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Tech and Tips

Huffing and Puffing – A quick look at the enduring legacy of the GMC ‘Jimmy’ blower

It was 1976 when Les Sustak hopped in his ’69 Camaro, picked up a buddy, and drove more than 300 miles from his home in Ontario, Canada, to suburban Chicago for one reason: Buy a blower.

“It was a kit from Gary Dyer’s machine shop,” says Sustak. “Mail order then wasn’t quite what it is today, and I was unsure about getting the blower and all the parts to Canada, so we road-tripped to Chicago for it.”

After installing the classic 6-71 huffer and sorting out the requisite tuning with it, Sustak made the rounds at the popular meets of the day, which eventually included the 1979 Street Machine Nationals. It was there that the Camaro caught the attention of a Hot Rod photographer and earned a place on the magazine’s cover. It would be the first of several magazine feature stories on the force-fed Z-28.“Back then, there weren’t too many cars with blowers,” says Sustak. “They were different and that’s what attracted me, because there was nothing like them to attract attention.”

Very true. A 6-71 blower bursting through a car’s hood is a visceral experience, from its visual presence to the shove in the back that only a few pounds of boost can provide. And a blower’s characteristic whine is a siren song for every red-blooded enthusiast, as nothing swivels heads faster at a car show than the sound of a Roots supercharger.

Nearly 80 years after pioneering lakes racers adapted the “Jimmy” blower for high-performance power building, it remains one of the most enduring elements of hot rodding and a cornerstone of an industry forged in ingenuity.

Supercharging, of course, existed before hot rods and street machines. The concept of boosting the output of a combustion engine by force-feeding it more air than it would naturally inhale dated back to the 1800s, and in the 1910s superchargers were experimented with on airplane engines. By the early 1920s, car builders also started experimenting with them, with Mercedes and Bentley largely leading the way.

In the simplest terms, a supercharger is an air pump, and a great many variants have been developed over time, but it was the two-rotor, positive-displacement design developed by Indiana brothers Philander and Francis Roots that took hold in the early days. Rather than boosting a combustion engine, however, the tinkerers from the Hoosier State originally intended the device to pump water.

As a positive-displacement pump, the Roots blower traps and compresses air, pumping its approximate displacement – the volume of air it moves – with each rotation. The faster it spins, the more air it moves. Crucially, there is no internal pressurization within the Roots compressor, so the supercharging effect comes from simply pumping more air than the underlying combustion engine can. As a result, the air pressure within the intake manifold rises and – presto! – you’ve got boost.

That’s the distinguishing difference between positive-displacement superchargers and dynamic superchargers, which use centrifugal or axial flow to increase air pressure. In fact, without internal pressurization, some lab coat types will argue the Roots design is not technically a supercharger. The term blower is more accurate, but perception is reality: It’s a supercharger. And vice versa.

The GMC application of the Roots blower came the late-1930s, when it was hung on the side of GM Diesel’s new Series 71 range of inline, two-stroke engines – with the “71” denoting the 71-cubic-inch displacement of each cylinder. That made a four-cylinder version a 4-71 engine, a six-cylinder diesel was the 6-71 and so on. The engines were produced from 1938-95 and in that time, GM Diesel and its spinoff Detroit Diesel produced everything from 1-71 to 24-71 iterations.

Lakes and drag racing were GMC blowers’ first hot rod applications, thanks to a plentiful postwar supply that made them very affordable.

A million and one uses were found for the durable yet relatively compact power plants, from heavy-duty trucks and buses to military vehicles, marine vessels, and countless static industrial applications. And while the blowers for were sized appropriately for the engines’ various sizes, they were never used to enhance power. Instead, GMC blowers were scavenge pumps used to draw fresh air into the engines and expel exhaust gases.

After World War II there was a seemingly endless supply of surplus military equipment offered for pennies on the dollar, including GM Diesel Series 71 engines and their parts. So, in 1948 an innovative early hot rodder and dry lakes racer named Barney Navarro picked up a surplus GMC 3-71 blower for next to nothing and plopped it on a Ford Flathead. It worked liked gangbusters, and fellow racers quickly took his lead. The rest is history.

The burgeoning and ever-inventive aftermarket industry developed the supporting hardware for adapting the Jimmy blower to gasoline engines, as it was not belt-driven on the Series 71 engine. Street-based applications were relatively uncommon, but a growing number of examples could be found as hot rodding grew. This 4-71 setup on the small-block Chevy in Scott Williams’ ’32 Ford roadster is a good example of what you might have seen in the early-’60s.

For the next 25 years or so the GMC blower was used pretty exclusively in dry lakes racing and drag racing. There were exceptions, but with little in the way of aftermarket of bolt-on manifolds or drive systems, street applications were pretty uncommon and required a fair amount of fabrication. That changed in the 1970s, when a proliferation of street kits from the likes of Weiand, Hampton, Dyer’s, and others plugged into the burgeoning street machine movement. Soon enough 6-71 blowers were bursting through hoods from sea to shining sea.

Like fashion and other cultural cornerstones, trends in hot rodding have waxed and waned in the 50 years or so since street machines and blowers were the Zeitgeist — but while multicolor graphics, yellow slapper bars, and chrome side pipes had their day like fondue parties, satin jackets, and 8-tracks, the GMC blower remains a touchstone of hot rodding. Even with more modern and efficient ways of building horsepower, many still choose the iconic 6-71 blower because there’s simply nothing like the visual or aural statement it makes.

By the 1970s, GMC-style blowers were becoming more prevalent on hot rods and street machines as several manufacturers were turning out intake manifolds and kits for popular V8s. This ’68 Olds 442 has a Hampton intake manifold for its 6-71 blower.

Les Sustak agrees. He still has his Camaro street machine and it looks just like it did on the cover of Hot Rod nearly 50 years ago – including the original Dyer’s 6-71 blower. It’s still shoving pressurized air into the small block engine and still turning heads with its spine-tingling whine.

“It’s still very streetable and always attracts attention,” he says. “Everybody still loves a 6-71 blower.”

Exactly.

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