The Centrifugal Supercharger Explained
Since we sell more centrifugal superchargers than any other type of supercharger (twin-screw or roots), it’s time we sat down and discussed exactly what the centrifugal supercharger is and how it works. Take a fresh breath of air, because this fairly detailed and technical. Don’t worry if you don’t understand exactly how it works – it really doesn’t matter. The important thing is that it makes ridiculous amounts of power, which in turn makes your car faster, which in turn helps you get chicks – and that’s what this is all about, right!??!?
The centrifugal supercharger compressor creates its boost via a very rapidly rotating impeller that draws air into the center of the supercharger compressor (head unit). (If you’re lost already, first read “Superchargers 101 – An Introduction to Superchargers“). The impeller design inside a supercharger compressor is very similar to a turbocharger’s compressor impeller. After drawing the air molecules into the center of the supercharger compressor, it throws them outward towards the into the supercharger scroll. The supercharger scroll acts as a chamber to collect the air molecules and channel them towards the supercharger discharge tube so they can be forced into the engine’s air intake. The diameter of the scroll increases as it moves further away from the center of the supercharger, which slows the flow of the air while increasing the pressure of the moving air.
The centrifugal supercharger compresses the air primarily at the point that the air leaves the supercharger impeller and is forced into the supercharger scroll. At this point the air is pushed through a venturi shaped bore. The compression peaks at the apex (narrowest point) of the venturi before being released into the scroll for discharge. This compression method allows the centrifugal supercharger to enjoy a fairly high degree of thermal efficiency, however, in order to produce significant amounts of boost, the centrifugal supercharger impeller must spin at very high rpms. In fact, the amount of boost produced by a centrifugal supercharger is proportional to the square of its impeller speed, which enables the centrifugal supercharger to make very high amounts of boost in the upper half of the engine’s powerband.
Due to the design of the centrifugal supercharger and it’s discharge scroll, the supercharger compressor can be mounted far away from the air intake. A discharge tube can be used to carry the compressed air from the supercharger compressor to the engine’s air intake, or to an intercooler, making it very adaptable and easy to install. Also, because the centrifugal supercharger essentially acts as a high-speed fan, air can flow backward through the compressor under certain circumstances like quick deceleration or while shifting gears (rather than a fixed-displacement supercharger – roots, twin screw – which only allow air to flow in one direction). Supercharger manufacturers exploit these benefits by easily making the mounting hardware (hoses, discharge tubes, etc.) for a wide variety of vehicles at a very affordable price, without requiring the throttle body to be moved or modified as is the case with fixed displacement systems.
While the centrifugal supercharger is efficient, easy to install, adaptable, and cool (produces fairly cool air charge), there are several drawbacks that should be noted. First, because the compressor’s impeller is gear-driven and spins at very high rpms (40,000+), centrifugal superchargers tend to be noisier (more whistley) than their fixed displacement counterparts (Powerdyne’s Silent-Drive superchargers eliminate much of this noise by using an internal belt to drive the impeller rather than a gear-drive system). This whistley noise doesn’t bother most people – in fact, we find that most of our customers like to hear their superchargers in action. The biggest drawback of the centrifugal supercharger is it’s inability to make high levels of boost at low engine rpms. Typically, a centrifugal supercharger will make it’s maximum (quoted) boost at the engine’s redline rpm and nearly nothing at 1500-2000 engine rpm. Boost builds exponentially with engine rpm, meaning that boost comes on very quickly in the upper half of the powerband. While this normally isn’t a problem for lighter cars with manual transmissions, it poses a significant problem to heavier vehicles, towing vehicles, or vehicles with automatic transmissions. If you have a truck, towing vehicle, or an automatic transmission, you may want to consider a screw-type supercharger, which makes full boost as low as 1500 engine rpm.
Compressor surge is a problem that affects all superchargers and develops when the supercharger is creating boost, but the throttle shaft is closed. This condition can occur under deceleration or while shifting between gears, and can cause the car to sputter and chirp. Under surge, the compressor forces air into the closed throttle body until the pressure inside the throttle body is higher than the amount of pressure being created by the supercharger, and the air pops backward through the supercharger. At that point, pressure is released inside the throttle body and the compressor forces air back through the scroll and into the throttle body, which again has nowhere to go, and the process repeats. While surge normally is not highly damaging to the engine it is certainly annoying and can cause damage with time. To eliminate these problems under surge conditions, a bypass valve (sometimes called an anti-surge valve) is used to release the excess pressure. The bypass valve is actuated using intake manifold vaccuum, which opens the vent valve and releases pressure in the air-intake. Air is either released into the atmosphere (blowoff valve) or recirculated back through the supercharger compressor (bypass valve).
SuperchargersOnline.com carries centrifugal superchargers from the following fine manufacturers: ProCharger (ATI), Paxton, Powerdyne, and Vortech. If you you think a screw-type supercharger would better suit your needs, try Whipple Industries, Kenne Bell, or Jackson.