'We like the idea of a big centrifugal under the hood, especially if we can blow through a simple carburetor and avoid the tariff of EFI. And centrifugals deliver power on demand. Just think of a centrifugal as a beltdriven turbocharger without all that plumbing. It offers similar efficiency numbers compared to a turbocharger when it comes to compressing air without adding excessive heat. Packaging it is far easier than a turbo, too. The blower sits low off to the left or right of the engine, driven by a belt, and it can squeeze under almost any hood line. Why let the whole world know the deal? Leave that to the show-car guys.
Beltdriven centrifugals offer plenty of opportunity to make serious power. Our guide offers more than just cursory beltdriven recommendations. We took the plunge and stuffed an ATI PowerCharger on a 540ci Rat to make stupid power just to show you how's its done. Its the kind of engine that sets up the 6 o'clock news reporter to ask, "How much horsepower does this engine make?" Then he has to ask you again because he thought you said 900 hp. That's when you'll know you've arrived.
How Big?Just as with carburetors and cams, blower sizing is one of the more important questions you need to address when buying any kind of supercharger. Blowers move air, but how efficiently they move and compress it depends on proper sizing.As an example, ATI (ProCharger) offers 16 different centrifugal supercharger sizes, from the P600B that will move up to 1,200 cfm, to the monster F-4 that will push a massive 4,300 cfm-and according to ATI, up to 2,300 hp.
But not every engine needs a mongo blower. When searching for the right blower, the basic application starts with your engine's normally aspirated horsepower level. Smaller engines perform best with smaller blowers that can move air quickly, while larger-displacement engines need larger blowers to move more air. A big unit like an F-4 on a mild street 302ci small-block would still make power, but a smaller blower would actually perform better and make more torque.
Base engines that make 400 hp normally aspirated will demand a smaller size blower, something more like ATI's small P-1SC rated at a max airflow of 1,200 cfm and a max of 30 psi, or perhaps a D-1 that can help make as much as 925 hp. You can expect a blow-through centrifugal to add between 35 and 65 percent power to an engine. So if your engine is in good shape and makes 400hp, adding a D-1 for example could add another 200 to 250 hp to make 600 to 650 hp depending upon how well your cylinder head/cam/exhaust systems work. We've also seen centrifugals add 100 percent power, but that was with excellent heads, cam, and exhaust components where the engine was designed to be used with a blower.
Because a centrifugal is basically a beltdriven turbo, very high impeller speeds are a requirement. Centrifugals spin between 50,000 and 70,000 rpm, generally 8 to 10 times faster than engine speed. This is accomplished by combining the step-up ratio between the crank and blower pulleys and also from an internal ratio increase. Internal step-up ratios can vary even within the same blower size from 4:1 to 5:1. Higher impeller speeds do not necessarily equate to more boost. This is all tied in to the size and design of the supercharger. As you might have guessed, centrifugals do not make "instant" boost compared to a Roots but that may not be a detriment. Consider that boost at lower engine speeds can easily create more torque than your chassis can handle, so "instant" boost may not always be necessary.