially true when comparing a transverse mounted engine with a longitudinal one. There were some early K04 turbos around which will fit the K03 manifold. But most will not. Different generations of engine will also have slightly different setups, but most KO4's can be setup to work in place of a K03/K03s.

Horse Power

In terms of power, the K03 has the least juice meaning that with proper add-ons to accompany, it will top out at 190 bhp. You may get more but the life of the turbo will be reduced.

The K03s has higher power output. Most people have achieved 215 bhp with K03s which is the safe limit, any more and turbo life will be compromised. However, in some exceptional cases, people have claimed to have exceeded 250 bhp on this turbo so there is quite a margin here to work with.

This means that the K03s gives a safe performance boost of at least 25 bhp as compared to a K03, more if you are prepared to reduce the life of the turbo or happy to do extra maintenance work.

K04 Offers Much Better Performance

Contrary to the K03 and K03s, the K04 is relatively larger and offers much better results.

With proper hardware support, the K04's performance figures can even be taken up to 350 bhp but the safe limits for the KO4 sits around 220hp.


K03 and K03s turbos were smaller in size than the K04. This meant that although they provided relatively lesser power, the overall space required for their installation was lesser than that of the K04.

Turbo limits - safe should retain factory reliability and longevity, the Max however is pushing to the limits and will certainly shorten the turbos lifespan.

Turbo Model Safe Max Tuned Max Tuned
K03 190hp 220hp
K03s 215hp 250hp
K04 around 270hp* 350hp

Safe limits will vary slightly between engines, typically the 2.0 TFSi will produce more power, thanks to the extra cc, compared with the 1.8 versions, so the safe limits will be sllightly higher than those shown above.

*Please bear in mind the K04 did vary and there were quite a few variations of these, which causes a lot of confusion regarding safe limits. Heat is a big problem so the ones with the temperature sensor are generally more suitable to the higher power levels.

Early K04 turbos can struggle at 250 on some engines, later ones sing happily to 300.

Newer Line Of VAG Group Turbochargers

While the K03, K03s and K04 were the previous generations of VAG turbochargers, newer versions include the IHI IS12, IS20 and IS38 turbos.

The IS12 is the stock turbo installed on Audi and VW 1.8t MQB engines whereas the IS20 has been fitted with the 2.0t MQB engines.

Why Is IS38 A Great Turbo?

The IS38 is significant because this bolt-on turbo can produce up to 370 hp (with the proper add-ons). It is a highly sought-after turbo from IHI, owing to its high reliability.

Advantages Of IS38 turbos

This turbo has a significant advantage in that it is factory installed in several vehicles like the VW Golf R, VW Arteon, Audi S3 and TTS.

As a result, the IS38 is an excellent choice for those seeking a dependable option in terms of high performance and longevity.

Other impressive features of the IS38 include:

  • High turbo output at greater RPMs;
  • It is a relatively easy turbo to install due to its bolt-on nature.
  • Availability of a large number of hybrid models with improved housing and impeller designs that contribute to higher performance figures.

A High-Performance Replacement For IS12 & IS20

Because of its performance improvement, many car enthusiasts have replaced the IS12 and IS20 turbo with the newer IS38.

Upgrading The Turbo

Why Upgrade Your Car’s Turbo?

Here are a few reasons why people change or upgrade the turbos on their vehicles:

  • For increasing a car’s performance upgrading the turbo is a good option and one of the most significant power adders
  • The performance of turbos degrades with time, necessitating their replacement
  • Newer turbos offer more add-on options to car owners and take advantage of new developments in technology.

Choosing The Best Turbo Upgrades

With so many options available, determining which is the best part is a critical step and we would urge you to check your supplier carefully, there are a lot of fake and poor quality turbo units out there.

Fit a cheap turbo upgrade and we guarantee you'll be doing the work again about 6 months later.

Further to that, choosing a turbo that can offer the following additional features is a step in the right direction:

  • A billet Aluminum compressor wheel (light and strong)
  • VNT Vanes (Maximizes the boost available)
  • Balanced and flowed internals
  • An uprated wastegate actuator (sharpens throttle response and maximizes your power gains)

Keeping such minor details in mind can mean the difference between a successful project and money wasted.

Remapping MUST be carried out after a turbo upgrade

It is pertinent to mention that for swapping the stock turbo with a more powerful aftermarket part, the vehicle’s software often needs to be upgraded.

Failure to do so can cause the engine to run lean, or go into limp home mode throwing error codes. It is also the only way you can fully realize the potential gains from your upgrade, a stock ECU will only slightly trim the fuelling and you'll be wasting the opportunity.

Turbos To Replace The Original Ones

If the need arises and you decide to replace the stock turbo, here are the options to go for:

K03 replacements

If you want to upgrade the stock K03 on your vehicle, some examples to consider include the following but please note there are differences between the design of inline and transversely mounted engines turbos, so don't get them confused:

Stigan 847-1001 having SKU 40-30002 SG

This turbocharger can fit a large number of vehicles and is pretty much a drop in replacement. Some examples include:

  • 1997 to 2006 Audi A4 1.8 liters;
  • 1998 to 2004 Volkswagen Passat 1.8 liters;
  • 2005 Volkswagen Passat 1.8 liters Gas Engine.

BorgWarner 53039880029 having SKU 40-30002 BW

TorqueCars are big fans of the BorgWarner turbos and highly recommend them.

