Turbo Selection – E85

Regardless of the turbocharger that is selected an E85 specific tune will be considered for use.

To gauge how a K04 setup may benefit from running on E85 I compared dyno charts from tunes produced by EPL, 034, and GIAC.

The EPL tune used 93 octane fuel, the 034 tune used 91 octane, and the GIAC tune used E85. Specific boost profiles and summary of aftermarket components employed by the respective vehicles was unknown.  Additionally the data comes from different dyno’s under different conditions – minor differences should be ignored – any major differences could be legitimate indicators of how the pump gas (91 and 93) cars compare with the E85 powered car.

Overlaying the three dyno charts produced the following composite:

91 versus 93 versus E85 Based on prior information about gains available to the B5 S4 by using E85 fuel the significant increase in wheel horsepower is not surprising, though still impressive.

What is a surprising is the shift in peak torque shown by the E85 fueled car, occurring nearly 1200 rpm later than the gasoline powered vehicles.

The reason for this is unknown, but one possible explanation is a slower onset of boost pressure with the GIAC tune as compared to the EPL/034 tunes.  If this is a cause, why it was done, if it was a decision, or if it is an characteristic of using E85 fuel with the K04 turbo’s, is another unknown.

Getting back to the question of turbocharger options, and weighing the results above, the next question I have is how does E85 fuel affect and RS6 size turbo with regards to spooling up the compressor.  If a similar delay is caused that would make the RS6 turbo even less responsive, and less desirable to me if an E85 tune is something I anticipate using often.  On the other hand, if the RS6 turbo is less effected due to an already delayed boost onset, as compared to K04’s, the delayed torque onset results shown above would be less important.

This new information is helpful but has raised more questions for me than have been answered.

The options remain:

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

Turbo Selection

Some updated information was brought to my attention concerning one of the remaining candidates, the Project B5 RS6-X turbo.

I had thought the compressor wheel on this turbo was 45mm at the inducer and 56mm at the exducer.  I learned that it is 42.6/56 which is the same as the RS4-X (and FrankenTurbo F21), but the turbine side is the larger RS6 size.  What this means is that the Project B5 RS6-X turbo is dimensionally the same as the BW RS6, but at a fraction of the cost.  That makes it an interesting option in the mix.

Next: F21 vs K04