They look awesome, whine, and make huge power. Yes, we're talking about superchargers, and in the world of hot rodding they are the ultimate in cool. Unlike a normally aspirated engine that relies on both atmospheric pressure (14.7 psi at sea level) and vacuum to fill the cylinders, a supercharger-also known as a blower-feeds both air and the delivered fuel at high pressure into the cylinders. The result is a level of volumetric efficiency surpassing that of a normally aspirated engine to produce more power for a tire-frying experience. So actually, the topic of supercharging is really about increasing volumetric efficiency.
Like all other forms of engine and vehicle modification, a supercharger unit should be matched to the vehicle's performance level, type of driving, and budget for optimal results. This means improving the cooling system, fuel system, ignition, engine components, drivetrain, suspension, and other parts to support and enable the added power.
So here's your short course in macro superchargers: how they work, the different types, and the components you'll need to change on your vehicle. So hang on because the info will blow you away.
Blower Basics
Superchargers fall largely into three design categories: Roots, centrifugal, and screw, the most traditional being the Roots. As noted, the key to making good power in an internal combustion engine is to increase the amount of air and fuel stuffed into each cylinder before every power stroke. This improves the combustion that produces the heat that is transferred into energy, translating to greater force on the piston and the connecting rod so that crankshaft turns with added power. This is basically the same reason larger-displacement engines make more power and torque than smaller ones; there is a larger supply of air and fuel within the cylinders.
Roots
A Roots-style blower is probably the most commonly noticed supercharger at cruise-ins. On some applications these stick out of the hood with one or two carburetors attached. These units act as an air pump so that the compression of the inlet charge (boost) takes place inside of the manifold and cylinders, external from the blower. Because every full rotation of the Roots compressor element generates a specific amount of air pumped from the inlet side to the exhaust outlet (directly into the intake manifold), the Roots style is considered a positive-displacement blower.
Inside the case of a Roots blower are intermeshing, rotating rotors. A crankshaft-mounted pulley spins a drive pulley through a belt (typically cogged on larger blowers to eliminate slippage). The drive pulley, mounted on the front of the supercharger, is connected to internal gears that turn the rotors. The spinning of the rotors compresses air and fuel supplied from either the carburetor(s) or a fuel-injection system mounted above the blower case. This air/fuel mixture is pumped between the supercharger case and rotors.
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A static compression ratio of about 8:1 is a good compromise for noncomputer-controlled engines running on premium pump fuel. More power is available by reducing compression to 7:1, but the engine won't be as responsive at low rpm when not under boost. Static compression ratios above 9:1 typically limit boost and max power potential. |
This supercharger (shown disassembled) is a Weiand 6-71 unit from Holley. This type of supercharger (traditional Roots) uses a special intake manifold and pulls air from the top down into the motor. | |