Roots superchargers use a pair of intermeshed rotors spinning in opposite directions to move air. The rotors are shaped into axial lobes that create air pockets. They are intermeshed so that the left and right rotors grab air alternately, moving the air away from the top center of the unit outward and downward along the curved inside of the housing of the unit and back toward the center, where it is fed into the engine. One rotor cannot generate enough force to easily turn the other, so they are driven by a gearing system at the end of the rotor.

Because the left and right rotors deliver air in alternating pockets, the air stream out of a Roots blower arrives to the engine in pulses that “stack up” on top of one another. This pulse effect can be damaging to the internal parts of the engine if not controlled. To minimize this effect, the rotor lobes are twisted from one end to the other in order to deliver the waves of air over a longer time frame.

Roots superchargers are Positive Displacement (PD) superchargers. A PD blower is one that puts out the same amount of air per revolution, regardless of the rotational speed. The faster it turns the more air it pumps. However, the amount of air pumped per revolution remains the same, regardless of the speed of the rotors.

Another type of positive displacement pump is an automotive cylinder. The displacement of an engine is fixed, so the amount of air pumped through the engine dependsentirely on the RPM. The fact that the Roots blower and the engine are both PD pumps means that the engine and the supercharger share a linear relationship. When the engine speed is doubled, the amount of air is doubled—through both the engine and the supercharger.

This linear relationship explains why a Roots supercharger is able to maintain a relatively constant boost throughout the RPM range, unlike a centrifugal type supercharger.

A Roots type supercharger can be configured to provide a higher or lower level of boost by changing the rotational speed of the blower in relation to the engine. As with any belt-driven device, changing the size of either pulley will result in a different ratio between the driving pulley (the crankshaft) and the driven pulley (the blower). As a result, pulleys of varying sizes are available which allow the user to change the boost level by swapping the pulley on the supercharger.

Roots type superchargers are often confused with twin screw superchargers. However, twin screw superchargers actually compress the air while moving it, while Roots blowers are simply air pumps. Because of this, Roots superchargers operate under only moderate compression. To increase final boost ratios, multiple-stage blowers have been developed that increase compression through a series of boost cycles.

Among the three types of supercharger, Roots blowers are the least efficient. Although they are capable of moving large amounts of air, it requires a relatively large amount of force to do so. This is force robbed from the crankshaft, which reduces the available power to the wheels. The relationship between the blower and the crankshaft is usually referred to as “parasitic” because of the way the blower steals power from the engine.

Thermal efficiency is another consideration with a Roots supercharger. The clearance between the two lobes enables small amounts of heated air to escape back towards the intake, which heats the incoming air. Additionally, because compression occurs as the waves of air stack up on one another, it occurs after the air leaves the supercharger, in the hottest part of the intake. Hot air is potentially damaging, but it’s also another example of parasitic loss: hot air is not as dense as cool air, so combustion is less efficient.

To address the problem of thermal efficiency, nearly all Roots superchargers require an intercooler between the blower and the engine. Even so, the best a Roots blower can expect is about 65% thermal efficiency, and that occurs at a very low RPM range. Efficiency at 6500 RPM is usually no better than 55%, although the newest and most efficient models such as the Eaton TVS can reach thermal efficiency ratings as high as 75%.

Roots-type units are the simplest in design, which makes them the most reliable of all the supercharger types. They can also move vast amounts of air, giving them the best gain potential. But they’re not incredible efficient, and they tend to be heavier than twin-screw and centrifugal superchargers.

By Vanessa Wilson