Home » There’s More To That Ridiculous GMC Twin Six V12 Engine Than Meets The Eye: COTD

There’s More To That Ridiculous GMC Twin Six V12 Engine Than Meets The Eye: COTD

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A for sale listing can probably tell you a lot about a person or their vehicle. Someone who refuses to tell you about a car’s condition could be setting you up for an unpleasant surprise when you arrive. Coolant jugs, oil bottles, and spare parts in the vehicle can hint at previous or perhaps current issues. Sometimes, it’s what’s in the background that will tell you something about a seller.

This morning, Thomas told us the story of a rare survivor GMC ‘Twin Six’ engine. Hat tip to Nick Hernandez! The engines were two 351 cubic-inch V6 engines forced together in an unholy 11.5-liter V12 marriage. It’s about 1,500 pounds and 53 inches long with four cylinder heads. All of this for a mountain of torque, 630 lb-ft, to be exact, but just 275 HP. It’s a rare engine with around just 5,000 units ever getting made.

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You can buy that mammoth engine and if you flip through the pictures, you’ll find that the seller is probably a hardcore gearhead. Look at that garage!

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Mike Harrell wins COTD today for noticing what we didn’t:

A measure of the difficulty to be encountered in rebuilding these engines is that the person with an Austin A70 Hereford Countryman parked on the lawn is giving up on it.

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Wow, look at that!

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Readers Matt Wishart and Clammie both pointed out another fun fact: The GMC Twin Six makes an appearance in Mad Max: Fury Road in the 1932 Ford three-window coupe named “Elvis.” The vehicle was built by Ian “Elvis” Davis from Rod Bods Downunder. The initial vehicle was a red hot rod before it was modified again for the movie. From Australia’s Which Car?:

Not too long after the producers of Mad Max: Fury Road came knocking and the rod went off on a trip that would take it to the other side of the world! But don’t worry, that glossy body and mint interior didn’t get wrecked in Africa. “We removed the body, interior, wheels, the grille, the headlights and all the jewellery from the V12 car,” says Elvis.

“I put on a new body on it, plonked in a bucket seat and fitted some basic wheels to the car and that is what they used in the movie. A customer then ordered a turn-key car using the shiny body and trim, so I built a new chassis and fitted it with a small block Chev, it’s a nice car! The GMC motor in the movie car blew up at some point – you can’t rev them very high for very long – but I’ve managed to get it back.”

But that isn’t where the story ends. “We actually ended up making two extended ’32 coupes for the film,” says Elvis. “For starters, they had to roll the car at one point, so they needed another to finish the close-ups. So we built another extended chassis and body, and they took a mould from the V12 and fitted a fiberglass dummy motor over the top of some other basic engine. We also made four chassis for the bare metal ’34 Chev coupe that one of the main baddies drives, the one with the big cross fitted onto the front.”

Have a great evening, everyone!

(Top Image: Warner Bros. Pictures)

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Cerberus
Cerberus
10 months ago

That low hp figure is reached at 2400 rpm and peak torque at 1600. HP is just torque times rpm divided by 5252. The more rpm, the greater the HP figure will be, which is why weak weedwhacker engines with peak hp at really high rpm often aren’t all that strong in spite of the numbers and is why you don’t see them in vehicles that perform heavy work and are expected to do so for extended periods of time with a high ability to shrug off non-perfect fuel, oil, cooling, and timing before overhauls. As an example of a well-known opposite, a stock dyno chart I found of an S2000 shows hp of about 43 at 2400 rpm, yet peaks at about 210 whp around 8400 rpm. If this GMC engine could efficiently breathe at moderately high rpms and the reciprocating mass wasn’t an issue, the hp number would also be high. Let’s say that peak torque of 630 lbs./ft. was at 5200—a more relatable automotive engine number—the HP it would make at that engine speed would be almost 624 (the formula also tells us that at its real peak HP of 2400 rpm, the engine is still making about 602 lbs./ft, but let’s say it had its peak hp at 6400, which is more relatable to modern regular car engines, and still held that 602 torque. Well, we would end up with a peak hp of about 734). Of course, a peak at higher rpm would make it less useful for a work vehicle that needs immediate power to move a large load off the line and constant access to maintain speed on hills, head winds, etc., which requires a lot of torque to be available at all operating speeds. Having to rev to a high rpm to get to that torque is not only going to result in poor performance for the job, but poor longevity and fuel efficiency as it would have to operate at those higher speeds at high load much of the time and the gearing would have to be very short to get the rpms up as quickly as possible. I’m not sure how it would even work without a TC automatic or slipping the hell out of a clutch. This is why you don’t see something with the characteristics of a Ferrari engine in a tractor trailer in spite having higher hp figures. Both types of peaky engines—high hp vs torque or high torque vs hp will have fairly narrow ideal operating ranges, but that’s what gears are for. In the case of a large trucks, it’s a lot of gears.

