boost

alright so ive had this question for a while and havent been able to figure it out. how is boost pressure different from supercharger to supercharger? for example, if you have an eaton m45 and an m90 and theyre both pushing 7psi, will the m90 be flowing more air? the first thought would be that since the eaton m90 flows twice as much air per revolution that it would then be flowing twice as much air at 7psi. but that doesnt seem possible, how can you have the exact same amount of pressure with twice the airflow? so then that means that to achieve the 7psi, the m90 must be rotating half the speed of the m45? will the m90 make more power than the m45 at 7psi?

also, since the amount the supercharger flows increases as rpms increase, then how do you have a pulley that "sets" the amount of boost?

Boost is nothing more than forcing in more air than an engine can pass through it. Due to intake system flow and camshaft design primarily, an engine only moves so much air at peak flow. Supplying more than that amount creates boost. Your two superchargers are moving the same amount of air, only one of them is rotating slower. Power will be very similar at the same boost (it does take more power to turn the smaller unit faster and it likely runs a bit hotter). The larger unit has more potential power if more boost is used. Superchargers also flow a certain amount of air per revolution, therefore, a pulley 'sets' the boost by changing the speed of the supercharger...faster = more boost.