Even today, the Roots-type blower is still the way to go in the top drag racing classes. When bolted on top of a huge-cubic-inch engine, the sky's the limit in regard to power, with 8,000 horsepower not being uncommon. At that power level it can take around 500 hp to spin the blower when it's making maximum boost.
Blow-Through Vs.Draw-Through Carb
In a draw-through application, typically on a Roots-style blower, the carb is located on top of the blower. As the blower operates, air is pulled, or drawn, through the carburetor and into the engine. The carburetor works about the same as on a naturally aspirated engine with the exception that its power valve has to be referenced to the intake manifold pressure. This makes tuning relatively easy. The main issue with this type of system is packaging in regard to hood clearance.
Centrifugal supercharger systems employ a blow-through system. Here the blower creates pressure before the carburetor and air is forced through the carb and into the engine. While it's a snap to package this type of system under just about any hood, the downside is setting up the carburetor properly. Setting up a blow-through carburetor takes quite a bit of skill. If you go this route, then the best idea is to find a company that has tons of experience dialing in blow-through carbs. (See the story by Dan Ryder and Mike Harrington in the next issue for more on building a blow-through carburetor.)
The Whipple Supercharger
While similar in appearance to a Roots-style supercharger, the Whipple supercharger really belongs in its own category as a screw-type compressor. The housing on a screw-type supercharger will have an opening at the top back (inlet) and lower front (outlet). As the screws turn, air is trapped in the lobes and pushed forward, compressed, and moved into the engine.
A screw compressor is a positive-displacement unit that uses a pair of intermeshing rotors to produce compression. The rotors, comprised of helical lobes, are affixed to a front and rear shaft. One rotor, called the male, will typically have three lobes. The other rotor, the female, has valleys machined into it that match the curvature of the male lobes. Typically, the female rotor will have five valleys. The rotors never touch, but are timed by a pair of gears operating in a lubricated chamber that's separated from the rotor chamber. With the three/five rotor combination, the male rotor turns three times to every one time of the female rotor.
The number of lobes on the male and female rotor will vary from one compressor manufacturer to another. However, the female rotor will always have numerically more valleys than the male rotor has lobes. Because of the number of male lobes, there are three compression cycles per revolution, which means the resulting compressed air has small pulsations compared to a reciprocating compressor. By the time the compressed air leaves the package it is, for all intents and purposes, pulsation-free.
Too Good For Its Own Good
Back in the mid-1980s Norm Drazy worked his butt off to develop a large screw-type supercharger. His invention utilized a four-lobe male rotor and a six-lobe female that turned 30-percent slower. His unit was called the PSI, and Norm was hoping to introduce it to Top Fuel dr ag racing. Like the Roots, the PSI fills from the top and discharges at the bottom; but unlike the Roots, the PSI employs internal compression. This is why the PSI requires less horsepower to turn than a traditional Roots supercharger.
The PSI on a little 385ci engine made 1,260 horsepower on alcohol at 28 pounds of boost. This was the same power as a 14-71 unit made on a 552 cubed engine running 36 psi. Not only that, the outlet temp was a comparatively frosty 85 degrees compared to 150 degrees for the 14-71. It was all sunshine and rainbows until the NHRA banned the PSI in Top Fuel. The PSI, and the new larger Whipple screw compressors, did find a home in Top Alcohol drag racing, where they have become standard fare.