400 small block on C50 dumper

[Arnulfo]

Going to check the truck out this Friday. Owner used it in his concrete business. 1970 GMC 4 speed with split rear. I am restoring a house and will use it as a dump truck. Owner said original motor died so was replaced with a small block 400. Says he is able to keep up with L A CALI traffic which I find hard to believe. Slow lane averages 75 mph.Your thoughts on this swap? Is the 400 a good choice? I am new to the big truck world. Any advice is much appreciated.

[C10 - C90 Bill]

The 400 is a good engine. Should work fine in your truck. The truck may have higher gears in it from the factory. Could be why it can run as fast as he claims.

[kwmech]

As much as like 400s, the total GVW that the truck is capable of would almost dictate a big block. "Keeping up with traffic" would necessitate different gearing. BUT how fast do you want to go with a fully loaded 50 year old truck? Most medium duty trucks in Calif are supposed to adhere to a 55mph restriction anyways--not that anybody follows these guidelines....

[Arnulfo]

Originally Posted by kwmech

As much as like 400s, the total GVW that the truck is capable of would almost dictate a big block. "Keeping up with traffic" would necessitate different gearing. BUT how fast do you want to go with a fully loaded 50 year old truck? Most medium duty trucks in Calif are supposed to adhere to a 55mph restriction anyways--not that anybody follows these guidelines....

Kind of what I was thinking on the 400 small block. Would like to be able to hold 60 mph on freeway without hurting motor. I do not want to go faster than that especially with a load. As you said no one adheres to the 55 mph. Will always be driven on slow lane but even there average speed around here is 65-70 mph.

[kwmech]

If I were you I 'd figure out gearing and RPM first. That is going to be your killer regardless of the engine. If you can keep it under 3000 at 60, you should be good

[special-K]

I like the 400 in a truck for the bottom end torque. Sounds like that owner didn't know how to boast about a C/50 dump truck. I'd be wanting to hear "It can pull a house down" or It could haul an acre of dirt if it fit" over hearing how fast it can roll. The 400 I had in my '72 K/20 ran all day on very little throttle. If I floored it there didn't seem to be much more. With 4.10s and 35" tires I'd roll in the slow lane at 60 mph barely on the peddle. I got the same mpg as the '72 K2500 with 350, 4.11s, and 33" tires (12.5)

[1976gmc20]

I can't imagine any 1970 C50 running 75+ mph

That said, I knew a guy with a ~69 C50 with a 350. He had a little self loader on it and way overloaded it and as far as I know he didn't break the engine. But he rarely took it out on the highway. He mostly used it to haul loads a few miles down to a place where he would reload onto a flatbed semi-trailer for transport.

My 1965 GMC C50 with the V6 was about a 50 mph truck, which was no big deal back in those days (1980s).

[C10 - C90 Bill]

Originally Posted by 1976gmc20

My 1965 GMC C50 with the V6 was about a 50 mph truck, which was no big deal back in those days (1980s).

My '62 GMC 6500 with the 478 would go 62 MPH. Back in the gas crunch days when they let trucks go faster than cars, I was passing cars like crazy, lol.

We were just talking the other day on another truck site that the 2-Ton trucks, in the middle between 1/2 to 1 Tons and the Heavy trucks, went the slowest for some reason.

[1976gmc20]

Originally Posted by C10 - C90 Bill

My '62 GMC 6500 with the 478 would go 62 MPH. Back in the gas crunch days when they let trucks go faster than cars, I was passing cars like crazy, lol.

We were just talking the other day on another truck site that the 2-Ton trucks, in the middle between 1/2 to 1 Tons and the Heavy trucks, went the slowest for some reason.

The V6 GMC engines had a reputation for throwing rods out the side of the block if you pushed them too fast, so I was pretty cautious

The medium duty trucks tended to be farm trucks, urban delivery trucks, or service/construction trucks. I guess they figured that nobody was going to be driving one very far on the highway.

Plus they were mostly just using the same engines as pickups, only with way lower gears to handle the heavier load. My 1958 Chevy 2-ton utility/boom truck had the 235 straight six with a 4 speed and a 2-speed. I don't know if I ever even got it up to 35 mph on gravel roads. That's the same engine my mom had in her 1959 Belair

My GMC just had a single speed axle. Since it had a two piece driveshaft already, I thought about splicing in an auxiliary 3 or 4 speed transmission. Given 12 or 16 gear combinations, I'm sure I could have found the right ratio for climbing any hill or getting tickets on the highway.

[C10 - C90 Bill]

An Auxiliary would be nice, especially if it had an Overdrive.

[1976gmc20]

Originally Posted by C10 - C90 Bill

An Auxiliary would be nice, especially if it had an Overdrive.

One could always turn it backwards and have several over drives

I drove a big ole 5x3 GMC for a little while (not my truck). I think the "brownie" was under/direct/over. I got along with it just fine; I think I mostly used direct and over as it was a U-shaped pattern. It was a great truck as long as somebody else was buying the gas!

[prateroad]

Old thread but I bought a 71 C50 someone had put a 400 smallblock in it. It sounded good running but when I timed it right, it started knocking. PO had retarded timing so it would labor and not knock for the sale. I thought it was a 350 until further investigation verified the 400 which is externally balanced by the front damper and the flywheel. I don't think GM made a flywheel for a 400 in a medium duty truck. It was shaking itself apart, I do believe. Put a 350 in it. Also found out on another C50 that you shoudn't use a regular pickup 350 engine for a C50 because it doesn't have the steel crank. That big flywheel and the torque load put on it broke the crank just inside the engine at the last journal. Might save someone some headache knowing what I didn't know then.