well it depends on the motor and mods of course. the lower the compression, the more boost you can run. if youre trying to make all the power you can and are running lower ompression, a nice centrifugal will be able to make 30+ psi of boost. i dont recall ever seeing a roots style making over 20 psi(except top fuel dragsters running like 12-71s). the amount of boost a roots can make is determined by the housing size, it can only move so much air. but because the design of centrifugal blowers, the limitation is impeller speed and bearing wear. of course centrifugals do come in different housing sizes as well, but think of a centrifugal as a belt driven turbo. the faster you spin them, the more boost they make. because it is power by a belt instead of exhaust gases, it doesnt have the same "lag" but does make very linear(rpm based, more rpm=more boost) power.
best way to explain the difference; roots boost is determined by throttle positioning, centrifugal boost is determined by rpm.
