Selecting the right centrifugal supercharger requires a little research since matching the system correctly to the engine's performance level is key. Centrifugal superchargers develop added power from maximum shaft speed, the overall compressor housing size, and impeller trim configuration. This influences the amount and rpm level that maximum boost occurs at on various engines. Centrifugal supercharger manufacturers can provide excellent help in selecting the proper system for your vehicle's performance requirements.
Give it a Boost
A Supercharger's boost is defined as the amount of air pressure produced to fill the cylinders over and above the air pressure in a naturally aspirated engine. The amount of boost and the rpm at which boost begins is a function of the throttle, the blower size and drive ratio (overdriven or underdriven), engine displacement, camshaft grind, and exhaust system. Assuming an even speed ratio between the engine and the blower, a larger blower will produce a higher level of boost than a smaller one on an equal-displacement engine. By operating the blower faster in relation to the engine's speed (overdriving) boost can be increased. Conversely, running the blower slower will decrease boost (underdriving) and allow a level appropriate for engine compression.
Just as you don't want a high static compression with a supercharger, you don't want a long-duration, big-overlap camshaft, either. Since the incoming air is under pressure, the intake valve opens as the mixture rushes into the cylinder. If the exhaust valve is open, a portion of the air/fuel mixture will be pumped out of the exhaust system by the supercharger. In general terms, a camshaft with less than 240 degrees duration (measured at 0.050 inch lift) will work well. Camshaft overlap should be minimized with lobe centers in the area of 112-115 degrees. Many street-blower cams are ground on a dual-pattern profile to allow the exhaust more duration than the intake (with the intake closed). This is because the blower pumps more air/ fuel mixture into the cylinder on the intake stroke, but the exhaust needs to exit on its own, so the exhaust duration is increased. Most camshaft manufacturers offer cams specifically designed for blower motors.
The amount of boost you can run is directly related to an engine's static compresssion ratio. When the boost is combined with the compression ratio, the result is the effective compression ratio. Typically, a 5- to 8-psi boost range (usually produced with the supplied pulleys in blower kits) will work fine for compression ratios in the 8:1 to mid-9:1 range (operating on 91/92-octane fuel). However, this will ultimately depend on other modifications to the car, manual or automatic transmission, gearing, operating temperature, vehicle load, and altitude. If detonation is encountered it can often be controlled with boost retard devices or by experimenting with different-sized pulleys.
Choosing a carburetor or fuel injectors is a crucial step when building a blower-specific engine, because under boost the engine will often need 40-50 percent more fuel and air. Unlike a normally aspirated engine that may suffer only low power from poor fuel delivery, a supercharged engine without enough fuel under power may run extremely lean and destroy the engine. Running too small a carburetor also means that you can't flow enough air to produce maximum boost.
As the rotors inside this...
As the rotors inside this Whipple unit mesh, the air is trapped between the rotors and the casing and is compressed. Finally, the center-lobe space becomes exposed to the outlet port, and the air is discharged into the manifold.
This Kenne Bell twin-screw...
This Kenne Bell twin-screw supercharger has been tested to increase power on the '04 Z06 Corvette engine an extra 195 horses on pump gas (600 hp versus 405 hp stock). That's at just 8 psi. Adding more boost raises the power significantly. These blower kits mount on top of the engine, face forward, and include a big throttle body, a cool-air system, and a billet fuel system.
As a rule, the maximum final...
As a rule, the maximum final compression ratio (effective ratio) should not exceed 12.4:1 for 91/92-octane pump fuel. Of course, expensive octane boosters and higher-octane racing fuel will allow you to run a higher final compression ratio.