When you up the CR its usually to help an already built motor run better. Higher compressions ratio for NA builds, lower for turbo. Thats general. I wouldnt suggest buying high CR pistons without other stuff already done (exaust, intake, cams, etc). And you'll need tuning. Otherwise if you tear the block down, replace them, and put it all back together without changing anything from stock, its gonna be a big waste of money.