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I see all these people getting 8.5:1, 9:1, and 9.5:1 pistons for boost applications but if you have a built block, what is the highest CR you would run... I have a friend that has a 400hp 1990 Prelude that is running 9:1... He was thinking about going to 10:1 or 10.5:1 ... Anybody wanna talk CR's for a moment??

 
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look at it this way... generally speaking, the higher the compression ratio, the more power you make. This works for forced induction cars too.

The problem with higher compression is the higher risk of detonation. When you lower the compression, you can run more boost. Since your compression is lower, you will make less power with the same amount of boost as compared to stock, but you will have the capability to run more boost overall, which gives you the potential for more power.

When you lower your compression, you effect many different aspects of your motor. If you are turbocharged, the lower compression will extend your spoolup time. Lower compression also effects fuel tuning. You will also be able to run more timing due to the higher detonation threshold.

On my friends SRT4, we swapped out the stock pistons for a set of 10.5:1 JE pistons. With these pistons, his GT35R was making 26 psi of boost while on his staging limiter (4000 rpm). Just free reving the motor, it was making over 20 psi.

 
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so on a properly tuned, built motor you could run lower boost levels and achieve the same power but you would have to be much more precise with the tune?