The ratio of the pulley on the blower compared to your crank pulley (or whichever pulley you use to run the blower determines how much PSI it will produce). Its more work to change boost settings on a supercharger then a turbocharger, and turbochargers are more efficient and can yield more power... but in either case you need knowledge to build the set-up yourself, especially the most important part of each set-up... What to tune it with and how. If you have money, look into either a opcon/autorotor twin screw supercharger, or a newer Garret ball bearing GT series turbo, gt28rs (this is the turbo i want later on if i ever build my bottom end) for example.
