GM Twin Six: 702 CID, 275 HP, 630 Torques

"Rusty Vandura - www.tinyurl.com/keepoppo" (rustyvandura) 05/08/2018 at 14:38 • Filed to: None

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benjrblant > Rusty Vandura - www.tinyurl.com/keepoppo 05/08/2018 at 14:45

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Not-clipped image.

Party-vi > Rusty Vandura - www.tinyurl.com/keepoppo 05/08/2018 at 14:50

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Yes these crude engines once relegated to driving oil pumps in the southwestern US are now worth oodles of money since everyone and their brother wants them for a ratrod build.

RamblinRover Luxury-Yacht > Rusty Vandura - www.tinyurl.com/keepoppo 05/08/2018 at 14:55

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Last time somebody posted on these I put a pencil to paper and calculated that with a custom crank and an overbore to the same size as the top end same-family V6, it would be straightforward to break 1000ci. That’s not even with an offset grind and custom rods.

Rusty Vandura - www.tinyurl.com/keepoppo > Party-vi 05/08/2018 at 14:57

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The writer says as much in the link. I thought it was kind of interesting. Lowest specific output of anything in GMC’s engine line.

HammerheadFistpunch > Party-vi 05/08/2018 at 14:57

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most of them got bought up by v12thunder

http://www.thunderv12.com/about_our_engines

Rusty Vandura - www.tinyurl.com/keepoppo > RamblinRover Luxury-Yacht 05/08/2018 at 14:59

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You’re punching way above my weight... But I mostly follow you.

Just fun stuff to look at. There’s a beautifully restored crackerbox GMC in the link.

I love the medium-sized trucks pre-1975.

Demon-Xanth knows how to operate a street. > RamblinRover Luxury-Yacht 05/08/2018 at 15:26

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Considering that the 478 V6 was in the same engine family. I’d love to make one of these spin to 5k and push out about 600HP and 750LBft

RamblinRover Luxury-Yacht > Rusty Vandura - www.tinyurl.com/keepoppo 05/08/2018 at 15:44

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The V6s that these were developed from ran all the way up to a 478. A simple doubling (478 x 2 = 956) would be easy given the correct custom crank, (assuming it was a good block) and a relatively small overbore* or offset grind** on the crankshaft or both would push it over 1000 cubes.

*.117" is not a small overbore by normal standards, but the engine has a bore of 5.125" (much larger than even a Cadillac 472), and the 478 was also built as a diesel. Diesel = substantial head gasket space between cylinders = it’ll take an extra sixteenth of an inch all the way around, no problem. Just have to find the pistons.

**If an extra 3/32 can be found at the top and bottom of stroke, that would do it.

Demon-Xanth knows how to operate a street. > Rusty Vandura - www.tinyurl.com/keepoppo 05/08/2018 at 15:46

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One thing to remember about this engine, it was made to be under load for extended periods of time. If you took a 283 of the same era that made similar HP, it would overheat and kill itself before the end of the first day at work. Many industrial engines and motors are rated as low as they are because they are built for a much higher duty cycle.

Example:

This is a Tesla motor that makes about 340HP, but the duty cycle at that load is quite low:

This is a 300HP electric motor for industrial use, that motor may have a 100% duty cycle and will deliver that power all day every day until the pump it’s attached to wears out:

RamblinRover Luxury-Yacht > Demon-Xanth knows how to operate a street. 05/08/2018 at 15:49

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What’s hilarious about you saying that is that the cylinders in the 478 are ridankulously oversquare. 5.125 bore, 3.86 stroke. That’s the same oversquare as a 302. It therefore *absolutely* has the surface area in the head to breathe well, and should rightly be able to rev like a demon if it only has good induction and heads that will breathe.

Demon-Xanth knows how to operate a street. > RamblinRover Luxury-Yacht 05/08/2018 at 15:55

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The 702 redlined at 2500RPM, the 478 went far higher to around 4500.

RamblinRover Luxury-Yacht > Demon-Xanth knows how to operate a street. 05/08/2018 at 16:25

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Sounds like it was all down to the bore and heads. Putting the 478 heads on would be needed to make a “big 12" regardless, so that simplifies matters.

Demon-Xanth knows how to operate a street. > RamblinRover Luxury-Yacht 05/08/2018 at 16:36

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You would also need to ensure the oiling system was up to the task. I know the 4.7 Dodge’s oil pump worked great upto 6000RPM where it would cavitate and stop working. It’s entirely possible that the oil pump and gearing was enough to drive a 6 at 4500, but not be able to supply enough oil at pressure to push a 12 beyond half that. But that’s one of those “start doing research” tasks that can be addressed with an external oil pump like those used in dry sump applications if needed. we’re talking a fairly odd ball engine without a lot of aftermarket support and effectively zero racing R&D.

Rusty Vandura - www.tinyurl.com/keepoppo > Demon-Xanth knows how to operate a street. 05/08/2018 at 16:41

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I get sort of shivery inside thinking about things like that.

AfromanGTO > Rusty Vandura - www.tinyurl.com/keepoppo 05/08/2018 at 16:48

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The trucks that had these V12s were interesting. It has been lengthened by almost a yard it looks like. (Almost a meter for those of you whose countries haven’t been to the moon. :P)

RamblinRover Luxury-Yacht > Demon-Xanth knows how to operate a street. 05/08/2018 at 16:49

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Look what Smeagol finds:

http://www.crankshaftcoalition.com/wiki/Building_a_GMC_702_V-12

You were right, oil pump modding is a must.

Rusty Vandura - www.tinyurl.com/keepoppo > AfromanGTO 05/08/2018 at 18:29

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Metric putdown: well played.

AfromanGTO > Rusty Vandura - www.tinyurl.com/keepoppo 05/08/2018 at 18:56

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Thank you.

Yet kinja didn’t post the pictures of the trucks with the GMC V12....

Party-vi > HammerheadFistpunch 05/09/2018 at 08:14

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Boo and hiss.