The centrifugal supercharger gets its name from the way it pumps air into the intake system. This type of supercharger has a belt driven impeller or compressor wheel that draws into the center of the supercharger and uses centrifugal force to force the air out radially and into a circular scroll that increases in diameter as it moves further away from the center of the supercharger. This slows the flow of the air while increasing the pressure of the moving air. This is quite similar to the way a turbocharger works, which means that this type of supercharger has more in common with the turbocharger than with positive-displacement superchargers. In fact the centrifugal supercharger's compressor wheel is quite similar to that of a turbocharger and some centrifugal supercharger manufacturers have even implemented compressor wheel technology that has been developed for the turbocharger! The circular scroll is also similar to that of the turbocharger.
A major difference between the centrifugal supercharger and the turbocharger is that the compressor wheel of the centrifugal supercharger is usually driven by a belt and step-up gears rather than by the exhaust gasses. The use of a step-up gear means that the centrifugal supercharger is not a positive-displacement supercharger. Instead its air-flow rate increase at the square of its shaft RPM, which is significant as the compressor wheel must spin at very high RPMs in order to produce significant amounts of boost. It also means that the boost pressure increases with RPM. Thus, the centrifugal supercharger will make high amounts of boost pressure in the upper RPM range.
The Pros and Cons of Centrifugal Superchargers
Unlike a positive-displacement supercharger, the centrifugal supercharger produces low boost pressures at low RPM by design. This is because high boost pressures at low RPM will result in excessively high boost pressures at high RPM because the air-flow rate from the centrifugal supercharger increases at the square of its shaft RPM! However, because it produces internal compression, the centrifugal supercharger has excellent thermal efficiency of 70-85%, making it the most thermally efficient type of supercharger. This improved thermal efficiency makes the centrifugal supercharger ideal for high boost applications. The other major advantages that the centrifugal supercharger has over the other types of superchargers are its compact size and its ability to free-wheel when boost pressure isn't required.
The compact size of the centrifugal supercharger makes it easier to install the supercharger in the engine bay. The centrifugal supercharger can also be mounted far away from the air intake, making it possible to incorporate an intercooler, and making it even more adaptable and easier to install. Its compact size also means that the centrifugal supercharger consumes less engine power, and because it requires less engine power, the centrifugal supercharger can use a relatively thin drive belt when compared with those of the positive-displacement superchargers.
In addition, the centrifugal supercharger is able to "free wheel", which means that it is not adversely affected when air flows backward through the supercharger, as may occur under quick deceleration or while changing gears. This means that the throttle body does not need to be moved or relocated to the front of the supercharger as is the case with positive-displacement superchargers. This means that stock throttle response will not be compromised. What happens when the throttle is closed at high RPM, such as when you change gears, is that the pressure between the supercharger and the closed throttle will build up until it exceeds the boost pressure being supplied by the supercharger. The air between the supercharger and the closed throttle will then flow back through the supercharger relieving the pressure. The reverse air flow back through the supercharger is called surge and doesn't cause any harm. However, it does contribute to the noise produced by the centrifugal supercharger. Though this noise caused by surging can be mitigated by installing a blowoff valve either at the supercharger outlet port or near the throttle plate. However, the main source of noise caused by a centrifugal supercharger is the result of the step-up gears that allow the compressor wheel to spin at speeds in excess of 40,000 RPM. Powerdyne counteracts this noise in their Silent-Drive centrifugal superchargers by using an internal belt to drive the compressor wheel rather than step-up gears.
But the biggest disadvantage of the centrifugal supercharger over its positive-displacement counterparts is the low boost pressures produced at low engine RPMs. As we mentioned earlier, this is because the centrifugal supercharger's air-flow rate increases at the square of its shaft RPM. The result is that a centrifugal supercharger will typically produce maximum boost at the engine's redline with hardly any boost pressure below 2,000 engine RPM. However, boost pressure does build quite quickly in the upper half of the engine's powerband.
The lack of low boost at low engine RPM means that the centrifugal supercharger would be suitable for quick revving, light cars with manual transmissions rather than heavier vehicles or vehicles with automatic transmissions. Thus, if you have a truck or a car with an automatic transmission, a positive-displacement supercharger, which makes full boost as low as 1,500 engine RPM, would be a better option.