From all I gather these things are low reving torque monsters that are just work horses. They are cool as hell lookin and sounding. Does anyone know what the compatible parts are from normal use engines? I'd also like any info on making it a high rev animal if at all possible. I figure the low rev crap is due to alot of duration, and a long stroke in the crank. Not a pro, not seeking the holy grail, but just found out about these things, and my brain started working again.(DAMN THING!) Thanks, Adam
That I know of there's no aftermarket support for them at all. They made the V6 version of this motor into the late 70s and it got a lot bigger, so parts from that might upgrade the V12 - it's just two V6 blocks siamesed, it even uses 2 pair of V6 heads and 2 V6 intakes on top. So pretty much anything from the V6 versions would work, except the crank most likely. This motor I think was originally designed by GM to be a 4-stroke V6 diesel, which would explain the low RPM/high torque nature of it. The diesel version is fairly rare, though. It would be interesting to convert the V12 over and see what kind of numbers it would make. You might not go very fast but I'm sure you could spin the tires until the rims melted. Or pull trees out of the ground several at a time.
The stroke isn't that long at 3-5/8 and less than many older V-8's.The stock HP is about 275 with 600 pounds of torque.The engine is more or less made of two 351 GMC V6's with a common block and crank. Here's some factory info;http://www.6066gmcguy.org/TwinSix.htm
I seen on in a junkyard about 10 years ago,when i sen the V12 badges I had to look since I did not know they were made. I would love to hear one one with 4 pipes and glasspacks. Jeff
I think the weight of the engine alone is well over 1000 lbs.The V-6's this engine is based on didn't take kindly to be over reved,so the V-12 is probably quite RPM limited.
Twin-Six 702 V12 275 hp @ 2400 rpm 630 lb-ft @ 1600-1900 4.56" bore 3.58" stroke 7.50 to 1 compression Sounds like this thing is just begging for a pair of blowers....
I did the exhaust on one of these in a county fire truck years ago. I wanted to put some "Smitty's" on it but they insisted on a stock muffler. Fortunately the fire truck had enough room for them. That thing would have sounded sweet going down hill.
I was looking into getting one of these a while back, wild motor thats for sure. In my search for information I talked with a family friend who drove Over-the-road (Tractor Trailer). GMC used them in their Semi's back in the day. They had two transmissions behind them, usually a 4x3 (first transmission had 4 gears and the second had three gears). He drove them for a number of years and he said they ran great except they got around 3 MPG. End of that thought.
The old Cooper-Jarret Trucking Co./Freight Line used them to pull triple 40 ft trailers on the Ohio Turnpike. From what I remember, that 3 M.P.G. is what killed them-they got parked before they were worn out.
I recall them actually weighing closer to 1200 pounds. That's JUST the engine. I wanted to build one for a while too but they are super long and super heavy so you'd need quite a chassis. It would be for looks only if you wanted to do it and I'm not sure that it would really look THAT cool. still...I'd like to see one done just to be sure...
The latest Blastolene car used a 702. These are the same guys who built Leno's Tank Car. http://jalopnik.com/384146/blastolene-b702 http://www.blastolene.com/B702.html
I have owned driven and worked on many of those GMC,s They came in 305,351, and478 V6 versions. The toroflow diesel V6 had 478 cu inch it would turn 3200 RPM ,high for a diesel. The pistons were heavy and had 5 rings. The heads would pop off the exhaust valves They had sodium filled valve stems. All got very poor fuel economy were heavy and hard to get parts for. They could out pull all the other trucks in their class. Might make a truck pull engine or put in a tractor . Never be any good in a drag race because they reved up too slow. OldWolf
Bob Bendtson of Bendtson engine adapters on the hamb board has a few of these including a restored running version etc. He showed it off at the hundret car show a couple years ago. They are for sale....must be 10 ft long..
There is another cool GMC engine that very few people know about that was the replacement for the 702. It was a 637 CID 60 deg. V8 basically made by adding two cylinders to the 478 V6. On another thread on here about odd-fire and even-fire engines the 90 degree V6's are mentioned, the 637 was the opposite, an odd-firing V8, the only one ever built that I'm aware of. I overhauled one once, it was the meanest sounding gas hog I've ever been around. Had a 2 barrel carb with each barrel about 2" in diameter (can't remember for sure, it's been alot of years.)
The Petersen Musuem in LA has a T-Bucket with one of these in it...Kinda pissed that my camera died before I could take a pic of it...It was pretty damn long...and they had machine 2 equally long valve covers to cover both sets of heads....it was pretty interesting...
there are a few cool videos of these engines running on youtube , the blastolene B702 car is on there also ,
That engine in the gas form would run forever. Could not ware it out, and could not pass a gas station ether. In the diesel form it was good for about 40000 miles and it was wore out. JUNK. They called them the Toroflo diesels.
I had the V6 version in an early sixties Chevy Suburban . Going downhill with a tailwind and the ignition off it still got crappy mileage . I think they only used the V12 version on fuel tanker trucks 'cause that's the only way you could carry enough fuel .
I have several of these engines. They are basically two GMC 351's with a common block and crank. They are about 5' long and weigh around 1400 lbs. in stock form, width is about like a SBC due to the 60 degree vee. The biggest problem with them as a hot rod engine is the heavy stock recipricating assembly of 3300 grams . The heads have some decent potential and the engines are built pretty solid. I am currently transplanting a mostly stock 702 into a '50 GMC COE, but am also working on a boosted 702 that uses Ross pistons, heavily modified CAT 3116 rods, o-ringed deck, etc. etc. I haven't decided what it will go in yet....
This topic was brought up some time ago. This is an industrial engine not suited to performance duty. It's obsolete and really difficult to do much with. The original GMC v6 from the early 60's is where you go to find bellhousings. They really don't make much power.
i ran into a guy at Goodguys that had a COE single axle tractor, former fire department owned, with a V12. It was very low mile and he had sympathectically restored it. Awesome truck. 5x2 trans and air brakes. He was restoring a trailer to pull behind and show off his large collection of restored GMC pickup trucks. Very cool.
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