I plan on getting an ls1 built for forced induction with 9 5 to 1 compression but i will be getting the procharger about 2 or 3 months later.Does the compression really make that big of a diff?If so how much hp will it be off from 10 5 to 1 cr?

 

It's not just HP you loose, things like throttle response and driveability come into play also. If you know for a fact that the blower will be there in the future, then I wouldn't have a problem building the motor and giving it a 3 month break in period.