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.
Article courtesy of Mac's Motor City Garage.