In the modern automotive industry, the V12 has reached its last leg. With tightening emission regulations, what was once considered the status quo for performance and luxury has been phased out by manufacturers over the past decade, with Mercedes and Aston Martin being among the last to embrace it.
For General Motors, it's been half a century since its final V12 was produced in 1965. But this V12 wasn't meant for the ultra-plush Cadillacs; rather, it was for the biggest GMC trucks on the road. Moreover, this was an era of engineered insanity for GM. In a time when the American marquee was toying with transforming the Corvette into a rotary-mid engine supercar and wanted the original Firebird to be a jet-powered racer, it's no surprise that the brand's unorthodox designs would find their way onto public roads.
Simply put, this motor was massive. With 702 cubic inches and nearly 1500 pounds of raw American power, it was an engine that weighed more than some cars it shared the roads with. Meet the GMC Twin Six.
Long before it only sold pickups, SUVs, and EV Crab-walking Hummers, heavy-duty was the name of the game for GMC. Rewind the clock to the 1960s; American roads were flooded with semis, dump trucks, and buses bearing the company's Red Letters. For the most part, these rigs would be powered by GMC's signature family of 60-degree V6s. Purpose-built for heavy-duty use, the engines ranged from 305 to 478 cubic inches in size. But with the rise of diesel in the commercial truck space, GMC knew its gas-fed engines could be in trouble. They needed a power plant with diesel performance while still using gasoline power.
That's where the Twin Six came in. Matching the bore and stroke of the 351 V6, the V12 engine gave customers 702.4 cubic inches — or 11.5 litres — of raw American power. Although rumored to be two V6s welded together (which is not the only time this has happened), the Twin Six had its own unique single-piece block and crankcase. 54 parts would still be interchangeable between the two families of engines, but plenty were unique to the twin, including the gargantuan four-foot crank, weighing in at 190 pounds. Just as big was the twin six's oil sump, holding an incredible four gallons of oil, as well as its massive water pump driving 118 gallons through the engine each minute for cooling.
It would go through fuel just as rapidly, achieving a mere three miles to the gallon. Thankfully, it made up for this in performance, cranking out 275 horsepower at 2400 rpm and 630 pound-feet of torque at just 1600 rpm. Most baffling, however, was its weight; at just a tick under 1,500 pounds, it outweighed the Mini Coopers of the era.
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Performance was never the true selling point for commercial customers. The biggest talking point was downtime; for trucks, that could mean lost profits. Reliability was the Twin Six's true advantage, as GMC claimed the engine only needed minor overhauls after 15,000 miles, with major maintenance not needed until 200,000 miles.
GMC would produce the engine for only five years, as an option in the 7000-series truck chassis. The motor would also be a popular choice for fire engines, not just for its performance or reliability, but its utility as well. GMC boasted in advertisements that the Twin Six could move 1,500 gallons of water a minute with unmatched affordability. That caught the eye of both firefighters and farmers, with the latter jerry-rigging them into irrigation pumps.
Unfortunately, the engine couldn't keep up stop the rise of diesel. Production of the Twin Six concluded in 1965, and it was replaced by a V8 of the same bore and stroke. In total, around 5000 examples of the mammoth engine were ever produced, with roughly 200 surviving today. While some are rumored to still be putting on work, many of these beasts have found their way crammed into rat rods and muscle cars for when the LS swap is just too boring.