Built for fire trucks and farm fields, this enormous engine rewrote what “American V-12” really meant.
Ask a classic car fan which company built the last production V-12 engine to emerge from Detroit, and chances are you’ll get “Cadillac” as an answer. That’s not a bad guess, considering the luxury marque offered a 12-cylinder monster of a motor as its top option throughout most of the 1930s. You might also hear “Lincoln,” given Caddy’s rival stepped up with a V-12 engine of its own at the end of World War II, installing it in the Zephyr from 1946 to 1948.
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The real answer, however, is considerably more obscure, especially if you’re not deeply immersed in the world of classic … trucks? That’s right. It turns out Lincoln’s engine wasn’t the last mainstream American V-12 by a long shot, because decades later, deep in the GMC division of General Motors, a group of eager engineers were breaking out their slide rules in a mad effort to combine two enormous six-cylinder engines together.
The unheralded behemoth they created, which became known as the Twin-Six, enjoyed all too brief a time in the truck builder’s showrooms. But the engine left an indelible mark through its gigantic proportions, Tyrannosaur torque, and bonkers specifications. It also set the tone for two future mega-cylinder monster motors that would appear in its wake nearly 30 years down the line.
GMC’s Brand-New V-6 Lays the Groundwork
The roots of the GMC Twin-Six are found in the brand-new V-6 engine that General Motors unveiled in 1959. Designed specifically to be used in commercial vehicles, pickup trucks, buses, and delivery models (like the still-not-civilianized Suburban of the era), it featured a 60-degree angle, inboard spark plug mounting, and a six-throw crankshaft that obviated the need for balance shafts. The watchwords in GMC marketing at the time underscored how smooth the V-6 ran and how easy it was to maintain, two factors intended to appeal to the fleet buyers it was aimed at.
As was common in that period, the GMC V-6 was available in a bewildering array of displacements, ranging from a modest 305 cubic inches (5.0 liters) all the way up to 478 cubic inches (7.8 liters). Smaller versions of the V-6 were found in pickups and Suburbans in retail showrooms, while everything else was installed in commercial models. There was even a version of the 7.8-liter engine that had an additional pair of cylinders grafted to it, making a V-8 that pushed displacement all the way up to 10.4 liters.
Six Plus Six Equals OMG
If GMC already had access to so many different versions of its V-6 (and V-8) design, why did it feel the need to push into 12-cylinder territory? The answer was likely commercial pressure from companies like Seagrave and American LaFrance, which offered enormous V-12 engines in their dedicated fire truck platforms through the 1950s.
Don’t forget that this was a time before diesel engines had truly caught on in American industry. This meant displacement was the easiest way to come up with the kind of torque needed for heavy-duty gasoline applications. What better way to produce gobs of twist—as smoothly as possible—than by increasing the cylinder count?
This was the guiding philosophy behind the Twin-Six, which took a pair of 351-cubic-inch 60-degree V-6 blocks and used them as the basis for a 12-cylinder block casting. It was more than just a conceptual similarity: You could swap more than 50 parts between the humble V-6 and its hairy-chested V-12 cousin, helping keep both production and maintenance costs reasonable.
Sketching out what should be shared was an easy calculus for GMC’s engineers to make. Not only was the block “doubled-up,” but the Twin-Six also borrowed two pairs of cylinder heads, a pair of carburetors, a pair of distributors (sharing the same drive), and of course a quad set of exhaust manifolds from the V-6 template. Altogether, the unit provided 702 cubic inches of displacement, or a whopping 11.5 liters, and featured a 4.56-inch bore and 3.58-inch stroke. Some of the V-12’s other specs were just as intimidating: 56 engine head bolts, a water pump pushing nearly 120 gallons per minute, and an astonishing 4-gallon oil capacity.
The advantages of the Twin-Six V12 were immediately obvious. Officially, output was rated at 275 horsepower, and you ran the risk of breaking the dyno with the engine’s 630 lb-ft of torque at a very low 1,600 rpm, an enormous figure for gas engines when the engine broke cover in 1960. A fair bit of that grunt was needed simply to motivate some of the weight of the Twin-Six itself, which weighed nearly 1,500 pounds. That’s only 1,000 pounds less than the current Mazda MX-5 Miata.
Paving the Way for Future Motor Mashups
The legacy of the GMC Twin-Six is somewhat obscure, but that doesn’t mean it was an unsuccessful engine. Intended for use in the most hardcore of applications—specifically those that required extensive, uninterrupted operation—it was a favorite among the same fire truck crowd courted by Seagrave and company, as well as agricultural pumping operations serving vast, open fields. The option lasted until 1966, with its end coming about as GMC introduced a wider range of diesel options, as well as a cheaper 637-cubic-inch V-8.
Today, the Twin-Six occasionally shows up in custom car builds from hot rodders who want to stand out from the crowd. They are thin on the ground, however; only 200 examples are known to have survived out of the 5,000 produced. This isn’t uncommon for industrial-type drivetrains, as there was little impulse to preserve old equipment once it was replaced by a fire house or farm.
The Twin-Six might have been the last production V-12 to emerge from Detroit (tantalizing concepts like the Chevrolet Corvette Falconer aside), but it was not the only engine to result from a mashup of two existing mills.
The ultra-rare Cizeta-Moroder V16T supercar arrived in 1991 with a 16-cylinder engine that shared more than a few parts with the Lamborghini Urraco V-8. In effect, the design sandwiched a pair of flat-crank eight-cylinder Lambo engines together to produce 540 horsepower and 400 lb-ft of torque from 6.0 liters.
More familiar is BMW’s V-12 engine that arrived at the end of the 1980s, which was used in both the 7 Series sedan and the 8 Series coupe. Dubbed the M70, its design relied on a pair of M20 inline-six engines tilted together, swapping out their original cast-iron blocks for an all-aluminum design (including cylinder heads) and using a single-overhead camshaft.
Even if you can’t use X-ray vision to see down inside the block, there are a few other tells in BMW’s engine bay to betray its twin-I-6 origins. Look closely, and you’ll discover a pair of ECUs—one for “each” engine—again borrowed from the M20 setup, though in the case of the V-12, it allows for the unusual ability operate each bank of cylinders independently should catastrophe strike. There are also a pair of distributor caps and crankshaft sensors.
BMW built several versions of the 5.0-liter design, starting with the 300-horsepower standard M70, all the way up to the S70 edition that provided the 850CSi coupe with a hefty 375 horsepower. Like the Twin-Six, however, the company eventually realized its cheaper-to-build and easier-to-package V-8 engine produced similar power, and the 12-cylinder was phased out by the end of the 1990s—but not before a radically rethought version of the engine, the 6.1-liter S70/2, put in a 618 horsepower performance amidship in the world-conquering McLaren F1 supercar.
