Home » The GMC ‘Twin Six’ Is An Insane 11.5-Liter Four-Head V12 Meant To Fight Diesel Engines

The GMC ‘Twin Six’ Is An Insane 11.5-Liter Four-Head V12 Meant To Fight Diesel Engines

Gmc Twin Six Topshot
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The modular engine renaissance is in full swing. From BMW sharing components for three-, four-, and six-cylinder engines to Jaguar’s Ingenium engine family, component sharing is so hot right now. However, we’re far from peak modular engine. Some would argue that happened back in the 1960s, when GMC put two V6 engines together to create the mother of all commercial gas-powered V12s. Oh, and did I mention that one’s up for sale on Facebook Marketplace?

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See, in the late 1950s, diesel commercial engines were gaining market share and GMC wanted to fight back with gasoline power. The answer was simple: Take two existing engines and sort-of glue them together to create something borderline obscene. After some thought, engineers decided on merging wo 351 cubic-inch V6 engines together to create a 11.5-liter V12. Granted, the V12 does sport its own unique block, crankshaft, and camshaft, but the cylinder heads are exactly the same as on the V6. Yes, this means the Twin-Six has four cylinder heads and four exhaust manifolds, along with the expected two distributors.

Oh, but the insanity doesn’t stop there. The crankshaft alone weighs 180 pounds, the wristpins for the pistons measure more than an inch in diameter, the oiling system demands 16 quarts of black gold, the water pump pushed two gallons a second, three thermostats helped keep this thing at operating temperature, and the entire engine weighs nearly 1,500 pounds. Engine length? Try 53 inches long.

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All this monstrous size added up to — wait for it — 275 horsepower at launch. That doesn’t sound impressive, but peak horsepower hit at 2,400 rpm. Peak torque was a burly 630 lb.-ft. from 1,600 to 1,800 rpm, an astonishing figure for a gasoline-powered commercial engine from the 1960s. GMC advertised this engine for use in both commercial vehicle and stationary applications, claiming it would run up to 200,000 miles between major overhauls in a period advert. Another claim? That the Twin Six was the cheapest way possible to pump 1,500 gallons of water per minute. Hello, fire department?

Sadly, the GMC Twin Six was no match for the diesel engine’s postwar popularity. From the venerable Detroit Diesel two-strokes to household names like Cummins and Caterpillar, oil-burners chipped away at the big GMC’s sales to the point where only 5,000 made it out of the factory between 1960 and 1965. However, this particular Twin Six is still kicking. The owner, located in Cedarville, Ohio, claims that it “Runs good with gas down the carb,” which seems alright for this $2,900 rarity.

Gmc Twin Six 2b

As it stands, surviving GMC Twin Six engines are extremely rare. Parts support falls into the dreaded “lol, lmao” zone, meaning that if this beast from a bygone era needs an at-home rebuild, you’re pretty much out of luck. Or are you? If you have money to burn, a little company called ThunderV12 may have what you’re looking for. This company will take a Twin Six, give it a rebuild and hop it up with new carbs, a new camshaft, a revised ignition system, stronger internal fasteners, a beefed-up oiling system, and off-road heads for an absolute redline of 5,000 rpm and nearly double the stock power on 87 octane. Oh, and that’s before bolting on turbochargers, superchargers, or nitrous oxide. Best of all, ThunderV12 also sells adapters to bolt this engine up to modern transmissions with Chevrolet bellhousing patterns. Imagine how usable this beast would be with a six-speed automatic behind it.

(Photo credits: Facebook Marketplace seller, GMC)

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AutoPartsGuyBuffalo
AutoPartsGuyBuffalo
10 months ago

http://6066gmcguy.com/

They also did a V12 twin 305!

My family has one of the V6 305s in a ’67 Fire Truck. Torquey engines.

Phuzz
Phuzz
10 months ago

Aston Martin did something similar to this to create the AML V12 for the DB7, by basing it on the Ford Duratec V6. (As with the Twin six, it’s a bit more complicated than just welding two V6 blocks together).
I only learned today that the Duratec design had come to Ford via Mazda, who had originally developed it in partnership with Suzuki and Porsche. So there’s a little bit of Porsche in Aston Martin.
Long article about it by one of the engineers that designed it here: https://www.designjudges.com/articles/the-origins-of-aston-martins-v12

Richard Truett
Richard Truett
10 months ago

Shit like this is why I love spending time here.
GREAT STUFF.

Brammachu
Brammachu
10 months ago

It’s already sold, which one of you Mad-Men bought it?

Matt Wishart
Matt Wishart
10 months ago

‘Mad Max – Fury Road’ features a coupe’ with a double six (‘Elvis’). It was originally built as a regular hot rod, with nice paint etc but was purchased by the producers and turned into ‘Elvis’.

JDE
JDE
10 months ago

Since so many of the parts interchange on the 305 to 478V6 variants, it sure would be interesting to join two 478’s with the various part used to make the big boy 702.

notoriousDUG
notoriousDUG
10 months ago
Reply to  JDE

You’d just have two linked 6 cylinders.
The 12 was a unique casting for the block and the heads not just two 6 cylinders put together.

Lockleaf
Lockleaf
10 months ago

I’m pretty sure rat rod bob built a rod with one of these in it, but it could have been some other Youtuber.

