6 71 blower dimensions

Most gearheads will instantly recognize the familiar GMC 6-71 blower, but its original application and backstory remain relatively unknown. Let’s explore. 

The GMC blower of history and legend is, of course, a type of pump known as a Roots blower. Two brothers, Philander and Francis Roots of Connersville, Indiana (no relation to Rootes of Great Britain; note the spelling) initially devised their machine in the 1850s to pump water, but it has countless applications for moving fluids and gasses, from underground mines to blast furnaces. In common use, a Roots blower can be as small as a matchbox or as big as a house.

One interesting aspect of the Roots blower is that its internal flow is the opposite of what we may imagine: around the outside of the rotors or impellers (above right). In automotive applications, a Roots blower typically has two, three, or four lobes per rotor (the GMC uses three in its original form). The Roots is a positive-displacement pump. That is, with each rotation it will pump its approximate displacement. When pumping air, it’s one atmosphere in and one atmosphere out with each turn of the rotors. There is no net internal pressurization in the blower itself.

The concept of supercharging is essentially as old as the automobile. Obviously, if we can pump more air through an engine at a given speed, we can burn more fuel and make more power. Numerous types of pumps are suitable for the job, including the Roots blower, and Mercedes was the first to offer a Roots blower on a volume production vehicle with its Kompressor models in 1921. But there were many others to follow, including Bugatti, Bentley, and Maserati.

A small but noteworthy point: Since the Roots is a positive-displacement device without internal pressure, supercharging is achieved by using the blower to pump more air than the engine can, thereby raising the air pressure in the intake manifold above atmospheric. For this reason, some insist that the Roots blower, unlike most other types, is technically not a supercharger—even though supercharging is the ultimate result. If we call the machine a Roots blower, everyone can be happy.

Above is the GMC 6-71 blower in its original habitat: mounted on the side of a GMC Detroit Diesel 6-71 engine. Introduced in 1938 and produced well into the 1990s, the 6-71 is a two-stroke, six-cylinder diesel. In GMC diesel nomenclature, 6 represents the number of cylinders, while 71 represents the cubic-inch displacement per cylinder. So the displacement here is 426 cubic inches, and that is the approximate displacement of the blower as well. The 71-series has been produced in versions of one to 24 cylinders, and each one has a blower (or blowers) of appropriate size. Here the blower does not serve as a supercharger but simply as an air pump. Since the 71 series is a two-stroke, the blower is used to pull in fresh air and push out the spent exhaust gas.

As we saw earlier, Roots blowers were originally found only on the most exotic and expensive cars—beyond the reach of the backyard mechanic. But that changed in 1948 when pioneer hot rodder Barney Navarro mounted a war surplus GMC 3-71 blower on the flathead V8 roadster he raced on the California dry lakes. Others followed, and now  thanks to General Motors, hot rodders had an affordable and plentiful supply of Roots blowers in a number of sizes, including 3-71, 4-71, and the 6-71, the latter being perfect for the new overhead-valve Detroit V8s. Regardless of size, all the GMC blowers have the same authoritative sound, somewhere between a growl and an angry whine.

Soon enough, the ever-inventive hot rod industry developed a number of adapters and drive systems, including gears, chains, multiple v-belts, and the most popular setup, the toothed Gilmer belt. Aftermarket cases, rotors, end plates with sealed bearings, and other parts also appeared, and complete turn-key kits as well. (Above, Weiand kit at left and Dyers kit at right.) There were also front-mount kits from Potvin, Cragar, and others  (see below) that echo the original Blower Bentley setup, though the conventional top-mount system with Gilmer belt proved to be more practical.

In ’70s drag racing, the 6-71 size gave way to 8-71 and larger blower displacements and today, NHRA racers in Top Fuel and Funny Car use blowers of extrapolated 14-71 dimensions as defined by the current rules. On a 6-71, the impellers are not quite 15 inches long while the 14-71’s are a full 19 inches in length. But the design itself is based on the original GMC two-stroke blower.

To tell the truth, these days the GMC 6-71 blower is increasingly obsolete as a performance booster. There are newer and better alternatives including the turbocharger and the Lysholm twin-screw supercharger (which resembles a Roots blower but isn’t). Still, hot rodders continue to embrace the venerable 6-71. For looks and sound, it’s difficult to top.

