I'm gearing up for a pre-emptive rebuild.
I've got a 88 GTU with a S4 TII swap in it.

I will be buying a S5 longblock to rebuild as a back up engine.

I'm wondering if anyone has run a S5 high compression motor with the S4 ECU?
If so, Did it run lean? I would assume so since the comp is higher and the S4 maps are for lower comp.
Any dyno runs where you followed AFR's? How bad did it get?

I will be premixing so the OMP isn't a problem. I will also be running full S4 electronics(injectors/TPS/BAC/etc).

I don't want to throw in the S5 because then I'll have to keep the OMP to prevent limp mode.

I was thinking that IF the S4 ECU can't handle the higher comp then I could either go Haltech(perfect money is not a problem world) or Rtek 2.0.

The Rtek should be able to handle some fuel map manipulation no prob for this since it's basically the same motor.

If all else fails and I end up broke I'll probably just throw in some S4 rotors like I'm doing now.

Hmmm. Maybe I'll streetport it as well. Is there anyone here on S4 rotors and SP running a Rtek 1.7 well? or was a SAFC needed?

 

Compression ratio has no effect on air/fuel ratio. It's just squeezing the air and fuel more, not changing how much there is.

The actual CR increase is only small (~6%) so unless you were very close to detonation in the first place it'll be fine. If you're worried about then make sure intake temps are kept down, as that increases resistance to detonation. Make a proper CAI and consider a FMIC.

 

My motor is s5 internals and ported all to hell and the stock s4 ecu handles it fine.

I'm NA but the turbo should be no different, well, except the porting part atleast.