GM once built a V12 engine that weighed more than an original Mini Cooper: Jalopnik

In the modern automotive industry, the V12 has reached its final stage. With the tightening of emissions regulations, what was once considered the status quo in performance and luxury has been phased out by manufacturers over the past decade, with Mercedes and Aston Martin among the latest to embrace it.

For General Motors it has been half a century since the last V12 was produced in 1965. But this V12 was not intended for the ultra-luxury Cadillacs; rather, it was for the largest GMC trucks on the road. Moreover, this was an era of artificial madness for GM. At a time when the American brand was toying with transforming the Corvette into a rotary mid-engine supercar and wanted the original Firebird to become a jet-powered racer, it’s no surprise that the brand’s unorthodox designs would find their way onto public roads.

Simply put, this engine was huge. With 702 cubic inches and nearly 1,500 pounds of raw American power, it was an engine that weighed more than some of the cars it shared the road with. Meet the GMC Twin Six.

Long before GMC only sold pickups, SUVs and EV Crab-walking Hummers, heavy-duty was the name of the game for GMC. Turn the clock back to the sixties; America’s roads were flooded with semi-trucks, dump trucks and buses emblazoned with the company’s red letters. These machines would be powered largely by GMC’s signature family of 60-degree V6s. The engines were purpose-built for heavy duty use and ranged from 305 to 478 cubic inches. But with the rise of diesel in the commercial truck industry, GMC knew its gas engines could be in trouble. They needed a power plant that delivered diesel performance while still using gasoline energy.

That’s where the Twin Six came into the picture. Matching the bore and stroke of the 351 V6, the V12 engine gave customers 702.4 cubic inches (or 11.5 liters) of raw American power. Although rumored to be two V6s welded together (which isn’t the only time this has happened), the Twin Six had its own unique one-piece block and crankcase. There would still be 54 parts interchangeable between the two families of engines, but many of them were unique to the twin, including the massive four-foot crank, which weighed 190 pounds. Equally large was the Twin Six’s oil pan, with a capacity of no less than four liters of oil, and the enormous water pump that drove 118 liters through the engine every minute for cooling.

It would burn through fuel just as quickly and only get three miles per gallon. Fortunately, it made up for this in performance, putting out 275 horsepower at 2400 rpm and 630 pound-feet of torque at just 1600 rpm. The most baffling thing, however, was the weight; weighing less than 1,500 pounds, it outweighed Mini Coopers of the era.

Performance was never the real selling point for commercial customers. The biggest topic of conversation was downtime; for trucks this could mean a loss of profit. Reliability was the real advantage of the Twin Six, as GMC claimed the engine only needed a minor overhaul after 15,000 miles, while major maintenance wasn’t needed until 200,000 miles.

GMC would only produce the engine for five years, as an option in the 7000 series truck chassis. The engine would also be a popular choice for fire trucks, not only for its performance and reliability, but also for its practicality. GMC boasted in advertisements that the Twin Six could move 1,500 gallons of water per minute with unparalleled affordability. That caught the attention of both firefighters and farmers, with the latter installing them in irrigation pumps.

Unfortunately, the engine could not keep up with the rise of diesel. Production of the Twin Six ended in 1965 and was replaced by a V8 with the same bore and stroke. In total, approximately 5,000 units of the gigantic engine were ever produced, of which approximately 200 remain today. While some are reportedly still working, many of these beasts have found their way into rat bars and muscle cars for when the LS swap is just too boring.