http://www.automotiveforums.com/t150554.html



specifically this part



So say that you are running 8.5:1 compression pistons. To get an effective compression of 18:1, you are going to need to run approx. 16.5 psi of boost.
If you were running 9.5:1 compression pistons, then you would only need to run about 13 psi to achieve the same amount of power.