Related

The GMC 6-71 diesel engine with corresponding 6-71 blower was frequently used as a truck engine, marine engine, or generator power source or a pump power source. These units are 7.0 Liters, 426 cubic inches, and rated at 190 HP. The case and rotor length for the 6-71 blower is 15.0 inches. The 6-71 was/is adapted for use on hot rod engines, Bonneville type cars, race cars, etc. When these units were no longer adequate for air flow, race car blower companies started making longer blower cases and rotors with their own castings, maintaining the nomenclature such as 10-71, 12-71, 14-71, and even 16-71.

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I had an interesting email exchange with a very knowledgable and astute racer that I thought I would share. I’m happy to see he noticed the differences:

I noticed an error in the “Superchargers for IHRA” article…

“One way of looking at where all of that power came from is to look at the displacement of the blower. The 6-71 displaces a little over 400 ci per revolution (6 x 71 ci).”

…This is not correct. As mentioned in the article, the name of the supercharger comes from the diesel engine it was designed for, I.E., 6-71 = 6 cylinders, each cylinder displacing 71 cubic inches. The displacement per revolution of the supercharger can NOT be determined by following the above, as that merely leads to the displacement of the diesel engine it was originally designed for. Weiand superchargers(who produce GMC style roots superchargers obviously) says that their 6-71 moves 411 cubic inches of air per revolution, which is actually pretty close to the 426cu in that you would get doing the above math, but they say their 8-71 moves 436cu in, which is a long way from the 568cu in you would get by doing the above math.

Figured I would let you guys know, just trying to thwart off some potential disinformation(obviously unintentional).

In response, you are correct in your analysis. In racing though, things are not always as they appear if you try to extrapolate.

The 6-71 designation of the blower was derived from a 6 x 71 diesel as the article says. The blower ran at one to one engine speed. It ran at atmospheric pressure. It was not used for boost. The intake valves on the engine would open and the blower would move the air into the cylinder. It was a two cycle engine, so it displaced its size every revolution. The blower was matched to the engine size. So it displaced 6 x 71 cubic inches every revolution.

There was a 4 x 71 as well. I assume it was from a 4 x 71 diesel. And I believe the 4-71 blower size conformed the same as the 6-71.

Then the 8-71 appeared in racing. The rotors are 1 inch longer than the 6-71. I do not know if there was an 8 x 71 GMC diesel. The displacement of the 8-71 racing blower is around 450 ci. The article says that designation does not conform to the original 6-71 derivation. The article says that the rotors are simply 1 inch longer for the longer designations. Our FI Racing Secrets book provides more info on actual blower sizes for larger blowers with and without rotor seals. Those sizes were determined from the blower manufacturers for baseline blowers.

There is a variability in the actual amount of air displaced by blowers from different manufacturers and different designs. We received requests from many racers to do a book on blowers. In my initial research, I found out that the actual air pumped by the Roots blowers is dependent on many factors. Leakage is one big one. Stiffness of the parts change the leakage. Number of runs on a blower increase the leakage. Outlet opening design changes reversion in the blower and that effects leakage. The amount of manifold boost changes the leakage.

The big factor now explored by the manufacturers and racers as well is the design of the inlet. That is a big influence on the amount of air the blower displaces. One manufacturer told me they found the inlet design to be all trial an error for specific engine combinations.

That later info is all beyond the intent of the article. I think what we say in the article is all true though. You just cannot extrapolate as you indicated.

As of now, we have no further plans yet for doing a blower book.

The numerical designation for the original GM Roots type blowers were based on the engine displacement the blower was designed for.

Example;

6-71 blowers were designed for:
6 (cylinders) 71 (CID per cylinder)
This, however, does not signify the actual displacement per revolution of the supercharger. The actual displacement per revolution is determined by the rotor length and diameter.


The data for GM based Roots blowers are:-

6:71 small diameter
Rotor dia=5.505", length=14.975", displacement per full turn of rotor=339CI.

6:71 big diameter
Rotor dia=5.778", length=14.975", displacement per full turn of rotor=411CI.

8:71
Rotor dia=5.778", length=15.905", displacement per full turn of rotor=436CI.

10:71
Rotor dia=5.778", length=17.000", displacement per full turn of rotor=466CI.

14:71
Rotor dia=5.778", length=19.000", displacement per full turn of rotor=521CI

This is theoretical displacement.

Retro or high helix will change this considerably.

A worn blower will reduce this a little.

A very good tight new blower and a Teflon stripped blower should be about the same.

Bob