When superchargers are talked about they are typically classified into two different groups - Centrifugal and Positive Displacement. For the most part, we all know what a Centrifugal Supercharger is, no matter if it is a turbo or a blower. Centrifugal compressors are easily identified by their snail shell shapped housing. The big question (for some) centers on what exactly is a Positive Displacement supercharger?

Without a doubt, a Roots style supercharger is Positive Displacement. These are the superchargers that we all came to know and love as we grew up. They have been, and still are, used by many forms of drag racing as well as a very common use on street rods - typically on Chevy motors. These have also gained popularity lately on factory built cars - again, mostly GM. All this having been said, these are perhaps the worst designs of all supercharging - especially considering today's technology. Perhaps their biggest downfall is their very low efficiency.

This is where the Whipple supercharger (screw compressor) comes in. The only question at this point: is the Whipple is a true Positive Displacement supercharger? I realize that it's easy to just jump on this and say that it is, but before you do, think about it for a second. The term Positive Displacement means that the air is trapped once it enters the blower and must exit the unit as it spins. That and the fact that the two sides of the supercharger are completely separated from each other. That is to say that if the blower isn't spinning, air will be trapped on either side. Now, does this still sound like the Whipple?

Keep in mind that even though the Whipple has a very quick rate to boost, it is not the same as a Roots style. A roots is always trying to make the same amount of boost, regardless of rpm. It compresses all of its air in the intake manifold, which is completely unlike the Whipple. Every rotation of a roots is designed to displace a certain amount of air. The Whipple actually acts more like a centrifugal in this respect. It does its compression in the supercharger itself. Also like a centrifugal, it must build compressor speed before it can make boost; although this is where the Whipple starts to separate itself from the Centrifugal. It begins to make full boost by about 2500 to 3000 rpm, depending on the unit and the application. It will then maintain this level of boost until the demands of the motor exceed the ability of the supercharger.

On the standard Mustang kit from Kenne Bell (Whipple), the car will usually reach full boost by about 2800 rpm which it holds until about 5200 to 5400 rpm. By comparison, a centrifugal supercharger (not a turbo) will start to make boost way down low (just like the Whipple), but won't reach full boost until redline for the motor. Essentially, the Whipple offers a relatively constant level of boost, where as the centrifugal is always building boost. I'm not trying to say which is better, just what each supercharger will offer.

Back to the question of Positive Displacement. As I said earlier, a roots style supercharger's inlet and outlet are completely separated from each other. This is definitely NOT true of a Whipple. While both types of supercharger will "free wheel" when the drive belt is removed, the Whipple's inlet and outlet are not isolated from each other.

All this has to makes you wonder if the term "Positive Displacement" is really fitting for the Whipple. Even though it acts a lot like a roots, its properties are more like a centrifugal. As to what it should be clasified as, perhaps it should have its own. To say that it is a Positive Displacement is actually an insult to what is truly a superior design. I suppose what you decide to call it is up to you, I just wanted to offer something to think about.