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Centrifugal & Roots Blower - Blown Away

Superchargers: The Power Of Forced Air Induction

By Mike Petralia

Horsepower is an aphrodisiac. To some people, building an engine that could sustain the earth's rotation with its crankshaft is better than almost any form of physical gratification. To others, it's just about making the big power. Either way, supercharged power is the best way to get the job done. And there's no better way to supercharge an engine then by bolting a blower on top of it. We researched street supercharger kits and learned quite a bit from a few dyno tests, as well, so why don't you relax a bit while we explain the power they can give you.

What Is A Blower?
The term "blower" is a misnomer when spoken in bench racing language. Although the big 6-71 aluminum huffers (referred to as "Roots-style" blowers in the aftermarket), we see poking through the hoods of hot rods were once used as air movers, or "blowers" in a literal sense, when bolted onto an engine, its purpose changes slightly. A little history on blowers might help. According to Pat Ganahl, author of the book Street Supercharging, the history of the roots blower can be traced back more than 125 years.

First patented by the Roots brothers in 1860, their original bi-rotor gear pump was not very successful driving a water wheel at their woolen mill. But they later found it to be very good at pumping large volumes of air at relatively low speeds and put it to use as a blast furnace blower in a local foundry. Since then, the Roots design has been used in a wide variety of industrial applications, as well as being fitted to internal combustion engines since the early 20th century. The first recorded example of supercharging an automobile in the U.S. and the first supercharged car to win a race were both accomplished by Lee Chadwick of Pottstown, Pennsylvania.

In 1907, Chadwick and engineer John Nichols mounted a centrifugal blower to their 1,140-cid Great Chadwick Six, producing a certified top speed over 100 mph. The first noted example of a Roots blower being fitted to a hot rod was in the '30s when the Spalding brothers bolted a Mercedes Benz blower on their flattop Ford.

The original Roots design is still in use today, although with several adaptations which have made for more efficient use in an automobile. The Roots-style blower once ruled the dragstrip, too, with every Top Fueler and Funny Car running a big aluminum or magnesium 14-71 on its Hemi. Now, most teams have switched to the more efficient, and more expensive, screw-type compressors that go from 0 to lots of boost in an extremely short period of time. But to keep the purpose of this story directed at what's common on the street and local tracks today, we'll stick to the Roots and centrifugal-style blowers.

Roots Explained
As mentioned earlier, the term Roots comes from the name of the guys who basically invented it-whereas centrifugal blowers are named so because of how they operate. Most of the big Roots blowers you see today are direct descendents of a GMC Detroit Diesel blower. These blowers use twin two- or three-lobe rotors, spinning opposite each other inside a precisely machined case to move a large quantity of air at a slow speed. They were originally designed for use on diesel trucks to boost the power output of their slow-spinning engines.

Professional and amateur racers, the automotive OEMs, and the performance aftermarket have all adapted Roots/GMC blowers to work on both new cars and custom-built hot rods. In general Roots blowers are categorized by the size of the original diesel engine it was fitted to. The most common, the 6-71 blower, was first used on an inline-six-cylinder GMC diesel with each of its bores displacing 71 ci, (426 cid total). And the 8-71 GMC blower was fitted on inline-eight-cylinder diesels, also with bore sizes of 71 ci.

There were also smaller and larger diesel versions like the 2-71, 3-71, and 4-71, as well as versions for V-series diesels like the 8-V, 12-V, and 16-V engines, but the diesel blowers bigger than the inline 8-71 really didn't translate successfully to automobile use. The drag racing aftermarket, using the same Roots technology, invented the bigger 10-71, 12-71, and 14-71 size blowers. There were also some smaller, less-known diesel engine blower combinations like the inline 53-series that the OEMs and aftermarket have wisely adapted as the under-the-hood-style superchargers in use today.

Centrifugal Blowers
The centrifugal blower also has "roots" as an industrial air mover, although its history has not been so nearly well documented. Centrifugal blowers have been in use on high-performance automobiles since the '20s and actually found their way onto quite a few OEM dealers' lots. Factory-built Duesenbergs, Studebakers, Packards, Fords, and others have all worn an ancestral descendent of the Paxton supercharger we know today. The blower was offered as an OEM option to the buying public. Unlike its slow-spinning Roots cousin, centrifugal blowers move air by turning a small wheel, called the Impeller, inside its housing at very high speeds.

By Mike Petralia
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