compression

[Str8in]

So let talk about compression. What im planning on doing is using a 20g turbo ive been told to use at least a 9:1 compression piston. Ive heard when running high boost you should use low compression pistons because the turbo increses compression. Am i wrong thinking that I need a lower compression piston and that a 9:1 is good to run a 20g on? I mean wouldnt i have problems if its a 9:1 running 93 octane fuel on a 20g turbo pushing about 20lbs of boost?

[scottsee]

thats high compression. i dont know if thats good with a 20g. i wouldn't think twice with a 14b, t25 or 16g.

[JoeWagon]

You are trying to guess pistons for a given boost, which is backwards. You are on the right track -- use some general advice and choose pistons, but when you tune the car, you will find what boost level you will use. If the car can run 25psi with no knock, run that. That said, 8.5 or 9.0 are reasonable compression ratios for pistons. If I were you, I'd get the 8.5. I believe either the 1g or 2g is 7.8:1 stock.

[kjewer1]

Yes, 1g is 7.8:1, 2g is 8.5:1. For compariosn my current motor ended up at almost 10:1, presumably because of questionable cam timing, and it didn't last long My next motor is going to be 7.5, just ot be safe. I'd rather have to move a little more air to make up for low compression. And with a 74 lb/min turbo currently only run as high as 64 lbs, I have plenty of extra capacity. With a 20g 9:1 would be ok on race gas, but on pump it could get risky. When I was running 25 psi daily on pump gas, I believe the motor was around 8.5:1 (ross pistons, 180 psi static compression). With good intercooling and tuning I didn't have any trouble with knock, and that motor was fine until something went wrong one night on race gas. But thats another story.