A positive diplacement type blower (often called roots type) "pushes" a relatively constant volume of gas from the inlet to the discharge at a relatively fixed pressure differential. Increasing the speed increases the volume it can push, but the pressure differential is largely a function of the physical size of the components - rotors and case size. That's why PD blowers produce boost (pressure diff) at low rpm, and why the boost pressure varies little with speed. They don't internally compress the air. So flow through a PD blower is relatively constant regardless of pressure difference, for a given speed of course. Installing a smaller pulley increases the rotating speed of the 'pump' - and usually this results in a pressure increase, but not nearly as dramatic as what happens when you increase the speed of a centrifugal.
A centrifugal compressor is nothing more than a rotating air-foil. It compresses the air in exactly the same way that an airplane wing creates lift. As speed of the rotating foil increases - both volume and pressure increase -- until the airfoil stalls. Then both pressure and flow drop off.
Clear as mud?
