the old style roots blowers take more power to run, because of the big belt, and the size of the rotors and such, a centrifagul supercharger uses a small V belt or part of the serpentine to power a very small vane rotor. I think with centrifagul superchargers ( I know with turbo's u do ) is boost refrence the fuel pump if you use a blow through type system, if you run a draw through, you cannot intercool because the air going to the intercooler will have the fuel mixture in it ), by opening up the vent on the pump, and running a line from it to the preassurized side of the intake, so when your boost comes up, you don't blow the gas back into the tank. if you run 15 PSI of boost and you need 6 PSI of fuel preassure ( or so ) so, you would need 21 PSI of fuel preassure at 15 PSI boost, boost refrencing does this for you. I will be turbocharging my 200 I6 in the Falcon, and should end up with around 170-180 HP at the wheels, about 210 at the flywheel, lot better than 120 HP.