Is the Power FC the wrong EMS to be using if some one is to down the E85 route? What EMS would you guys recommend for an S4 TII?

 

Just because E85 is being used does not disqualify using a PFC.
Since E85 requires a higher fuel flow rate, larger injectors take care of that.

But if you want an ECU that can run and adjust fuel flow bases on how much alcohol is detected
by an alcohol detector, then you need a more modern ECU.

 

What are the biggest primaries you can run on the PFC, can secondaries be too large? Isn't it harder to get an idle with larger primaries on the PFC?

 

550 is stock for gas. 725 runs OK on gas.

It is very simple basic math/science.
Comparing BTUS of E85 to gas, and then get the proportionate larger injectors.

For the secondaries you can run 2 or 4 injectors.