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Roots Type Superchargers Explained

History


The roots style supercharger is the oldest type of supercharger and dates back to the early 1900's when it was first used as an industrial air-moving device. In the past 30 years or so, however, the roots style supercharger has undergone drastic changes and has become so efficient and quiet that it is now commonly used as a forced induction system for automotive applications. The roots style supercharger, while still the least thermally efficient supercharger design (versus centrifugal and screw-type designs), has found a home on board top fuel dragsters as well as on modern Mercedes, Ford, and GM passenger cars as an original equipment power adder.

How it Works


The roots type supercharger is two counter-rotating meshed lobed rotors. The two rotors trap air in the gaps between rotors and push it against the compressor housing as they rotate towards the outlet/discharge port. During each rotation, a specific fixed amount of air is trapped and moved to the outlet port where it is compressed, which is why the roots type supercharger falls under the broader category of fixed-displacement superchargers (like the twin screw supercharger).

Advantages & Disadvantages


The roots type supercharger is known for its ability to produce large amounts of boost while spinning at very low speeds. On an automotive application, a roots type supercharger can often make it's full (peak) boost by 2000 engine rpm. This characteristic has contributed to its success and popularity on the top fuel racing circuit and has made it ideal for use on smaller 4 and 6 cylinder engines that traditionally struggle in the lower half of the rpm range (and is why Jackson Racing uses a roots type Eaton compressor). Another advantageous characteristic of the roots type supercharger is its simplicity of design. The roots type supercharger has very few moving parts and spins at low rpms, making it one of the more reliable and durable supercharger designs.

The big disadvantage to the roots type supercharger is its thermal inefficiency - or its nature to produce high discharge temperatures - which robs power from the engine. With a roots type supercharger, an intercooler is almost always a necessity to bring the air charge temperatures down to an acceptable level. This poor thermal efficiency can be attributed to the fact that it has no internal compression (compression is done after the air leaves the discharge port). Additional heat is created by compressed (hot) air that leaks backwards past the rotors and heats up the temperature of the inlet charge.

Conclusion


The roots type supercharger is the oldest type of supercharger and still has its place in the automotive world on dragsters, smaller engines, and trucks - all of which are need power in the bottom half of the rpm range. Most major manufacturers have steered away from roots type superchargers likely because they create so much heat, even at low levels of boost. Like the screw-type supercharger, it is also difficult to create very high levels of boost with a roots type supercharger. Nonetheless, several manufacturers (Magnuson/MagnaCharger, Saleen, Allen,, Jackson - all use an Eaton roots compressor) have been able to design automotive supercharger systems that make good use of the roots type compressor's advantages while overcoming its shortcomings. If you do purchase a roots-type supercharger, expect incredible power gains right off of idle. You can also be assured that you will have one of the most simple and dependable superchargers available, which is why automobile manufacturers (GM, Ford, Mercedes) generally choose roots compressors for OE applications. An intercooler will most likely be necessary at boost levels above 6psi with a roots supercharger.

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