GMC blower

Today’s Top Fuel dragsters cross the line at over 300mph in motorsport’s rawest display of power. But the mechanical catalyst of this blood-and-thunder action was originally designed for a meek and mild purpose, as Keith Howard explains

As automotive contrasts go, you might think there are few starker than a diesel truck engine and the fire-breathing V8 units in a Top Fuel dragster. One is designed to plod for mile after mile at a maximum 3000rpm or so; the other is a 10,000rpm, 5000bhp, nitromethane-guzzling behemoth whose ephemeral and all but explosive bursts of activity occupy a few seconds at most. Chalk and cheese? Tortoise and hare? Certainly, except one of the key ingredients of the classic drag engine, first adapted to the cause in the ’50s, was originally made for a dowdy diesel.

Well, not that dowdy, perhaps, because the Detroit Diesel rates as one of the best and diesel engines in American history. It saw service in the iconic Greyhound buses, in numerous commercial vehicles and in many mobile and static military applications. Unusually, it was a two-stroke, so to remove exhaust gases as a charge of fresh air was admitted to the cylinder required a scavenge pump. The device GMC built for the purpose was in effect a large, Roots-type, positive displacement supercharger although its job was to suck rather than blow.

What the drag racers did was turn that sucker around, literally. By bolting a GMC unit- affectionately known as a Jimmy -to the induction side of their engines and using it as a blower, they harnessed even more energy to fling themselves down the quarter-mile.

GMC manufactured its scavenge pumps in five different sizes, designated 3-71, 4-71, 6-71, V8-71 and V8-92. Pragmatically, the numbers indicated the type and capacity of engine for which each was intended. The 3-71 was for a three-pot, each cylinder of which had a capacity of 71 cubic inches (1163cc), the 4-71 for a four-cylinder equivalent, and so on.

For the drag racer these figures meant one thing: a GMC blower could move an awful lot of air. Much more than any other supercharger readily available to them, let alone at the bargain prices the GMC units commanded as war surplus items following WWII and the Korean conflict McCulloch, Latham and SCOT superchargers were also used in dragsters during the ’50s, but these were originally designed for street cars. So, even allowing for the American motorist’s traditional appetite for cubic inches, they were able to flow only a fraction of the air volume, and hence develop much less manifold pressure, than the larger Jimmies. Whereas with these smaller blowers boost pressures were typically 6-8psi, with a GMC unit 15-18psi was readily attainable. And if you needed more boost, well you could always trade up from a 3-71 to a 4-71, or a 4-71 to a 6-71.

If anything, the GMC blowers were on the large side for those early supercharged dragsters, whose engines would readily melt their sand-cast pistons if pushed too hard. So it was common to run the Jimmies at somewhat less than crank speed. And they were originally deployed in what seemed, with hindsight, rather odd ways. When Don Hampton, now the supercharging guru at Hampton Blowers in Downey, California, pioneered the use of a GMC blower in a drag car in 1953, he mounted it ahead of the engine (a 354 Chrysler Hemi) driven by a chain. The classic dragster format, with the blower mounted atop the engine, breathing through large, forward-facing throttle butterflies in a top-mounted cowl, arrived a little later.

With the blower in its natural position over the engine inlets, some tied to drive it with multiple V-belts but slippage was, predictably, a constant problem. Flexible toothed belts, borrowed from machine applications, soon became the standard fitment. Chevy, Olds, Caddy and Ford Flathead engines were all given the GMC treatment in those early years as well as the original Hemi, but when the larger 426 Hemi arrived in the early ’60s that swept all before it With piggyback GMC blower,it became the standard means of propelling yourself 440 yards in the span of a few heartbeats.

Drag racing is all about excess, so there’s an inevitability about even the 6-71 GMC unit eventually being a bottleneck in the search for yet more horsepower. GMC never made a 10-71 or 12-71, let alone a 14-71, but these all appeared on successive generations of dragsters as resourceful engineers like Hampton devised ways of making the blowers even bigger, and extended the GMC nomenclature as they did so. The National Hot Rod Association, the sport’s main governing body, called a halt with the 14-71 in the name of safety.

Initially these outsized units were made by using V8-71 rotors in an extended 6-71 casing, then even larger versions were made by joining additional sections to the ends of V8-71 rotors. The hollow aluminium rotor castings then flexed, though, making it difficult to hold the tight tolerances required for maximum boost pressure and minimum friction. So the cast rotors were replaced with solid aluminium items machined from billet. Hard anodising of rotors and case helped prevent aluminium galling on aluminium if and when the tolerances were lost; later, replaceable Teflon inserts to rotor tips and flanks helped improve sealing and reduce friction still further.

To save weight the original GMC cast aluminium casing was replaced with after market magnesium castings during the late ’60s, but mag proved too plastic at high temperatures, prompting an eventual return to aluminium. Some even machined casings from solid in the mid-90s, something Don Hampton scorns as a symptom of billet-mania. In truth, billet casings hold tolerance less well because in a good quality casting the metal’s grain structure is superior.

It’s a testament to the GMC design that, with the benefit of this inspired improvisation, it has managed to keep pace with the dramatic increases in engine power down the decades. A top A/G S class gas dragster of 1960 would develop about 500bhp at 6000rpm and cover the quarter mile in around 11.5 secs with a terminal speed of 130mph. Today’s best Top Fuellers run the blower at 40 per cent over crank speed to generate 5000hp at 10,000rpm and scorch the two furlongs in 4.5 secs with a terminal speed of 320mph-plus.

Don’t blink; you’ll miss it.

Thanks to Robin Moore