compression ratio

Why is the compression ratio better for holding high boost when it is lower? And on Dsms is it still safe to run 15 psi on 8.5:1 compression ratio? lets say you have supporting mods and a stock turbo.

It doesn't 'hold boost' any better, just remember that more compression at some point equals knock. If you lower compression, you can run more boost before you hit knock (the point where you can't make any more power) for that situation.

15psi on a stock turbo comes nowhere near the max compression before knock, so yes, it's safe.

ok cool thanks a lot Joe...but when you start increasing boost udner 20psi with a bigger turbo, why do some people say it will decrease the longevity of the car?

any power mods that you do to any car will decrease the life length! thats a given fact! see if you start cranking up the boost it makes the engine work harder... there for when it has to work harder it wears out quicker! understand?

All of the guys running serious boost (over 30 psi) are actually running INCREASED compression ration. Typically 9:1. I'm a hair under 9:1 and I run 25 psi on pump gas with no issues (the car is setup for that of course). The theory is there, but in real life on a properly set up car, you dont have to hose the compression ratio jsut to avoid knock. Thats just another bandaid. But naturally there are limits, and you dont want the CR of a honda if you intend to run high boost and low octane fuel.

The thing to remember is knock is from heat. Heat is from cylinder pressure. Cylinder pressure is airflow (power), heat (boost/inefficiency/intercooling), compression ratio.