I found out about these researching my gmc's 305, which was missing when I got it. Never actually seen one, only read about them. Very cool!
here are a couple links: http://thunderv12.com/ these guys have done a ton of work to make a hotrod version of the v12.. http://67-72chevytrucks.com/vboard/showthread.php?t=254111 this guy installed one in a 61 gmc truck, there are some done pics about page 15, and a video of it running with zoomies on one of the earlier pages... also a pic:
That's an Australian car, owned and built by Elvis who you can see in the pic. Only recently finished but I believe he had it for sale.
don't need high revs when you got lots of torque and high gears, it's got all the torque in the world to turn the tall ones, put one of those 702's together with a 5spd with some tall hwy gears and a rear-end that's just as up there and you got a ride that'll make the speedometer spin like a helicopter. more than one way to skin a cat.
I like that idea . So where have all the giant cube engines gone hiding? It doesn't need to be Brutus aero engine big (24!liters) but that is my weakness in dream builds
I believe I have the opportunity to buy two of these 702 V12's. The offer was presented to me. They are both complete and still in the trucks untouched. Not sure if I want the headache involved...
Check the photos from the Jalopy Showdown and the JalopyRama (i think), there is a guy in Maryland with an old COE, chopped and channelled all to hell and gone with one of those engines behind the cab. If i recall right the valve cover look like a 'w' motor so it look like a pair of 348"s or 409's bolted together. Sparkplugs come from the top down, get to them from between the intake runners.
I remember a 351 V6 my grandpa had in this contraption we called the "dune buggy" up at the deer shanty. It was an old panel truck that had been hacked down to a frame/cowl/dash with the frame shortened to a ridiculously short length. Front tires were old bald truck tires, rears were steel truck rims welded into larger tractor rims with tractor tires. Rear fenders were construction signs and there were bus seats sitting sideways over each rear wheel. Front seat was a 60's GM bench and it had a Camaro hood covering the engine hinged with some old barn-door hinges welded to it. The engine was a friggin' monster. First "car" I ever drove, and you didn't shift, and you didn't touch the gas. Just let out the clutch and it would chug through the woods going through damned near anything that got in the way. Ungodly low-end torque. Carried seven guys to their deer blinds for decades. Still ran when they scrapped it out in favor of 4-wheelers.
worked for GMC Truck and Coach from 1965-1971 and these engines were worked on daily in our shop...most had been converted to propane as it was a cheaper fuel at the time..3 MPG was correct, but all the gas pot trucks pulling max weight were getting about 3-5 mpg..that's what pushed the move to diesel...I remember a board in the sales room that had pistons,rods,valves from the GMC V6, IHC, Ford,and Chevy...the GMC parts were massive and HEAVY in comparison and I will say one thing....IT WON'T LIKE HIGH RPMS and it don't need rpm,just gear high and go......albeit, it is still hard to make a silk purse out of a sows ear
You could lighten the rotating assembly, and change cams-that would go a long ways toward it. I would be very interested in one of these engines, if anyone knows of one at a reasonable price.
