In the world of positive-displacement pumps, there are a variety of shapes and styles. As we mentioned before, a piston engine is one example-you will likely find a piston pump pressurizing the compressed air tank in your garage. Refrigerant in your air conditioning system is also pushed around by pistons in your A/C compressor. Rotary pumps are another example: The oil pump in your engine is a scaled-down version, while the Wankel rotary engine found in Mazda's RX cars can make pretty good power. Vane-style pumps, like those in your power steering system and automatic transmission oil pump, are just two more examples of positive-displacement pumps you can find all over your car.
However, most of these types of pumps are not good superchargers. They generally cannot move enough air to meet the demands of an automobile engine and still be sized small enough to fit within the confines of an engine compartment. Decades ago, the industry latched on to a different design, the Roots-style pump, because it can move very large quantities of air-enough to feed a hungry V-8.
See, you can't help but look.
The ubiquitous Roots blower is the oldest and most common supercharger used in automotive applications. Its visual appeal is undeniable: Like a sucker punch to the jaw, nothing says horsepower like an oversized Roots supercharger. Though it was adapted by the transportation industry to help power two-cycle GMC diesel buses, the Roots blower has a history that dates back before the golden age of the automobile. Originally conceived by brothers Philander and Francis Roots, the device was designed as an air pump for blast furnaces. It was quickly adapted for use in a wide variety of applications, from ventilating mine shafts to pumping fluids in plumbing applications. A Roots-type pump was even used to provide the air supply (insert '70s soft-rock band joke here) to the Federal Signal Thunderbolt air raid sirens common throughout the country during the Cold War.
Roots pumps are extremely simple machines. A case houses two rotors that are machined to tight tolerances, though the rotors usually do not touch each other, nor do they touch the case. The rotors are connected by a pair of gears, and a shaft extends out from one of the gears and is driven by the crankshaft. That shaft turns the gears causing the rotors to spin in opposite directions. An opening in the top of the case allows air to fill the empty space in the cavity of the rotors. Then as they turn, the rotors trap that air between themselves and the case of the pump, pushing it around and down through the case and out the other side.
The simplicity of the Roots supercharger may be its ultimate shortcoming as well. Dustin Whipple of Whipple Industries tells us that Roots pumps are a "no pressure" design. "They were never intended to pressurize air-only to pump it," he says. As a result, the Roots supercharger is one of the most inefficient. More about that later.