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I also had the same question awhile back and read a really good page explaining it. I can't explain it myself, but i can explain your pulley question. Even though a pulley will produce more boost than another, they are both limited by what the blow off valve allows them to produce. lets say you have a centrifugal blower making 6 psi. You then add a different pulley that is capable of 10 psi. The supercharger is now making 10 psi, but it is beeing "bled" out of the system via the blow off valve (BOV) back down to 6psi. If you want more boost you can adjust the BOV (some have a dial, others require stiffer springs).