The Roots supercharger, or the Eaton supercharger as its also called, is the oldest type of supercharger around, having been designed by the Roots brothers in 1859 as an air pump for use in the mining industry. The Roots or Eaton supercharger is a positive-displacement supercharger that consists of at least two lobed rotors housed in an aluminum casing. The rotors are meshed together and geared to rotate in the opposite directions. As the lobes turn, air trapped in the space around the lobes and is forced along the inside of the casing until it is discharged into the intake manifold. Being a positive-displacement supercharger, it moves air at a fixed rate in relation to engine RPM; hence a larger capacity Roots supercharger is required if you want to achieve higher boost levels.
Since the 1950's, engineers at the Eaton Corporation have been redeveloping the Roots-type supercharger. In the mid 1970's they made significant breakthroughs when they developed the twisted rotor for the Roots supercharger to improve its thermal efficiency to 50-60% and reconfigured the Roots supercharger's outlet port to reduce noise levels. The Eaton Corporation has also improved the fuel efficiency of the Roots-type supercharger. This resulted in the Roots supercharger also being referred to as the Eaton supercharger and it made the Roots supercharger an attractive option for both car manufacturers and hot rodders. They are also popular with muscle cars and hot rods as the Roots-type supercharger is usually installed on top of the engine and sticks out of the hood of the car.
However, the Eaton supercharger and the Roots supercharger does not compress air but simply moves air at a fixed rate in relation to engine RPM. In this sense it is nothing more than an air blower with air compression taking place externally. In other words, the Roots supercharger produces boost pressure by stacking more and more air into the intake runners and into the intake manifold. Thus, with the Roots and Eaton supercharger, boost pressure is the result of more air being forced in to the intake runners and into the intake manifold and boost pressure only increases after the air is discharged from the supercharger. Hence the Roots supercharger is also called an external compression supercharger. This external compression is also a major contributor to the relatively poor thermal efficiency of the Roots supercharger.
The Pros and Cons of Roots-Type Superchargers
If you're looking for a cost effective, low boost supercharger with excellent boost at low RPM then the Roots-type supercharger may just be your best option. However, these types of superchargers do suffer from internal leakage, which reduces it efficiency at low RPM. Furthermore, the Roots supercharger draws the most engine power of all types of superchargers and also has the least thermal efficiency of all superchargers. However, its simple construction with few moving parts makes the Roots supercharger one of the most reliable types of superchargers you can find. The Roots supercharger also doesn't suffer from surge as it is a positive displacement supercharger, which means that it moves air at a fixed rate in relation to engine RPM. This also means that a large capacity Roots supercharger is required if you want to achieve higher boost pressures. However, if you're looking for boost pressure of over 12 psi, you'd have to look elsewhere as the poor thermal efficiency of the Roots supercharger becomes a major problem at higher boost pressures. Although, they work wonders on exotic fuel dragsters that are used for short bursts of around 15 to 20 seconds at a time and were boost from low RPM is more important than thermal efficiency, they are not ideal for constant high boost applications.
When implementing a Roots supercharger, you must install a bypass valve and relocate the throttle body ahead of the supercharger's inlet port. If you don't move the throttle body, the supercharger will build up pressure between the supercharger and the throttle whenever your foot is off the accelerator, such as when you're idling, decelerating, or changing gears. When the pressure between the supercharger and the closed throttle exceeds the boost pressure being supplied by the supercharger, the air will be forced back through the supercharger. However, air can only move in one direction through a Roots-type supercharger. If the air tries to flow back through the supercharger, the supercharger will seize and will destroy the drive belt. This can also cause the throttle plate to buckle and get jammed in the throttle bore as the pressure will not be released. Of course, moving the throttle body further away from the intake valves will make the engine less responsive to throttle input, but that's just the cost of running a positive-displacement supercharger.