Originally posted by Keyser:
I'll give you a for instance...my old tour was at the shop 2 days ago. It's a 2.5 with 9:1 pistons and kinger heads, so the compression would be like 8.8:1. It did 325 to the wheels at 12psi with 91 octane crapoline. A 3L you can basically add 50hp to the equation would be my guess. If you have 93 octane I'd do 9:1 with or without the heads done.



I think it depends on a number of factors like intended use for the engine.
I made a nice spreadsheet chart to show how to pick your compression ratio and boost pressure. Feel free to download it from my website link at the bottom of the page.

For a street turbo, I'd go with 9.0 to 9.5 to 1 range. For a more drag race setup I'd probably go 8.5 to 1.

For autocross type of racing where you need to be in and out of the throttle you wouldn't want a low compression motor. You'd be decelerating and then when you needed to punch on a little throttle you wouldn't have that instant response that you are looking for.
In this case I'd suggest 9.5:1 up to stock 10:1 with a low pressure turbo application like what I run (not that I race). This is what I did: At 10:1 with 7 psi, a decent tune, a cheaper priced turbo and your basic performance mods done to intake and exhaust, you're looking at 300 wHP no sweat; on 91 octane even.
If I ordered pistons, it would be 9.5:1 for a 3L.
If you have a 2.5L and you did Kinger's heads where he also machined out the shrouding in the combustion chamber, then I'd go anywhere from 9.5:1 to 10:1. However, 12 psi with this setup is probably the limit on 93 ocatane fuel; or some fun playing with the timing using 91 oct.


Former owner of '99 CSVT - Silver #222/2760 356/334 wHP/TQ at 10psi on pump gas! See My Mods '05 Volvo S40 Turbo 5 AWD with 6spd, Passion Red '06 Mazda5 Touring, 5spd,MTX, Black