High-Helix And TVS
As we mentioned before, Roots superchargers don't make pressure in and of themselves. They generate boost by feeding more air into the engine than the engine can consume, essentially pressurizing the intake manifold with extra air. They become less efficient at higher boost levels because air begins to leak past the rotors back up through the supercharger, causing turbulence in the case. Turbulence increases the friction between air molecules, and we all know that more friction equals more heat. This is why we stated earlier that Roots superchargers were not very efficient. Traditional GMC-style Roots superchargers, i.e., the familiar 6- and 8-71 models, are great for drag racing. They operate best at low pressure levels-wide-open throttle at high engine speeds-but their efficiency can fall to around 30 percent when pressure levels in the intake manifold increase.
By design, Lysholm-type superchargers have a clear advantage here. By twisting the rotors and routing the air longitudinally through the case, turbulence and friction are greatly reduced. Plus, because they compress the air within the rotors, they are delivering high-pressure air into the intake manifold, which also helps keep air from blowing backward through the case. They are more efficient at higher boost levels and are ideally suited to street cars operating in part- to medium-throttle at higher boost levels.
Don't count out a Roots supercharger, though. A lot of R&D has been done to the GMC blower, and today's Roots blowers are much more efficient than their predecessors. Over the years, engineers have messed around with the rotor design, adding extra lobes and a 60-degree twist called the High Helix. All these improvements have boosted the efficiency of the pumps.
Photo by Whipple Superchargers
Recently, Eaton took things to a higher level with the introduction of its Twin Vortices Series (TVS) superchargers. Eaton is an OE-level supplier with an operating budget we can only dream of. The company added even more twist to the old Roots design, making rotors with a 160-degree twist along their length. Like a Lysholm, the TVS superchargers draw in air from the back of the case, pushing it out of an opening on the bottom of the opposite end. Like a traditional Roots design, though, the rotors still turn away from each other, and the air is still pumped between the rotor and the case. But the higher degree of twist prevents leakage past the rotors better than the old Roots design did.
At the end of all this is the issue of efficiency. So which supercharger type is the best? That obviously depends on your application and the type of driving you do, but we do have at our disposal several tools to help judge which is the best-we can measure a supercharger's efficiency. The two efficiency ratings to consider are the volumetric and adiabatic efficiency numbers.
Volumetric efficiency measures how much of a substance a pump can flow versus how much it actually does flow. We're familiar with this term as it applies to the automobile engine. Under perfect conditions, a 5.7L engine should pump 5.7 liters of air for each turn of the crankshaft. This rarely happens in the real world, though. Unless you have a big carburetor and exceptionally good-flowing cylinder heads, chances are an engine turning at 6,000 rpm will not be filling its cylinders as completely as it could. Atmospheric pressure can only push air in the intake, through the ports, and past the valves so quickly.
Photo by Magnuson Superchargers
The same principle applies to superchargers. While they are busy forcing air into the engine, superchargers rely on atmospheric pressure to push air into them. Things like rough castings, poorly designed rotors, and inlet and exhaust ports all affect airflow through the supercharger and can diminish its volumetric efficiency.
The adiabatic efficiency rating deals with the issue of heat. The easy explanation is to say adiabatic efficiency is a measure of how much a supercharger heats the air before delivering it to the intake port. That's too simple, though. Like the volumetric number, adiabatic efficiency measures the actual increase in heat versus the ideal. For any machine to do work, you can expect a certain amount of heat to be generated-usually due to friction. Bearings generate friction as they turn and air molecules generate friction as they tumble around inside the supercharger-friction is unavoidable. One can measure all these parameters to predict how much heat should be generated by this machine as it works. Then you measure the actual difference and divide the two to come up with the adiabatic efficiency rating for that given supercharger.
Like turbochargers, compressor maps are available for superchargers, showing the rpm and pressure levels they are the most efficient at. Though they look confusing, spend some time learning how to read them. They really aren't difficult to understand.