Wo-Nelly

I was just wonderinf how it would affect my a/f ratio. It has already been tuned and I make very close to the 12.5:1 and this is with the stock 10:1 compression. Now if I were to go with 9:1 compression and not retune, how would that affect my a/f? It shouldn't do anything to timing or does it?

InTeGrA B18b1

your going to be running richer maybe bogging out. slower spool, less power. more lag

TurboR1391

You should have you car re-tuned cause your fuel map is gonna be off.

APZGSR

more fuel you need to re-tuned. It hapend to me

Mr. Softee

yes, it will.

Boostfed

Well I don't have A/F charts to show but I had a Ported and polished head with .5mm over intake valves and 1mm over exhaust valves and head milled set up was 10.4:1 CR 28psi made 581whp and then I put a stock head on it and valvetrain stock valves 9.5:1 CR and made 573whp 28psi, the torque was lower at 355ft-lb versus 380ft-lb , I wish I had retuned it to see how much more whp with the stock head.

xtal

Ummm.. that's not how EFI systems work.

All the computer does is guarantee for a given amount of air (read from the MAP sensor), it tries to ensure that the mixture is ideal in closed loop mode, based on information from the o2 sensor - or runs the injectors from a set map in open loop mode. All of the air flowing into the engine is mixed so that it has a set mixture. The throttle controls the limiting factor in combustion, and that's the amount of air.

If you lower the compression, there will be more volume in the cylinder when ignition happems. You will get less power because the charge will not be as dense. This is independant of the mixture. You could lean it out to try and get some power back.

In other words, your a/f isn't affected at all, but your power will be down. If you've modified the map so that you were running very rich to combat detonation or lean conditions, the negative effects of those conditions will be compounded at the lower compression.