This left-side view shows an alternate accessory drive configuration with a crank-driven fan and twin-belt water pump drive. And here’s the number we were all waiting for: 630 lb-ft of torque at 1600 to 1900 rpm. The gross brake horsepower rating was 275 horsepower at a lazy 2400 rpm. Bore and stroke were 4.56 inches by 3.58 inches, respectively, yielding a displacement of 702.4 cubic inches. Cooling fan diameter was 25 inches.Ĭompression ratio was 7.5:1.
Note the belt-driven air compressor and the triple-belt drive for the DC generator and mammoth water pump, rated at nearly 120 gallons per minute. The ignition (not visible) employed a pair of distributors on a common drive. This view of the right side of the engine clearly shows the paired cylinder heads, manifolds, and carburetors as borrowed from the 351 V6. While the GMC V6 family also included a diesel variant, the not terribly successful ToroFlow series, there was no diesel version of the Twin Six, only a gasoline model. There were 54 shared components in all, reportedly.
The one-piece crankshaft forging weighed nearly 200 lbs., while the cylinder heads, intake and exhaust manifolds, and so on, were taken from the 351 V6. The 702 CID Twin Six version was essentially two 351 CID V6 engines on a single cylinder-block casting (see above). Offered in 305, 351, 401, and 478 cubic-inch versions, the heavy-duty V6 was used across the GMC truck line in specific applications through 1974. Introduced in 1960, the Twin Six was based on GMC’s 60-degree V6 engine series, also unveiled that year. GMC’s V12 came along toward the end of the gasoline truck era, when diesels were quickly taking over the industry but there was still a demand for large-displacement gasoline engines. Seagrave and American LaFrance, to name two, offered V12 gas engines in fire trucks for decades.
Now, when we think of V12 powerplants for highway use, we usually recall the 12-cylinder luxury cars of the classic era, such as Cadillac, Packard, and Lincoln, or exotic sports cars like Ferrari or Lamborghini. But in fact, there’s a solid historical precedent for V12 engines in large trucks. The fabulous Blastolene B702 proudly sports a GMC V12, and why not? After all, bigger is usually better in the hot rod world, and there’s no replacement for displacement. Not a common sight even in the GMC trucks it was intended for, the General Motors Twin Six of 1960 through 1966 was largely forgotten by gearheads until just a few years ago, when it suddenly captured the imaginations of extreme custom car builders, most notably Randy Grubb of Blastolene Special fame. Here’s more on this fascinating dinosaur.
Presenting one of the largest gasoline engines ever produced by General Motors: the 702 cubic-inch Twin Six used in GMC trucks of the 1960s.
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