The 1966-72 GMC 637ci 60-Degree V8 - GM's Largest On-Road V8 Is Virtually Unknown!

People talk about how "under powered" the GMC V6, Twin 6 and 637ci V8 engines were, not realizing that the engines absolutely can make WAY more power than they released with, but that would shorten the life of the engines and increase maintenance costs. These engines were GIANT, over engineered to be simple, reliable and made to last. By tuning them to turn slow and make a ton of low end torque, the engines can get big loads moving more easily and not be stressed to the max when the engine is being run hard. These were never meant to be performance engines, they were meant to do work. And that is something lost on a lot of folks today that are used to big rig torque and power in their 3500 series pickup.

I will always marvel at the old GMC monster engines, like their gigantic V6s for trucks.

Yep, ran into a 637 GMC in an old Cab over Fire Truck back in the early 2000's.... I'd never seen one before or since.

I worked at a GMC dealer,77-82.Did two long block changes on the 637.The dealer got two new factory long blocks from GMC.They were in the J model tractors with Fuller RT613 trans.Heavy piece of iron.Sure ran smooth.On the two barrel carb it would pour the gas into the engine.

For those that haven’t seen the V6 version in person, the engine shown at 3:00 is the size of a big block chevy and heavier. Heck the water pump on my 305 looks like it weighs 50# by itself. Cool old engines. Thanks Adam. 👍

The 702 twin-six was not coupled together. It had one casting of the block, it had one forged crankshaft, 4 separate heads, 4 separate exhaust manifolds, 2 carburetors and 2 distributors

The Turo Flo injectors were located were the spark plugs where in the gas versions,and the V-6 Turo Flos,also incorporated or utilized a balanced shaft .I think the V-12.was discontinued in 65. When I worked at an International truck dealership in 74-77 the serviced a fleet of cabovers that had the GMC V-8,I remember driving those trucks,you could roll the clutch of idle with out a bit of protest,those engines had amazing torque.If you ever drove those GMC medium or heavy duty trucks,you never really ran high RPM,they were ment to be ran at low engine speed,a great family of gas engines in my eyes.

Had a 534 ford to pump irrigation water and it was a lemon apparently. It was sized right according to hp required but just couldn’t do the job without constantly breaking down and laboring like it was going to blow up any second. Well, it soon did blow up. We replaced it with a Ford 460 turbo charged and that thing ran like a champ for over 25 years. Only needed valves replaced as it was run on natural gas.

I had a 305 and a 478 v6. They made Toro Flo diesel out of these. GMC also made a huge straight 6 truck motor. IHC made huge gas V8 and 6 in trucks also .

I worked with the Medium Duty team at the GM Truck Group in Pontiac 25 years ago. Back then, the gasoline powered medium duties were important to customers who operated in areas where diesel fuel was not as readily available. GM offered the CAT diesels back then, but the 454 gasoline engines were popular in medium duty. GM had many customers who preferred the gasoline engine for their application.

Large dump truck a local farmer had featured one of these, did tune up on it and was amazed at how smooth ebgine ran, was only one Ive ever seen. Big motor!

Heavy duty truck engines are really fascinating to me! Always happy to learn about GMC v6s and their related derivatives.

Hello frien, I'm an old retired mechanic. In the early 60s, Ford offered a 534 V8. I believe it was 534. I worked for a construction company, We had 2 Ford F550 single axle tractors with 534s. The intake was massive. I live in Florida. The used these same engines in orange groves. Back in the 50s & 60s farmers combated the cold by using short towers with a propeller on it to push the cold out of the orange groves. You still occasionally still see these towers, now a relic. They were powered by 534s. It may have been classed industrial, but they were in trucks. Note, the 550s that had these had a bubble top. The roof was higher than conventional. They were referred to as high hats.

I fixed up a 64 GMC and resealed the engine. No need to rebuild as these were 2X overbuilt as compared to a Chevrolet engine of the same time period. I added Holley Sniper to the 305 and you could lug it to 600rpm without protest. Super cool engine. Also GMC rated the engine as Net power not gross power which was common practice so they made more power than what they were given credit for. Special grey cast iron, hardened valve seats with rotators, and I believe 3 compression rings and one oil control ring. High flow oil pump and coolant pump. That 305 I believe took over 9 gallons of coolant to fill.

I’m fascinated by the 7.8L (478 ci) V6 engine made by GM back in the day.

I like how you didn’t forget about American motors cheers

We had Jeeps with the 401 in them and they would power them just about anywhere and was super reliable as long as they are not over revved and beat to death constantly.

I remember working on these engine in Coca Cola delivery trucks with my pops. Had to run over head adjustments every 15 to 20 k miles because some had solid lift cams and lifters and that was done with engine running, valve cover off (of course),, bit of an oily mess. They would tac out so fast cause of the stroke in these old boat anchors. Brings back memories.

I got a chance to work on one of these back in the eighties. Took the heads off ground the valves and seats. I remember thinking how big that hole was when a 6" extension fell down into the cylinder😮

I've only seen 2 of the 637 V8's in GMC dump trucks in my 67 years!😄As for big V8's, Ford also had a monster 'Super-duty' series for medium to heavy duty trucks (and marine use) from 1958, all the way till 1981. There were 4 different versions. A 401, 477, 534, and a twin-turbo 534 developed for Seamaster Marine, called the 'Super Seamaster 534-TI-B' that made 400 hp. and 657 ft. lb. of torque! They were big, slow turning V8's that maxed out around 3400 rpm, but made gobs of torque in the low-rpm ranges where it was most needed for heavy loads.😉