Originally posted by bnoon:
Boost of any kind isn't directly related to RPM. Boost is more closely relate to load and total engine restriction. Full throttle and 3000 RPM holds more boost than part throttle and 3000 RPM because the load isn't the same. Reduce restriction after the boost device, boost goes down, yet HP goes up, even though the load and RPM stay the same. These hold true to either SC or turbo...
Want more boost a low RPM? Get a turbo that spools up faster (either smaller in size or just more efficient), or put a smaller pulley on the SC to spin it up faster. You can make an SC achieve max boost at 2700 RPM too if you want to. Although the boost devices are not directly related to engine RPM, they are direcly dependant on their own RPM to move air. The fan trim size and shape being equal, the turbo and SC will move the same amount of air. The SC losses power through it's drive belts and pullies, the turbo through it's exhaust restrictions. Drive either without the input tube hooked up and boy, do you know it!!!
The silly part is, everyone is hashing the exact same points here. 
My apologies brad, I should have been using the term airflow instead of boost. But I figured that in a given scenario, boost can generally correspond to airflow enough to make the point here. Keep in mind, were have been speaking of a single, hypothetical system, not modifying the system during the time of discussion, unless otherwise noted.
As far as efficiency goes, turbo has it hands down though, a supercharger loses far more through drive losses than a turbo does through increased exhaust restriction (especially compared to a typical stock exhaust system)
as far as moving the same amount of air, yes, the trim size and compressor side shape, etc. determine the flow capabilities, but, the rpm of the "boost device" determines how much flows at a given rpm. The fact that a turbo is free to rev up to its optimal rpm well before an s/c gives the turbo a great advantage as far as torque and hp curves.
The rpm of the s/c is mechanically constrained to the rpm of the crank, so, engine rpm X drive ratio = s/c rpm at that engine rpm. You can change the drive ratio (pulley change) to increase the rpm of the s/c at a lower engine rpm, to increase low-end air delivery, while using a blow-off to limit boost pressure on the top end. Keep in mind that this spins the blower MUCH higher at the high engine rpm which increases drive losses in the s/c drive, and decreases the adiabatic efficiancy of the s/c itself (more heat in the intake charge)
