Having decided that a twin turbocharger setup would be the path that I followed the next question I began to consider was the size of turbocharger to install.

At one end of the spectrum were the Borg Warner K03 turbochargers that were already installed on the car.  The K03’s had a couple of things going for them, install would be a breeze, requiring absolutely zero work and the costs would be the lowest of any option.  Doing something along the lines of a Stage 2+ setup or even a Stage 2+/E85 tune would be straight forward.  I already have on hand 72# Bosch EV14 fuel injectors, an EPL 85mm MAF housing (likely unnecessary), and an installed RS4 fuel pump.  Supporting the K03’s with a bunch of go-fast bits that were sufficient for substantially larger turbochargers might be a fun investigation.  While I might try this, ultimately I’ll be putting something else on the car – K03’s are out.

The opposite end of the spectrum is occupied by the TiAL R770 turbochargers.

I’ve decided that there’s no point in looking for anything larger than the TiAL R770’s and anything remotely close to this size is probably a distant second given how much attention was paid to matching this turbocharger pair with the 2.7T motor.

In the R770’s favor was the fact that the most horsepower I was going to derive from my car would be via this pair of turbochargers.  Even if I chose to operate at less than what these turbochargers were capable of the experience of driving with them would likely be positive.

One downside to acquiring the R770’s at this stage would be the need to obtain a new set of downpipes with the split design for the wastegate as shown in this image on the 034 Motorsports website.

Another reason to consider passing on the R770’s was the cost, about double that of some of the turbochargers I was considering.  I decided I’ll hold off on the R770’s for a later date.

The middle ground was congested with several options:

• Borg-Warner K03
• Borg-Warner K04
• Project B5: RS4-X
• FrankenTurbo F4H-BT
• FrankenTurbo F21
• Borg-Warner RS6-K04
• Project B5: RS6-X
• JHM RS6-R
• LOBA Motorsport LO530P
• Silly Rabbit Motorsport RS6 K24
• TiAL Sport s605.2
• Garrett GT28RS
• TiAL R770
• Single Turbo Kit

Whew! Choosing from amongst all of those options was going to take some thought.

Next: Turbo Selection Easy Cuts

Presently I am in the process of collecting baseline performance data on the twin Borg-Warner K03 turbochargers that were equipped on the B5 S4 as delivered from Audi.  Once I have completed this process I’ll be removing the K03’s to install a different turbocharger.  The upcoming posts in this blog category will touch on some of the considerations and decisions I made during this process.

There are quite a few turbocharger options now available for the 2.7T twin turbocharged engine found in the Audi B5 S4.  Twin turbochargers aren’t even a requirement anymore as single turbocharger setups are becoming more common.  From the get go I’ve decided to stick with the twin turbocharger set, the main reasons being familiarity and ability to continue to use components that I have already acquired.

One process I have followed during upgrading has been to make incremental changes to the car, building on what already exists and only replacing parts when a weak link has been identified.  By following this process I am able to keep an eye out for opportunities to obtain upgraded parts at a discounted price, either through sales or used part classified ads.  One downside to this method is that if I decided to drastically change the car’s setup, for instance swapping from the dual turbochargers to a single turbocharger setup, there may be parts already acquired that would be overcome by events (OBE) and no longer of use.

In terms of cost and performance and single versus dual I have not felt there is a significant benefit one way or the other and with total cost a consideration, meaning I am not going to go crazy building a motor capable of setting world records for the 2.7T platform, the currently available dual turbocharger options meet my requirements.

Decision One: Twin Turbocharger configuration

Next: Turbo Size Decision