This turbocharger typically fits the following vehicles or similar:-

  • 1997 to 2006 Audi A4 1.8 liters
  • 1998 to 2004 Volkswagen Passat 1.8 liters
  • 2005 Volkswagen Passat 1.8 liters Gas Engine.

K04 Replacements 

If you want to replace the stock K04 turbo, here are a few good options to consider:

Stigan 847-1435 having SKU 40-30002 SH

This Stigan High-Performance New Turbocharger typically fits the following vehicles or similar:

  • 1997 to 2006 Audi A4 1.8 liters
  • 1998 to 2004 Volkswagen Passat 1.8 liters

BorgWarner 53049500001 having SKU 40-30003 AW

This BorgWarner Airwerks Series High-Performance New Turbocharger can be used for the following vehicles - mainly transversely mounted engines:

  • 2000 Audi TT All Models
  • 2001 Audi TT
  • 2000 VW Golf 1.8 liters
  • 2001 VW Jetta 1.8

IHI Genuine Turbocharger replacement

If your vehicle’s stock IHI turbocharger has gone faulty you can replace it with something like the SKU 9VA02.

This turbocharger can typically fit the following vehicles or similar

  • VW Jetta
  • VW Passat
  • VW Golf

Turbo Power Limits With Modifications

K03s Remapping

With a Stage 1 (Air filter, exhaust, bolt on mods) Remapping, expect to have power in the range of 210 to 220 bhp with your K03s turbo.

However, things can be taken a step further with Stage 2 Remapping (cat replacements, Fuelling improvements & intercooler upgrades), which can produce power ranging from 230 to 250 bhp.

Keep in mind that if you opt for a Stage 2 remapping, an intercooler will be required to lower the air temperature or you'll be stuck with a much lower power figure.

K04 Remapping

The K04 turbo has a peak power of around 250 to 260 bhp with Stage 1 modifications but we have seen some achive higher figures than this.

Nevertheless, such serious performance requires extensive modifications such as:

  • An upgraded High-Pressure Fuel Pump;
  • An upgraded FMIC (Front Mount Intercooler);
  • High-quality Fuel (98+ Ron);
  • A High-performance clutch (your OEM clutch will slip).
  • A Decat Exhaust System;

IHI IS38 Remapping

This turbo can reach up to 370 bhp with engine tuning/remapping making it the ideal choice for Audi Seat Skoda and VW fans!

The IS38 is a highly regarded turbocharger for Audi and Volkswagen vehicles, and with remapping, it provides best-in-class performance.

Swapping An Audi/VW Turbo To A Different Model

Although people are constantly adding newer and better turbo versions to their vehicles, swapping a turbo is not as simple as it appears.

Swapping an VAG group turbo to a different model is also no different. While the steps required for doing the process differ from one car to another, some of the necessary steps might include:

  • Fuel injectors replacement
  • Altering the diameter of the exhaust plumbing
  • Changing the fuel pump
  • Putting a new boost controller
  • Adding intercoolers or other cooling mechanisms
  • Changing the wastegate
  • Putting less restrictive catalytic converters
  • Replacing mufflers

The above list may appear frightening at first, but which of the above steps must be performed depends on the car in question.

In addition to the aforementioned steps, the design of the engine bay plays a significant role in determining how difficult the entire process of swapping an Audi turbo to a different model will be.

You may need to reroute exhaust and intake pipework, mount a larger intercooler and there may be space constraints.

Hybrid Turbos

Why Do People Use Hybrid Super Chargers?

While several manufacturers, including Stigan, IHI, and Borgwarner, offer turbos of various sizes and capacities, many people feel the need to develop hybrid turbochargers.

Hybrid turbochargers are improved versions that are designed by combining high-performance parts from various manufacturers.

The sole purpose of these hybrid turbochargers is to achieve better performance than OEM turbochargers.

How Different Is A Hybrid Turbo From An OEM Turbo?

To achieve exceptionally high performance, a hybrid turbo can have a single part replaced or, in some cases, all parts of the OEM turbo might be replaced.

Hybrid turbos are unique in that they are not constrained by fixed numbers. While such turbochargers have essentially the same appearance as stock ones, the modified inner parts ensure significantly enhanced performance.

Benefits Of Hybrid Turbos

Better Performance

Hybrid turbos provide improved response and higher performance figures because they are created by combining the best in class parts, resulting in a much better result than stock turbos.

A properly designed hybrid turbo ensures that the extra pressure and flow are achieved while the turbine is operating at a safe speed. This means that the longevity of the parts is not compromised.

You Get What You Pay For when it comes to hybrid turbos. Several trade offs have to be made in the case of stock turbos, but the biggest advantage of hybrid turbos is that you get what you pay for. For most cases higher quality parts machined to tighter tolerances add power, spool up more quickly and last longer.

Be wary though as some cheap import hybrid turbos are very compromised and will often only last a year or so. Ask around for peoples experiences, just looking a table of power gains or dyno graph will not allow you to fully understand the quality of the turbo you are fitting to your project.

Please join us in our forum if you want to discuss turbo upgrades in more detail, and please use the comments box below to pass on your turbo tips to our readers and visitors.

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One Response to “Full guide to VAG Turbochargers”

  1. Jacob Missall says:

    This was a great article full of awesome information. thanks

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