There’s not been much activity in the turbocharger selection area recently, but that has changed with the receipt of this data showing a F21 equipped B5 S4 operating with E85 fuel and measured on a Dynapack dynamometer.

It’s good to see a F21 car operating the turbocharger at a boost level consistent with what I believe they are capable of, steady 23-24 psi out to redline.

Details about the car: 2001 Audi B5 S4 on Stock Engine, FrankenTurbo F21 turbochargers, 72 lb injectors, 85mm MAF and intake, 2.5″ catless downpipes, Silly Rabbit Motorsport Intercoolers, tuned by Epic Tuning Group on AU Tuning’s Dyno.

And some results of the F21’s on 91 octane fuel.

In terms of selection criteria these results show that F21’s are going to be able to push the edge of my comfort zone with the stock motor.  With 93 octane instead of 91 I’d expect the horsepower and torque to nudge up another 15-20.

The contenders still are:

• Borg-Warner K04
• FrankenTurbo F21
• Project B5 RS6-X

The current contenders consist of the Project B5 RS6-X, FrankenTurbo F21 or Project B5 RS4-X, and Borg-Warner K04.  These three options span the range near the stock motor limit with the K04 filling the lower end, F21/RS4-X the middle, and RS6-X the upper end of Horsepower potential.  The RS6-X could be considered crossing over into the “upgraded rods needed” region if tuned accordingly, but for the purposes of my intended use I would have them operating below that threshold.

To better assess the tradeoffs that each makes I collected a half dozen dyno charts for Borg-Warner RS6 turbo’s and determined the median value for the set.

A few shortcomings to keep in mind; the six RS6 dyno’s are for a turbo design that is different from the RS6-X, the RS6-X uses a different compressor wheel from the Borg-Warner design.  I do not have a dyno chart for the RS6-X so I am using the BW RS6 as a substitute.  Another shortcoming is that the F21 dyno chart is for a setup that falls short of what the F21 is capable of past 5000 rpm – a better optimized F21 car would likely have greater torque output past 4000 rpm than what is shown below.  Lastly, more data to compare is always better and in this instance there is only one K04 car and one F21 car, though they were both tuned by EPL so hopefully they are somewhat illustrative of what a car equivalently equipped and tuned by EPL would put down on a dyno.

I am only interested in comparing torque at this point, I expect the top end horsepower results to follow along with the increasing dimensions of each turbo – K04 followed by F21/RS4-X, and then RS6-X, subject to pressure drop variations in supporting components such as downpipes, exhaust, and intercoolers

The turbochargers compared:

Comments: There are definite trade-offs arising from the turbocharger selected from these three choices.