Which Supercharger?

[olddog]

A roots style blower is considered a positive displacement blower. It is not 100% efficient because there is clearances between the rotors. There are some designs where they put Teflon on the rotor tips to seal better.

A centrifugal blower is not positive displacement. At a zero delta pressure they move a ton of air, but as the delta pressure builds the air flow will drop all the way to zero. The pressure that the blower can build is proportional to the tips speed of the fins. Therefore the faster you spin it the more pressure it can push. On the other hand, the slower you spin it, the less pressure it can push.

On an engine, a roots style blower is moving a certain volume of air per revolution. The engine displaces a certain amount or air per revolution. If the blower is moving about twice the amount of air that the engine is ingesting, the pressure will be about 2 atmospheres, which is about 14.7 psig. So at any rpm, the engine will get roughly the same amount of boost.

On a centrifugal blower, at low rpm the blower cannot make much boost at all. If memory serves it is a square function curve. So, let's say you can make 14.7 psig at 8000 rpm. At 1/2 that (4000 rpm) you can only make 1/4 the pressure (3.7 psig).

Full out both blowers will perform the same. Down low, a centrifugal is like having no blower at all.

A roots style still has slippage (losses) and that does make heat. At pressures less than 10 psig it is not worth considering. However the twin screw compressors out there are much more efficient. They take less power to turn and make less heat. If the price was similar, I would go twin screw compressor every time and twice on Sundays.

PS
A positive displacement blower will give you a nice flat torque curve.

A centrifugal will give a torque curve that looks like the right half of a "U". If your cam and heads quit breathing fairly quick, the two will combine to give a curve looking like "/ " (only a little flater).