Centrifugal & Roots Blower - Blown Away

Superchargers: The Power Of Forced Air Induction

Mike Petralia Nov 1, 2001 0 Comment(s)
Sucp_0111_02_z Centrifugal_roots_blower Blower 1/11

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.

Sucp_0111_06_z Centrifugal_roots_blower Pro_street_blower 2/11

The Weiand 177-cid Pro Street blower we tested is fashioned after the original GMC 53-series diesel blowers. It uses two-lobe, straight-cut rotors and is durable enough for everyday street use.

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.

Sucp_0111_10_z Centrifugal_roots_blower Superchargers 3/11

Regarded as "mini-blowers," the popularity of small, under-the-hood superchargers has grown immensely in today's power-hungry world. We tested this particular Weiand bower, which displaces 177 ci of air per revolution of its rotors. We found the best power with it spinning 115 percent faster (2.15:1 overdrive) than the engine, which is very common in small 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.

Sucp_0111_11_z Centrifugal_roots_blower Centrifugals 4/11

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 that's 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 600 percent overdrive. But they're designed to live and work up there.

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.

Sucp_0111_12_z Centrifugal_roots_blower Cutaway 5/11

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

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.

Sucp_0111_16_z Centrifugal_roots_blower Sealed_enclosure 6/11

The carburetor sits inside a sealed enclosure when used with a centrifugal blower. We witnessed tests of the new street blower from Vortech on a 383 built for cruising.

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.

Sucp_0111_19_z Centrifugal_roots_blower Additional_fuel 7/11

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 best with any high-power blower system, and both Holley and Demon carbs are making out-of-the-box carbs for these kinds of engines.

The only other part we changed was to go to a bigger, 950-cfm Holley HP blower carb. This was definitely a good move because with the blower pulling tons more air, the engine now wanted more than 970 cfm according to the Superflow dyno printout! Sitting on top of blowers is one place where a bigger carb is definitely better. Since we were still running strictly pump gas, we backed total ignition advance down from 36 to 28 degrees because just like with nitrous, too much timing can kill a supercharged engine.

Mini-Rat Supercharged

3000 563 322
3500 575 383
4000 600 457
4500 608 521
5000 604 575
5500 588 615
6000 559 638
6500 529 655
7000 469 625
PEAK HP 656 @ 6700
PEAK TQ 610 @ 4400

As we also learned with our nitrous test this month, there's a limit to all that's good making horsepower on pump gas. We pushed past that limit and the Mean Little Rat suffered the consequences. We think the damage was caused by a combination of too much combustion chamber heat and our using the wrong pistons for the application. When running power-adders on big-block Chevys, it's pretty common for small-bore, 396-402-cid pistons like these to fail at the weak spot next to the intake valve relief above the top ring.

Although our pistons were forged, they were not the low-silicon 2618-aluminum alloy forgings recommended for blower use. Regardless, there are now holes in the tops of pistons No.4,6, and 8, and we don't think any amount of race gas or reduction of timing could have totally eliminated the inevitable destruction like this. Bottom line, if you're going to run hard with a supercharged, pump gas motor, lower its compression to 9:1 or less with low-silicon, 2618-alloy forged race pistons.

Centrifugal Spins Up Some Power
This is one of the first centrifugal blower tests we've witnessed that was not designed to make max power for racing. Instead, Vortech's engineers put together a street blower package for use on carbureted engines. The engine test we witnessed was a 383-cid small-block belonging to our June '01 cover-car's owner, Doug Kruse. It will power Doug's awesome '57, and he didn't want to go EFI because everyone else in his neck of the woods already had. Unfortunately, at print time we didn't have all the specs on Doug's powerplant, but suffice it to say that his motor was properly built with the blower in mind.

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