"Blower speed is determined by the belt-driven pulleys used on the front of the engine. The faster you spin the blower, the more air the blower moves. Unfortunately, this also creates heat. Lots of heat. Additionally, while internal clearances on Roots blowers are better than they'’ve ever been, internal leakage still occurs, which decreases the blower’s efficiency. The most popular of the Roots blowers are the 6-71 and 8-71 superchargers most often seen on Pro Street cars.
Centrifugals have become the fastest-growing supercharger segment in hot rodding, mostly for packaging reasons. The centrifugal is really little more than a belt-driven turbocharger. Centrifugals work on the principle of using an impeller spinning at extremely high speeds to accelerate the air and pass it through a diffuser to slow it down, creating pressure (and increasing temperature) as a result. The air is then piped to the engine intake.
One significant difference between a centrifugal supercharger and a Roots blower is that the centrifugal is a true compressor rather than an air mover. This means that the air exiting the centrifugal is already under pressure. This is measured in the centrifugal’s rating of 70 percent adiabatic efficiency versus the Roots’ less efficient 50 to 60 percent rating. One advantage of the Roots blower over a centrifugal is the Roots is a positive-displacement blower, which means it can come up on BOOST ALMOST INSTANTANEOUSLY ! , while centrifugals require time to “spool up” to create boost. As you can see, there are a number of variables that make the decision a little tougher.
A recent addition to rodding’s family of superchargers is the screw supercharger. The screw supercharger is a device originally designed as a large industrial air compressor. Norm Drazy was the first to employ the screw supercharger in drag racing, while another ex–drag racer, Art Whipple, has worked with the Auto Rotor Company to produce a series of street screw superchargers.
Designed as a dedicated compressor, the screw supercharger also enjoys a rating of high-70 to low-80 percent adiabatic efficiency as well as a compact size. The supercharger gets its name from its twin screws that inter-mesh inside a case to compress incoming air efficiently. The original Whipple blowers were small and intended for mild Chevy pickup applications, but Whipple has a larger screw supercharger that will be capable of 750 hp. " [END QUOTE]