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Blown Away

The Power Of Superchargers

Mike Petralia Sep 8, 2005

Horsepower is truly an aphrodisiac. To some, building an engine that can sustain the earth's rotation on the end of its crankshaft is better than almost any form of physical gratification. To others, it's just about making big power. Either way, supercharged power is the best way to get the job done. And there's no better way to supercharge your engine then by bolting a great big buffer on top of it.

The term "blower" is a misnomer when spoken in "benchracese". Although the big 6-71 aluminum huffers (referred to as "Roots-style" blowers) you're so accustomed to see poking through hoods were long-ago designed for use as industrial air movers, or "blowers" in a literal sense, when bolted onto an engine, its purpose changes. 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 at its intended purpose of driving a water wheel. But they later found it to be very good at pumping large volumes of air at relatively low speeds, so they put it to use as a blast furnace blower in a local foundry. Since then, the Roots air blower has been used in a wide variety of industrial applications, as well as being fitted to internal combustion engines in the early 20th century. The first recorded example of supercharging an automobile in the United States, 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 giant 1,140-cid Great Chadwick Six, producing a certified top speed of over 100 mph. Root's blowers started showing up on the streets around the 1930s after the Spalding brother's bolted a Mercedes-Benz blower onto their flat-top Ford.

Fast-forward more than 70 years, and the original Roots design is still widely in use today, although with several adaptations made to better suit 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, many Top Alcohol teams have switched to the more efficient--and quite a bit more expensive--screw-type compressors that go from 0 to obscene PSI boost in an extremely short period of time.

As mentioned earlier, the term Roots simply comes from the name of the guys who basically invented it, where as 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 relatively slow speed. They were originally designed for use to boost the power output on diesel trucks. Professional and amateur racers, the automotive OEMs, and the performance aftermarket industry 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 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, such as the 2-71, 3-71, and 4-71, as well as versions for V-series diesels like the 8V, 12V, and 16V-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 its own larger 10-71-, 12-71-, and 14-71-size blowers. There were also some smaller, lesser-known diesel engine blower combinations like the inline-53 series that the OEM's and aftermarket have wisely adapted as the under-the-hood-style superchargers still widely in use on lots of V-6s today.

The centrifugal blower also has its "roots" as an industrial air mover, although its history has not been as nearly well-documented. Centrifugal blowers have been in use on high-performance automobiles since the 1920s, and actually found their way onto quite a few OEM dealers' lots. Factory-built Duisenbergs, Studebakers, Packards, Fords, and others have all worn an ancestral descendent of the centrifugal 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.

Both blower designs have merit and both have their 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 289-cid small-block, you'd have trouble slowing it down enough to make livable 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 engine, 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 through a pair of carbs, or maybe just one big one, and pressurizing the fuel/air mix in the intake manifold. 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 fair 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 EFI work on Roots blowers and make carburetors work well with centrifugals so we can all live happily ever after. Today, you can bolt either blower onto your 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 every time you run down the track. Also, unlike turbos, which are a similar, but still different enough form of supercharging to warrant their own separate story, there's very little exhaust system modifications needed when bolting a blower on. We think you'll agree that supercharging, in any form, is the key to ultimate power.


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Regarded as "mini-blowers," the popularity of small, under-the-hood superchargers has grown immensely in today's power-hungry world. This particular Weiand blower displaces 177 ci of air per revolution of its rotors. It tends to make best power, spinning 215-230 percent faster than the engine (2.15:1 to 2.30:1 overdrive), which is very common in small blowers.

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One of the best aspects of "mini-blowers" is that they can be stuffed under many hoods for concealed power. They can also be made to work well with EFI, like this MagnaCharger under the hood of an '04 Chevy Trailblazer.

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Magnuson Products has incorporated this mini intercooler inside its intake manifold for GM's LS-series engines. It adds a bunch of power, but it also requires a separate reservoir with it's own external heat exchanger to be plumbed into the system to keep a constant supply of cold water available.

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Another popular option for Roots-style blowers on the street is hard-anodizing the rotors for added wear protection. The anodized coatings can also limit heat buildup inside the case to keep boosted temps down. Shown here is Magnuson's EFI MagnaCharger.

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Although they've been used on cars for decades, centrifugal blowers have really come into the spotlight just in the past 5-10 years-- perhaps due to their inherent usefulness on an EFI engine. Centrifugals must spin at incredibly high rpm to make boost. It's not uncommon to see one with a final-drive ratio of more than 500 percent overdrive, but they're designed to live and work at that speed.

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This cutaway shows how a centrifugal blower gets its oil. Unlike Roots blowers, which typically carry their own oil, most centrifugals use pressurized engine oil to lube and cool their gears and bearings. A pressurized oil line plumbed from the engine sprays oil through the brass fitting (arrow) directly at the gear junction.

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Whatever type of blower you're running, the proper amount of additional fuel must be added at the right time or you'll suffer total destruction. Carburetors must be specially prepped to work with any high-power blower system, and both Holley and Demon are making out-of-the-box carbs for these kinds of engines.

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Ignition advance, or more precisely, too much advance, can be deadly to a blown engine. While there's no steadfast rule as to how much timing to pull out with a blown engine, you should always start on the conservative side.

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Spark plugs and your own ears are the best indicators of your blown engine's health. When advancing the timing or increasing the boost for more power, listen carefully and check the plugs often for little black specs on otherwise clean porcelain that will indicate dangerous detonation.

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Another way to keep an eye on detonation is by installing a knock sensor. All the new cars with EFI have them and a few are available in the aftermarket. An automatic boost retard box like those from MSD also works well to keep your blown engine from pinging to death.

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Both centrifugal and Roots-style blowers make boost dependant on how fast they're spinning. You can change the drive speed simply by changing pulleys. Drive ratios are calculated by dividing crank pulley diameter by blower pulley diameter. Ex: A 7-inch-diameter crank pulley with a 3.5-inch blower pulley equals a 2:1 ratio, or 200 percent overdrive.


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