Nothing quite approaches automotive nirvana than the act of bolting a supercharger to a Chevy engine. While some would argue, especially on the Internet, that running a blower is "cheating" and not as pure as making big power from a naturally aspirated mill, they are missing some of the huge benefits of superchargers.
As the name implies, superchargers increase the atmospheric pressure inside the combustion chamber. More pressure equates to more air and thus more power. Sounds simple, but there's more to it.
In a naturally aspirated engine, the intake valve opens and the downward stroke of the piston creates a partial vacuum inside the cylinder. The differential between the cylinder and atmosphere causes air to move into, or "charge," the cylinder. A supercharger increases the pressure inside the intake manifold so that the pressure difference between it and the cylinder is even larger. This causes a larger mass of air (air charge) to enter the cylinder. More air means that more fuel can be burned, and the result is what we all crave-more power!
A highly refined version of the classic Roots-style supercharger is this Magnuson unit. Instead of having nearly straight lobes, the rotors are twisted into a steep helix shape. While still commonly referred to as a Roots-type unit, it's actually more of a hybrid of the Roots and axial designs. Instead of air being drawn in through the top like a typical Roots unit, it's drawn in through the inlet port and pushed into the engine along the rotors' axis. Thanks to a vacuum-referenced bypass valve, the inlet pressure is equalized with the manifold pressure. This pretty much negates any parasitic loss while cruising and makes for a very efficient design. The Magnuson can also be run with or without an intercooler, but it will make more power and run on lower-grade fuel with the intercooler. The newer Magnuson units feature the improved Eaton TVS four-lobe rotor arrangement.
The compression added to an engine by a supercharger is measured in pounds per square inch (psi) and is referred to as "boost". If a supercharger is putting out 10 pounds of boost, then it is pressurizing the engine's intake to 10 psi over atmospheric. Most superchargers found on production cars are in the 4-12-pound range, while all-out race applications can put out 20, 30, or more pounds of luscious boost. The limiting factors are money and the engine's ability to stay in one piece under the strain. On a normal gasoline-fed engine it's common to see a 50 to 70 percent increase in torque from a supercharger putting out around 7 psi. And that's part of the allure of superchargers: big gains from a bolt-on system without the need to ever fill a nitrous bottle.
Not long ago that, the Roots design was considered the least efficient of the three basic supercharger types. Today there have been huge engineering advancements, specifically by the Eaton Corporation. Its new TVS superchargers are very efficient compared to older models. The newest design, introduced for the superchared LS9 in the '09 ZR1 Vette, features a four-lobe-per-roter design compared to the typical Roots arrangement of three lobes. Like other Roots systems, the supercharger is especially good at delivering boost at low engine rpm.
But why add on a supercharger instead of just building a bigger engine? The simple answer is efficiency. Engines are great at turning high-pressure gas into rotational energy, but are somewhat limited by atmospheric pressure, which is constant. One way to make more power using the Earth's atmosphere is to increase the size of the cylinder. If you can cram in more atmosphere, then you can make more power, even with a smaller cylinder. Using a supercharger makes an engine react like it has way more cubic inches, and that's a very good thing indeed.
Types
There are several different types of superchargers on the market today: axial-flow, screw, centrifugal, and Roots. Axial-flow compressors consist of rotating, aerofoil-shaped vanes. Since this type is typically seen inside jet engines, we'll skip it for this discussion, though we've always wanted to build a jet-powered Chevy. Then again, who hasn't?