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You’ve always seen superchargers on modified cars ripping up the tar at the drag strip. You’re thinking of fitting one yourself but then you discover that there’s quite a few different superchargers on the market. Which one to choose? Let me help you decide.

When it comes to superchargers, there are two different types of superchargers: positive-displacement superchargers, such as the ones fitted to hot rods and dragsters, and the newer centrifugal superchargers. Positive-displacement superchargers such the Roots supercharger have two or more counter rotating lobes or screws encased in an aluminum housing while centrifugal superchargers are more like turbochargers in design. To complicate matters even more, positive-displacement superchargers can be divided into Roots-type and screw-type superchargers. You also get vane-type superchargers but those are not used in automobile applications.

Positive-displacement superchargers have a few things in common. First, they pump air into the intake manifold at a fixed rate in relation to engine revs and supercharger size. Therefore, they have no over boost problems, but if you want more boost, you need a bigger supercharger. Second, the throttle body must be placed ahead of the supercharger as the supercharger will continue building pressure while on idle and, with the throttle plate closed, the pressure will build up between the supercharger and the throttle body. You must also fit a bypass valve to balance the pressure on either side of the throttle plate. Unfortunately, moving the throttle body further away from the cylinder head will reduce throttle response. Third, the supercharger must be placed above the engine making installation a bit cumbersome. And forth, they tend to add a lot of heat to the intake air but this can be reduced by using an intercooler.

Centrifugal superchargers are much smaller and have much more in common with a turbocharger that even their housings are similar to that of a turbocharger. Centrifugal superchargers use an impeller wheels in stead of lobes or screws to pump the air. They also uses step-up gears to spin the impeller wheel much faster than the rotors or screws in positive-displacement superchargers. But this means that they must ’spool’ up before they create boost, and they don’t create boost at low revs, such as at idle. Therefore the throttle body don’t need to be relocated. Also, because centrifugal superchargers use step-up gearing, their displacement is not proportional to their size and a smaller supercharger can be used to create greater boost. Because they are much smaller, centrifugal superchargers can be mounted at the front of the engine rather than on top of it. However, this supercharger is extremely noisy.

So if you are going to use an intercooler, and you want low rev boost at the expense of throttle response, I’d go for a positive-displacement supercharger. A twin-screw supercharger is more efficient than a Roots supercharger but is more expensive. On the other hand, if you want high boost at high revs, and don’t mind the noise, a centrifugal supercharger is the way to go.