The numeric designation on the Eatons is there cubic inch displacement. 90 = 90cid per 360 degrees of rotation of the pump. 112 = 112cid. I hear rumors of a 140cid Eaton soon to be released. Now, in virtually every application these superchargers turn faster than the engine rpm due to the pulley overdrive ratio. This ratio varies depending on crank pulley vs blower pulley diameters. Also remember that a 4 cycle engine must rotate 720 degrees to move its rated displacement.
The above said, and not considering the less than 100% volumetric efficiency of either an engine or a SC... here is what happens.
The a 4 liter engine can "breathe" say 2 liters per rotation and the M112 breathes 1.84 liters per revolution (61 cid per liter). If you turn the SC at the same speed as the engine... nothing much happens boost wise, in fact the SC may be a restriction and of course requires crank power to rotate so the net result is a loss in power. Now, if you spin the SC at 2 times crank speed the SC is now trying to force 3.68 liters into an engine that is only 2 liters of capacity (again, a 4 liter engine). The result is there is a measureable pressure in the intake manifold (boost). How much boost depends on MANY things but for the sake if rediculous simplicity lets say 15psi atmospheric pressure, a pressure ratio of 3.84 / 2.00 = 1.92 - 1.00 (atmosphere) = 0.92 X 15psi = 13.8psi of boost. Again, this is NOT a suggestion that this is the VERY boost level that this combo would make... just an illustration to show what is going on.
