How it works:
In order to understand how this calculator works, it is helpful to understand the basics of supercharger technology and why superchargers make more power (check out our tech section for more on info). A rough starting estimate to calculate HP gain is to take boost (say 5psi) and multiply it by 7% (a constant) to get your approximate power gain (in our example 35%). Why 7%? Because 7% is approximately how much more air and fuel gets into the motor with each pound of boost that is added. To get a more exact figure, take 1psi boost and divide by normal atmospheric pressure, 14.7psi at sea level. 1 divided by 14.7 equals 6.802%. Under ideal conditions, you can expect to burn 6.802% more fuel and air per pound of boost. This is the starting point for this calculator, however, as boost rises above 5psi, other inefficiencies kick in, so gains above 5psi become progessively less effective in making more power. When you need to retard your ignition timing, which is often necessary when running a supercharged engine to avoid detonation, other timing inefficiencies kick in. This is factored into the "HP Loss due to timing adjustment", which is an estimate as to the amount of ignition retard that will be required at a given level of boost. To offset these losses, you can run a higher octane fuel or an intercooler, both of which increase your detonation threshhold and allow you to run more boost without experiencing detonation. For more on detonation, check out "Detonation, Knock, and Pre-Ignition 101".

Check this out:
If you're slightly more technically oriented, you can also download a full-featured performance calculator that will calculate:
• Horsepower & torque from 1/4 mile track results
• Supercharger impeller RPM for some popular supercharger head units
• Injector size necessary to support a set horsepower level
• Fuel pump size needed to support a particular injector size and duty cycle
• Effective injector size if running non-factory fuel pressure
• More
Click here to download!
Calculator courtesy RACE Systems