A centrifugal blower is essentialy the compressor housing of a turbocharger driven off of the crank by a belt. In both cases you have a centrifugal air pump which draws air in at the center of its blades and flings it outward as it spins.



Just look at the compressor portion of the turbo, and that's how a centrifugal blower works. As the blades spin, air is sucked in at the center and then sped up and collected along the snail-shaped portion of the housing, which pressurizes the air.

A Roots-type blower is one of many types of "positive displacement" superchargers. To quote SCC: Positive displacement superchargers fill a chamber of a fixed volume with air at atmospheric pressure, and move that air to the high pressure side. The mass flow rate of the air thus moved depends primarily on the volume of the initial chamber and the speed of operation, i.e., how many times that chamber is filled and its contents squeezed out the other side, and is relatively independent of pressure ratio. Roots, Lysholm, G-lader, sliding vane and Wankel (yes, the rotary engine started out as a supercharger concept) compressors are all positive displacement superchargers.

More info on the Roots blower from http://www.cashflo.co.uk/Roots.html

A Roots compressor/blower is a positive displacement machine that uses two or more rotating lobes in a specially shaped cylinder, usually shaped like the figure of 8. The lobes intermesh with each other using timing gears and suck air in from one side of the figure of 8 to deliver the air on the other side.

There is no actual compression ratio built into the machine, it is simply an air mover. Compression is caused because the induced air is forced into a closed conduit, thereby causing the atmospheric air becomes pressurised after it has left the Roots blower.

The rotating impellers intermesh quite closely, but contact is avoided by using precision timing gears. Although these gears are splash lubricated, because they are external to the cylinder barrel, the machine provides oil free air.

Roots blowers are typically those sold by Eaton (rebadged by JR) and also the ones made by B&M you see on big-block American V8's. The Whipplecharger and Autorotor along with the blower used by Mazda on the Millenia are Lysholm type. VW used a G-lader on the Corrado G60 before installing the VR6 and calling it the Corrado SLC.