thoughts on using a muncie 4 speed in a c20

[doowopper]

Currently my truck has the SM465 trans that it was born with. I'm planning to swap in an LS that has an auto attached to it. I prefer a manual trans. Would a Muncie hold up under heavy loads and pulling a trailer?
TIA. Buddy

[68 C10 Driver]

Doowopper, I am putting a Muncie M20 4 speed in my truck to replace the 3 speed it had. I know the Muncies are great transmissions, I don't know about the "pulling heavy loads". How heavy a load are we talking? I guess it would all depend upon what the diff gears are in the rear end. Is your Muncie a M20 or M21?

I am getting rid of the 4.11 rear end gears in favor of something more highway friendly.

[doowopper]

I don't know what model it is yet. It's my son's. He's checking. Being a C20 I'm not bashful about loading it up with whatever needs hauling. I'd hate to go to the trouble of putting it in only to find out it's a weak link. I've got 4:11s.

[special-K]

I was going to say "Depends on how you'll use the truck". Now I see what you want to do and I don't recommend one. They are strong transmissions, but what you have is better. If I was stepping away from the engines these trucks were built to have I would also go with a modern transmission. I'd run what you have or even better, gain O/D to go with your modern motor, by using an NV4500 or newer manual. Or, go automatic with a bulletproof 4L80E

[LockDoc]

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Don't know if you have seen this thread or not but here is how I replaced the sm465 in my '72 Dually.

http://67-72chevytrucks.com/vboard/s...d.php?t=673588

Actually a pretty easy swap, using all stock parts except for the transmission......

LockDoc

[68Gold/white]

My good friend had a 66 C-10, LWB with a steel plate bed floor, it weighed a little over 5000 lbs.
It had a 400 small block that another engine builder friend built. Had a 3.42 8.5 rear adapted to the 66, coil springs. The truck owner was a legit machinist. He owned a machine shop. He could make anything.

The engine builder told the 66 owner that he was concerned about the pickup being a little too heavy for the transmission. The M20 was used because he knew it would not be abused. It did get a lot of spirited driving. Ran like a bat outta Hooterville!!!

A C20 takes more power just to power it down the road through the drive train, the towing of hauling a load would be too much for a passenger car C20.

Sooo my answer is also No....

[57taskforce]

Keep the 465 if you are going to use it as a truck.

[doowopper]

`Thanks for the replies gents.

[ghackett1]

As stated think about the use of the truck. I’ve got a Muncie M22 4 speed in my 68 and I love it but I don’t heavy or even medium duty haul in it, more of a scooting around town on good weather days truck — so for that it’s works great.

[72c20customcamper]

What trans is on the LS ? If it were mine I would go with 4l80 shifting while towing isn’t fun specially with a larger trailer . The 465 is a great transmission it’s made to be abused . The wide ratio M20 was for general street use can handle a fair amount of hp and torque

[Steeveedee]

If you use an M22 "Rock Crusher" you should be OK. They can take a lot of torque. They are pretty noisy, though that shouldn't matter in a truck. To some extent.

[KQQL IT]

2.56 1st vs a 3.56 1st with a 6.56 low gear.

[PbFut]

If it's a tow rig, go auto. Auto will add a bit more computer work. If you want manual go Tremic. Up to 6 gears, overdrive and very strong. The Muncie will be at capacity. A small cammed square port LS will eat up a Muncie unless you baby it.

[72c20customcamper]

Originally Posted by PbFut

If it's a tow rig, go auto. Auto will add a bit more computer work. If you want manual go Tremic. Up to 6 gears, overdrive and very strong. The Muncie will be at capacity. A small cammed square port LS will eat up a Muncie unless you baby it.

Never heard that . I’m running a Muncie behind a heavy cammed high compression 496 big block with 570 Hp and 540 pounds ft in my Chevelle trans is happy as a lark . A Muncie can handle upwards of 600 lbs torque depending on the input shaft which is the weak link . If you’re doing 5 k holeshots you probably will break and ear off . But I don’t thing many do that anymore with a 50 year old transmission

[68 C10 Driver]

My M20 set-up will be for highway cruising and maybe some light hauling, nothing that would over-task the Muncie. While my truck will essentially be a new truck when done, the hard use 'farm truck' application will be long over.

[Steeveedee]

In '70, one could buy a Cadillac Calais coupe with a 4-speed Muncie. The weight of that car is more than the weight of a 1/2T truck- 4600+ pounds. The base weight for a 1/2T fleetside long bed is 3600 pounds.

[PbFut]

Originally Posted by 72c20customcamper

Never heard that . I’m running a Muncie behind a heavy cammed high compression 496 big block with 570 Hp and 540 pounds ft in my Chevelle trans is happy as a lark . A Muncie can handle upwards of 600 lbs torque depending on the input shaft which is the weak link . If you’re doing 5 k holeshots you probably will break and ear off . But I don’t thing many do that anymore with a 50 year old transmission

Yeah probably over critical... I had a M21 in the race car and cleaned the teeth off 3rd gear first time out. Hot sticky Hoosiers place way more shock load on things. Went to a Jerico dog box with straight cut gears which is a very beefed up modified Muncie. I still had issues but it was probably my bad foot work in the heat of battle. The dog rings made things fun and fast. 440hp/tq in 2600 lb road race car. Lots of fun but don't miss all the repair bills. A shot of leftovers. All cracked or chipped. Sorry a little OT

[doowopper]

I've decided to stick with the slush box unless I happen to find an NV4500. Again, thanks for the input

[jumpsoffrock]

Originally Posted by doowopper

I've decided to stick with the slush box unless I happen to find an NV4500. Again, thanks for the input

https://www.ebay.com/itm/20441856852...cAAOSwrXxkzVvJ

This guy is in Ontario, I bought one from him in person, I slowly installed in my truck and drove it half a mile, got it into 5th, no issues. However I did not drive it enough to be 100% confident. I will wait and see.

It is fairly expensive kinda, but he seems to know what he's talking about. He was almost insistent that he opened the trans up for me, show me everything, and then he carefully showed my how he puts the top cover back on.


The bad news is that the original trans he had that I initially wanted ended up having a very small chip in a tooth of the input shaft, and he refused to sell it to me.
He only had one other to sell me.

Then he found some unknown issue with the shift tower in trans #2.
So he swapped the trans tower from #1 onto trans #2, made sure it worked ok, and away I went.

He was selling this transmission as a drop-in ready-to-run trans. I asked him that in person and he confidently said yes.