A Look Back at the 702ci GMC Twin-Six V12 Engine

[Bruce Nunnally]

Here’s a mythbuster: The GMC Twin-Six V12 is not two V6 engines welded together, although at first glance that’s the immediate reaction.

“The block is one-piece with oversized bore spacing for improved cooling, and it’s equipped with seven main bearings,” says Ron Quarnstrom, who is forming a strategy to dyno test this engine at his R&R Performance shop in Minnesota. “It also has a one-piece, hardened forged-steel crankshaft that weighs 180 pounds.”

Read more: http://www.enginelabs.com/news/a-look-back-at-the-702ci-gmc-twin-six-v12-engine/

[Cadillac Jim]

Those short straight exhausts may allow cold air to reach the valves, an that's not a good thing.

[Texas Jim]

My Father had one in a big truck in the mid 60's....

I remember that it had a totally different sound than any other engine did...

[Cadillac Jim]

There's nothing like a 60 degree or 120 degree V12. Not being conversant with big trucks, I associate the sound of the big Allison and Rolls Royce V12 aircraft engines in vintage aircraft and hydroplane racers with a magnificent roar. Even at cruise the sound exudes competence.

[SevilleDorado]

I have a GMC 302 cu.in. straight six in a 52 GMC 100 pick-up. Fenton headers, old school aluminum intake with a rochester 4GC carb - sounds awesome ! - not the same as a 12cyl. I am sure - just sayin.....

[Cadillac Jim]

The GMC 302 was a very popular street rod engine. You could easily put one in a Chevrolet, and the next time some kid in his daddy's 1953 Oldsmobile tried to kick sand in your face, a terrible thing happens to his ego. In those days, most people had been indoctrinated that any V8 could easily beat any I6 in a stop-light gran prix, but, cubic inches is cubic inches, and tuning is tuning, and most family cars were tuned for smoothness and throttle response below 40 mph. The early Chevrolet V8 and concurrently-produced six-cylinder cars had the same torque to the rear wheels in the lower RPM and speed ranges; the difference showed when you stayed in gear and wound the engine up to bring out the horsepower, but they were almost identical in normal driving. Of course, that old six was about 200 lb heavier than the newly designed V8.

In the stock car races common in the 1950's and 1960's on oval dirt tracks, the Fords commonly ran Mercury or Lincoln V8s. I once saw a Chevrolet roll over on its side after being broadsided in a turn and the hood came up; you could read GMC on the valve cover clear up in the grandstands. I went by the pit later and asked; it was a GMC 270.

Crane and Competition Cams still sell performance cams for the 270-302 GMC I6. The later 250 and 292 is preferred, though - those are still raced in South America.