Looking for to-the-wall power with a huge cool factor? Well, check out the world of superchargers. These persuasive units shove mammoth amounts of atmosphere down your engine's cylinders for a mighty combustion charge that enables awesome power and torque.
With a supercharger, the amount of air and fuel that can be packed into the cylinders greatly surpasses the volumetric efficiency of a highly refined normally aspirated engine. But just like all other vehicle modifications, you need to follow a performance theme that encompasses your budget, driving use, and performance goals. Then, with a little research and planning, you can have one of the quickest rides and coolest engine compartments in town.
In this story we're going to explore some of the differences in blower design and explain some basic principals. So hang on as we give your car's performance a boost.
It's What's Inside That Counts
Installing a supercharger on a stock engine generally works well as long as the static compression ratio is 9:1 or less and engine speed is limited to 6,000 rpm. With stock cast pistons, a cast crankshaft, and a small camshaft, you should limit boost pressure to 3 to 5 pounds maximum. Remember though, for maximum boost and horsepower applications (12 pounds or more), you'll need race-quality parts.
To run boost from 6 to 10 pounds on your street engine you should have:
* Forged blower pistons with a static compression ratio of 7.5:1
* Forged-steel crankshaft
* Four-bolt mains
* Steel harmonic damper
* Stainless steel valves
* Performance camshaft
* Roller-rocker arms
* Good-flowing heads
* Steel rods with quality rod bolts
* Chrome-moly pushrods
* High-output ignition
Today's superchargers are generally based on one of three blower designs termed Roots, centrifugal, and screw type. The Roots-type blowers are often the least expensive systems to install and are based on a design originally used on GMC diesel engines decades ago. By design, a Roots-type blower does not compress the air inside the supercharger but instead acts as an air pump. The compression of the inlet charge or boost occurs in the cylinders and the manifold. Because of this, a Roots-type blower is called an external-compression blower.
A centrifugal supercharger is sometimes mistaken for a turbocharger because of its shape, but instead of being propelled by exhaust gas, the centrifugal supercharger is beltdriven. During engine operation, centrifugal superchargers can run to extremely high speeds achieved by an additional internal step-up drive inside the blower. So as the impeller spins to a higher rpm, added boost is delivered to the engine's intake system.
From the outside, the screw-type blower looks similar to a Roots-type, but a screw-type blower uses rotors assembled with very tight clearances that interleave to pull in and compress air as it passes.
Give It a Boost
Boost produced from a supercharger (or turbocharger) is the amount of air pressure created by the supercharger and is a function of three things: the engine's displacement, blower displacement, and the speed that the blower is turned relative to the engine rpm.
Assuming a stable speed ratio between the engine and the blower, larger blowers produce more boost than smaller blowers on engines of the same displacement. As engine size increases, boost levels drop (if the blower speed and blower size remain constant). Conversely, as engine size drops, boost moves up. For a given-size blower and engine, boost will increase by turning the blower faster in relation to the engine's speed (overdriving), and boost can be decreased by running the blower at a slower speed (underdriving). You can get the blower to run slow enough to keep the boost down to a level appropriate for the compression ratio of your engine by changing pulley diameters.
The level of boost that an engine can safely accept is primarily determined by the engine's static compression ratio and fuel octane rating. But the key to any supercharger installation is to prevent detonation. Detonation in a blown engine is more destructive than in an unblown engine, and damage to piston ring lands will occur if a blown engine is continually operated while detonating. A handy device to have with a supercharged engine is an ignition system with boost-retard control, such as the Holley PN 800-450 Ignition.
Without enough fuel and air, your engine can't take full advantage of the supercharger and won't make maximum boost. Remember that in addition to fueling the motor, the carburetor also acts as a restriction to air passing into the blower and engine. Under boost, the engine may need up to 50 percent more fuel and air, so it's critical to match the blower to your carburetor(s).
Adding a blower to your performance car can not only increase its power, but also impress your friends when you pop the hood. If you'd like to see what's available specifically for your engine, check out the blower manufacturer Web sites listed in the Source box, and get ready for some supercharged fun. CHP