FYI

"All motors have a static compression ratio. This is the amount that the air inside the cylinder is compressed. It is a ratio of the cylinder volume at BDC to the volume at TDC. When a supercharger is added, additional air is forced into the cylinder effectively raising the compression ratio. The result of this is called effective compression. The formula for finding the effective compression is very easy:

((boost psi / 14.7) + 1) x motor compression = effective compression.

The effective compression allows a supercharged motor to be compared to a normally aspirated motor. For the most part, a supercharged motor with the same effective compression as a (similar) normally aspirated motor with the same static compression should have about the same overall power. "




http://www.motorsportsdigest.com/forced2.htm