Acid Tonic
Acid Tonic
10 months ago
Reply to  Cerberus

Whats interesting is how this engine behaves like a diesel. Doesnt like revs, has double or triple the torque compared to hp.

To think diesels make this kind of torque from roughly 4.0 displacement assuming 330ftlbs from a 2.0 turbodiesel.

Also since they run cooler they are happier under heavy loads for long periods without the need for as large of a cooling system compared to gassers that tow.

When I switched from smaller stock turbochargers (vnt15/vnt17) and went to a massive HY35 on my 1.9 the torque prior to 3000rpm significantly dropped but once it spooled at 4000rpm it made around 330whp until 6000 despite being a diesel.

The stock 90hp turbo was more fun below 3000rpm but fell on its face after that. Good for towing though. Compound turbo would be best of both worlds but gassers usually never do compound as its too hard to tune transients for knock prevention.

Good stuff.

Cerberus
Cerberus
10 months ago
Reply to  Acid Tonic

It’s definitely more like a diesel or an old aircraft engine than a typical automotive gasoline engine.

AC2DE
AC2DE
10 months ago
Reply to  Cerberus

This was a GREAT explanation of why industrial and large truck engines are designed the way they are. I’ve always wanted to put it into words, but never this well. Thanks!

Jeremiah McKenna
Jeremiah McKenna
10 months ago

I wonder how they blew up that engine? I saw the movie and how it was made, and a lot of those cars were thrashed.

Last edited 10 months ago by Jeremiah McKenna
Cerberus
Cerberus
10 months ago

11.5/2=5.75. 5.75*61 (~61 cu in/liter)=351 rounded up. 3.5*2=7, not 11.5.

Last edited 10 months ago by Cerberus
notoriousDUG
notoriousDUG
10 months ago

It is not two 351s; it is a unique block, head, and crank casting.

Jeremiah McKenna
Jeremiah McKenna
10 months ago
Reply to  notoriousDUG
Jeremiah McKenna
Jeremiah McKenna
10 months ago
Reply to  notoriousDUG

I believe is 2 351 v6 engines.

https://en.m.wikipedia.org/wiki/GMC_V6_engine

Cerberus
Cerberus
10 months ago
Reply to  notoriousDUG

Yes, the block is different, but it uses a lot of related parts, including heads, which is why there are 4 of them.

notoriousDUG
notoriousDUG
10 months ago
Reply to  Cerberus

You are correct, I was mistaken on the heads being one piece.
And they do share a bunch of parts but it is still a long way from being two engines linked together.

Phantom Pedal Syndrome
Phantom Pedal Syndrome
10 months ago

(comments)

Bongo Friendee Harvey Park
Bongo Friendee Harvey Park
10 months ago

(replies)

Mike Harrell
Mike Harrell
10 months ago

Hey, I was out yesterday. What did I miss? I… Huh.

Rafael
Rafael
10 months ago

(reads)

Rafael
Rafael
10 months ago
Reply to  Rafael

(replies to himself because dumb)

TOSSABL
TOSSABL
10 months ago
Reply to  Rafael

Not dumb: you made someone smile
Thanks: I needed it his morning

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