------------------- If you don't want to build them and the price is good, grab them anyway and re-sell them. Mart3406 ==============================
-------------------- About the stock 3000 rpm rev limit. Don't forget that these were commercial medium and heavy-duty truck engines that were expected to run at near maximum load most of the time and still go 150 to 200 thousand miles between overhauls - and to achieve that, they came fitted with engine speed governors that limited them to about 3000 rpm. The 3000 rpm supposed limit was probably a function of the governor more than the actual mechanical limitations of the engine design. As for or raising both the rpm range and fuel mileage, in addition to reducing the reciprocating mass and grinding new cam profiles, as was previously mentioned, I'll bet that adding a modern-day long duration and/or multi-spark CD ignition would also help quite a bit too. Better still - If you wanted to go a bit nuts and make some insane power - with or without raising the rpm level very much (although more or less forgetting you ever heard of the phrase "fuel mileage" LOL!) - how about - four T3/T4 hybrid turbos, mounted two per side, one on each of four stock exhaust manifolds, feeding into an intercooler and then into a fabricated tunnel ram-style intake manifold (or manifolds) fitted with a throttle body (or bodies), and fed fuel by twelve 55 lb./hour. EFI port injectors, driven by a homebuilt Megasquirt controller??!! I helped build a similar set-up using a *single* T3/T4 turbo and homebrew intake & Megaquirt EFI setup for my buddy's 144 cubic inch Pinto engine. That ended up dynoing at a bit over 400 hp on 22 lbs boost at a conservative and very street usable 5500 rpm. Multiply that same set-up times four and 1600-plus hp and 1200-plus ft/lbs of torque from a 702 cubic inch GMC V12 spinning at around 5000 rpm, would probably be achievable - and still with near stock idle and drivability!!! Mart3406 ===========================
I think everthing Mart3406 said is correct. You can safely turn 4000 rpm by turning the govener up. On of the weaknesses is the way those heads breath. A good, proper port job, with a good cam, would also really help. Joe Panek at Roto-Faze is probably your best bet for a custom billet, hi-po cam. He's doing two for me, for the FlatCad.
In the mid 1970's Safeway of Canada had a COE with a 702 used for cross country hauling with tandem trailers. Every 250,000 miles it would come in to Western GMC Truck Centre in Edmonton Alberta where I was working at the time for a valve grind on all 4 heads. I was there for 2 or 3 of them. After over a million miles the bottom end still had not been apart. Not sure what happened to it, it was still on the road when I quit in 1977. Tons of torque and bullet prove from factory. Wil
------------------ Mid-70's I doubt that it would have been a 702 GMC gasser. The 702 V12 wasn't installed in new trucks after the 1965 model year and was replaced by a 678 V8 version for 1966. Are you sure it wasn't a 12V71 diesel? The 12V71 was an entirely different engine, (think two 6-71 inline Detroits but arraigned in a 'V' configuration in a common block) and not related in any way to the 702 V12 gas enenine. Mart3406 ====================
It was in the mid 1970's that I worked at the dealership, the truck had been on the road long before I worked on it and probably after I left.
I think a good port job, new custom cam, electric fuel pumps, HEI, and dual 6-71's, or 4 turbos as mart3406 suggested, would be one mean, torquey, tire burning machine!
------------------- The relatively easy 1600-plus hp you could get with a homebrew "T3/T4 quad-turbo & Megasquirted EFI" setup would certainly help make up for at least *some* of the 'elephantesque' 1400-plus lb.weight of one of these monsters!! It wouldn't be "practical", "politically correct", or "environmentally friendly" (heck, any one of these are good enough reasons, just by themselves to justify building one!!!) or in any way cheap to build, And you'd need a hell of big car - probably one specifically designed around the engine - to put it in. Oh, but it sure could be fun!!! Mart3406 =========================
Why are you going to use CAT rods, the v12 have steel rods and crank in them. They would suck the ball out of the PCV valve and put them on top of the piston. The PCV valve was under the valve cover and screwed into the intake port. There is someone ( I saw him on youtube ) that's doing what you want to do with this engine. This man is cranking them up to around 6000 RPM. Timimg these engine where two dist. that came out the back of one main dist. The points were in the big dist.( 2 sets ) and you timed the front six with one little dist. and the back six with the other. The timimg had to be right or you would burn a hole in a piston. The gov. work off of oil pressure. Remember 2 intakes 2 carb. 2 govs. and 2 carb. linkage, and it all had to be right. If you look on youtube at v12 GMC you'll find a old man on the back of a Dodge flat bed showing his V12 with it running. I sold him this engine and the truck it came out of. I don't have any more V12, I kind of miss them. I am a old mechanica and I've owned, run, worked on all the old stuff.
I just found one of these engines for 300.00 but it is stuck. I'm wondering if it is worth buying. Owner says it ran 20 years ago.
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