Originally posted by warmonger:
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.



Well, my intended use is track use (road course/autocross), but since I plan on running the road course much more often and it's a 10 turn 2 mile course, I was looking for more power. The goal is 400-450 WHP ... what would you guys recommend? I will be doing the ADC T3/T4 setup. I was thinking 8.5:1 to be on the safe side when racing (also to push the car further on the dyno for [censored] and giggles). The car will be run on the street, but not too often. When runing on the track, I will be using 100 octane, so I think that I should be able to run 14+ psi on 9.0:1 without a problem right? For my street tune, 93 octane will be used and I'm looking for 325 WHP.

So, knowing my goals, what would you guys suggest ... 8.5:1 OR 9.0:1???

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