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Centrifugal & Roots Blower - Blown Away

Superchargers: The Power Of Forced Air Induction

By Mike Petralia

The Good And Bad
Both blower designs have merit and both have faults. Choosing the right blower is really a matter of defining your needs and realizing that neither is a cure-all power maker. The biggest difference in each blower design is the speed at which it operates. Of course, the speed it spins and the boost any blower makes is directly proportional to its size and the size of the engine it's mounted on. If you put an 8-71 on top of a 283-cid small-block, you'd have trouble slowing it down enough to make viable boost on the street.

Conversely, if you bolted that same 8-71 on top of a 632-cid Rat, it might have trouble making enough boost to satisfy you. Centrifugal blowers suffer in exactly the same fashion, but there are a wider variety of centrifugal blower sizes out there than there are Roots. You can probably choose the best blower size by contacting the blower manufacturers directly and asking specific questions. Don't overestimate your needs or the power-handling capabilities of your car, and you should be fine.

Both blower styles add power to your engine by forcing more air into it than normal atmospheric pressure would. Since the engine can only ingest so much air at any time, the extra air a blower pumps in will stack up in the inlet tract and can be measured as "boost." And just like how nitrous oxide injects extra fuel with the extra oxygen it sends into the engine, so must a blown engine get more fuel to burn with the extra air it pumps in. This is where Roots and centrifugal blowers have differed in the past. Roots blowers were best used on carbureted engines, pulling air and fuel together through a pair of carbs, or maybe just one big one, and pressurizing the fuel/air mix in the intake manifold-whereas centrifugal blowers have typically found their way onto EFI cars compressing only the air and mixing it with extra fuel downstream in the inlet tract.

Both systems work well, although centrifugals fare better in today's EFI world and are much easier to fit under a hood.

The lines separating Roots and centrifugals have blurred somewhat in the past couple of years with aftermarket companies working hard to make carburetors and centrifugals coexist. Today you can bolt either blower onto an engine and make a little or a lot of extra horsepower for a modest investment. But with that modest investment comes a list of enhanced benefits that some feel put superchargers way beyond other power adders. For one thing, there's never a bottle to refill when running a blower, and your tune-up won't change the more times you run down the track.

Also, unlike turbos, which are a similar, but still different enough from of supercharging to warrant their own, separate story, there's very little exhaust system modification needed when bolting a blower on. Check out the power we got from bolting a little Roots-style Weiand Pro-Street 177 blower onto our Mean Little Rat and also the results of a Vortech Racing dyno-flog on a customer's 383-cid carbureted small-block. We think that you'll agree that supercharging, in any form, is the key to ultimate power.

The Root Of Power
("Mean Little Rat Part Iv")
For those of you who missed the first three tests of our 408-cid "Mean Little Rat," let us recap briefly. The purpose of building and testing this over-bored 396 big-block was to see how much power could be made using nothing but off-the-shelf parts in an otherwise stock engine. We freshened the stock short-block with Speed Pro L2287F-60 forged pistons and polished a set of truck connecting rods. We installed a used, GM forged crank and balanced the whole assembly.

The top end got out-of-the-box Brodix OEFI oval-port cylinder heads and a variety of Edelbrock intake manifolds and Comp Cams Xtreme Energy street roller cams. The Rat made from 500 to 580 hp, burning nothing but pump gas in three different series of tests (see: Feb., Apr., and June '01 issues), and it never complained once. Then, to get still more power out of the same pump-gas combo we bolted on a 177ci Weiand Pro Street supercharger. This little blower packs a punch and had no trouble putting us over the 650hp mark.

To learn just how much power the little blower could pump we left just about everything exactly how it was from our last test in the June '01 issue. That meant that the Mini Rat still had over 10:1 compression going into this test, but we knew that the rather generous timing of the Comp Cams XR280R camshaft would probably keep us out of detonation.

One part removed from the last test was the Moroso vacuum pump-not because the vacuum pump wouldn't work with a blower, or on the street. To the contrary, the vacuum pump would enhance both the power and durability on the street due to the better ring seal it would help create. No, we removed the vacuum pump because we didn't have time to adapt its drive mandrel to the Weiand blower's crank-mounted drive pulley. But we figure power would have been up by about 15 more ponies if we'd been able to mount the vacuum pump.

By Mike Petralia
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