[MJN]

Technically the Kenne Bell is a Lysholm screw compressor it actually compresses the air in the blower case rather than just cramming it into the intake manifold like a roots blower. Because of this it is also more efficient than the roots, simplistically what that means is that it will heat the air less for a given boost and as we all know lower temps = denser air = more power. The roots and the Lysholm are both positive displacement superchargers. That is to say they move a fixed quantity of air per revolution no matter what the engine speed. That is why they are so good at providing boost and torque at low rpms. The centrifugals can also be very efficient depending on the design of the volute and the impellor, amongst other things, but they move air in a different way, more like a fan, and the boost increases exponentially with the impellor speed. So they provide little boost at low rpms but when they get spinning watch out because the boost can build in a hurry. One way around this excess boost issue with turbos is the use of a pop off valve that limits the boost. Both styles of blower (positive displacement / centrifugal)really need carefully sizing to the engine although the positive displacement blower is a lot more tolerant of over sizing than a centrifugal and will still be fun to drive even if it is too big for the engine. An oversized centrifugal or turbo isn't much fun, is very lazy at building boost, but when it does watch out. May be good for a track car but not so much fun on the street.