More importantly, the commercial V6 has the GREATEST factory valve covers of all time, which would also fit this v12. Check it out. FACTORY PLAID!!!

http://6066gmcguy.com/J-Engine/PlaidCovers-01.jpg

For reference: http://6066gmcguy.com/GMCplaid.html

Mike Smith
Mike Smith
10 months ago
Reply to  Lockleaf

You might be thinking of Waylon Wire’s Dirty Dozen: https://www.youtube.com/@1963impala2drWaylonWire
His channel is worth visiting and watching some of his stuff. His builds are very interesting and he’s a good’un.

318Charger
318Charger
10 months ago

An uncle had the V6 version in a grain truck.

Interesting thing about these engines. The spark plugs were in the intake valley instead of on the outside of the head.

Last edited 10 months ago by 318Charger
JDE
JDE
10 months ago
Reply to  318Charger

I suspect that had to do with the variants of these V-6’s. They could be 305, 351, 401 and 478 cubic-inch, and either gas or diesel in the V6 form. they also shared a large number of parts and pieces. The 409 has spark plugs on the exhaust side and the valve cover shape and basic displacement per cylinder of the the 305 version definitely seem 409ish, but the Diesel Toro-Flow intakes sat basically where the spark plugs would be in those recesses in the Valve cover.

Scott Wangler
Scott Wangler
10 months ago

That would be awesome in a rat rod

Griznant
Griznant
10 months ago

We had the GMC 351 V6 (half of one of these) in a “dune buggy” that my grandpa used to go through the woods. Basically a cut down commercial truck that had tractor tires on the back. Absolutely unstoppable torque-wise. You never shifted or touched the gas, just put in gear and let out the clutch. If you hit the gas it would rip through the trails too fast for comfort. A true stump-puller (and we used it for that too).

Mike Harrell
Mike Harrell
10 months ago

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.

Jbavi
Jbavi
10 months ago
Reply to  Mike Harrell

Underrated comment

Thomas Metcalf
Thomas Metcalf
10 months ago
Reply to  Mike Harrell

Good eye!

Timothy Arnold
Timothy Arnold
10 months ago

That was probably in response to the Detroit Diesel modular 2-stroke Series 71 engines that could be had in 1, 2, 3, 4 & 6 cylinder in-line, and V-6, V-8, V-12, V-16 & V-24 configurations, and what spawned the now ubiquitous 6-71 superchargers (among others).
https://en.wikipedia.org/wiki/Detroit_Diesel_Series_71

Clammie
Clammie
10 months ago

Made an account just to state that one of the Mad Max: Fury Road cars used this engine. If you’re like me, you assumed it was two V8s bolted together and was pleasantly surprised.

Another fun fact is that they blew it up trying to record sounds for the movie. They had to make a facade of the original and stick a small block underneath.

Alexk98
Alexk98
10 months ago

BUT will it fit in a Miata?

Data
Data
10 months ago
Reply to  Alexk98

The Miata may fit into it.

Alexk98
Alexk98
10 months ago
Reply to  Data

You’re not far off, and I’m pretty sure this engine fully dressed weighs more than my NA does

UnseenCat
UnseenCat
10 months ago

“Best of all, ThunderV12 also sells adapters to bolt this engine up to modern transmissions with Chevrolet bellhousing patterns. Imagine how usable this beast would be with a six-speed automatic behind it.”

With the right gearing, it could probably pass anything on the road…
Except for gas stations.

Michael Beranek
Michael Beranek
10 months ago

I believe they also made a V8 version. A 60-degree V8? How does that even work?

Mthew_M
Mthew_M
10 months ago

Ask Yamaha – they made quite a few of those for Volvo. Quite wonderful engines, too.

Maxzillian
Maxzillian
10 months ago

According to what I’m reading they just added a balance shaft to the engine. I’m guessing it was also an odd-fire.

Automotiveflux
Automotiveflux
10 months ago

Would look amazing in a hot rod

Clammie
Clammie
10 months ago
Reply to  Automotiveflux

It’s been done! Check out the story behind the Mad Max Elvis car.

Speedway Sammy
Speedway Sammy
10 months ago

Don’t forget the “ToroFlow” diesel version of the GMC 60 degree V6. Although I don’t think the V12 version got dieselized.

Bizness Comma Nunya
Bizness Comma Nunya
10 months ago

They made gas versions of this 60 degree engine family which were very durable, but thirsty (for obvious reasons).

However, when they decided to dieselify the gas motor…. it didn’t go well. It also didn’t really make any sense that GM had Detroit Diesel and had this GMC gas motor turned in to a diesel by GMC…. and it didn’t work well.

Speedway Sammy
Speedway Sammy
10 months ago

GM was such a big company back in those days that the competition between divisions was often stronger than competition with other corporations. Thus the GMC/Detroit Diesel conundrum. Same thing with the failed Oldsmobile diesel program, divisional hubris overwhelmed institutional knowledge available other places in the corp.

Ranwhenparked
Ranwhenparked
10 months ago
Reply to  Speedway Sammy

Yeah, they were more like a holding company with each division operating as sort of a stand-alone business

Bizness Comma Nunya
Bizness Comma Nunya
10 months ago
Reply to  Speedway Sammy

The olds diesel is a good example, spot on. It could have been so much better, and actually wasn’t bad at the end (especially the v6’s) … then it died.

CSRoad
CSRoad
10 months ago

Back in the day, I think the best thing about the Olds V8 Diesel was it produced a lot of good looking low mileage cheap pickups ready for an engine swap. If you are looking at a square body and you pop the hood and see hydraulic assist brakes, you are pretty much guaranteed it was once powered by the Olds Diesel.

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