The GMC Twin Six V12: 702 Cubes, 275 HP At 2400 RPM, 630 Ft. Lbs. At 1600 RPM

By
– Posted on October 4, 2021

(first posted 6/30/2012)   The GMC “Twin Six” 702 V12 is legendary, although it’s not really all that unusual. V12 truck engines were not unheard of in the US and Europe, and the GMC’s 60 degree V6 certainly lent itself to doubling up. But it’s acquired an outsized reputation, and is a cult object for certain hot-rodders and lovers of outsized engines, so let’s give it its 15 minutes of CC fame.

Let’s get the biggest myth out of the way first: these are not two 351 cube V6 “bolted” together. The 702 has its own block, but obviously shares many internal components as well as the V6 cylinder heads, valve covers and even intake manifolds and carbs.

And although they’ve ended up in some wild rods, these engines were available in the GMC steel tilt cab COE, exactly like the one that stood in front of me with a V12 badge at a red light, on old Hwy 40 outside Baltimore as I was hitch hiking, and then heard accelerating through its very many gears and straight pipes. Very memorable indeed.

Although it hardly sounded like a Ferrari; the V12 had the lowest specific output of the whole V6-V12 line: 250 or 275 hp at a low 2400 rpm; and 585 or 630 lb.ft. of torque at 1600 rpm. That’s quite a contrast to the HD 427 Chevy V8 truck engine, which made 260 hp @ 4000 rpm, and 405 Ft. lbs. @2600 rpm. Now that was something to hear at full chat.

Only about 5000 of the V12s were built between 1960 and 1965, and many of them were used as stationary industrial engines like irrigation pumps, where they ran at full (governed) throttle for weeks on end.

This was also the case with fire engine pumpers, which required high output engines. As with the irrigation pumpers, such, the GMC V12  effectively helped kill the legendary Hall-Scott OHC hemi V12 engines in those roles, because the GMC was considerably cheaper.

 

The 702 V12 has found its way into a number of “specials” including the Blastolene B702.

Here’s the B702 chassis and engine before that fanciful body was draped over it.

There’s even a company dedicated to the V12, ThunderV12. A rebuilt one starts at $10,900, if you’re thinking of upgrading your pickup. Yes, more than one has found its way under a GMC pickup hood.

So, now to hear one or two. Here’s a stock one, with just a bit of throttle blipping.

And here’s a ThunderV12, taken all the way to 5000 rpm, albeit very briefly. Enjoy.

The only thing left: imagine if GMC had used the biggest of the V6s, the 478 Magnum, to make a V12: 956 cubic inches and some 500 hp. A Thunder and Lightning V12.