i’m putting together a 67 camaro and i have 2 muncie 4 spd transmissions that i can put in. as far as i can tell one is an M-20 (late 1966) the other is an m-21 (around a 1968). both are course spline input shafts if that makes a difference. now i really am just wondering which is the better transmission, which will last longer or if they are the same internally and it makes no difference at all. my car is going to have a 350 in it and between 3.23 and 3.73 rear end (not sure just yet). its not a racecar by any means, but i do want to have some fun with it. thanks
me and my friend jeff did a tranny swap on his uncle’s 1970 Chevelle SS. He had it worked on by some shop out in java i believe. The guy took it out for a test drive and apparently beat the shit out of it, long story short he blew up the Muncie M21 that was in it and split the case down the side and blew a big hole in it. The chevelle was a stock 396 car and until this time, was numbers matching and never restored with 21k orig miles. What a shame. The Muncie is physically a small transmission and didnt seem like it held up very well, although i am no expert by any means.
M20 = generic model
M21 = better, close ratio
M22 = best “rock crusher”
The primary Muncie 4-speed transmission was marketed under Regular Production Option (RPO) M20. M20 was the generic RPO for a 4-speed, so note that the Saginaw 4-speed was also marketed under RPO M20, but with slightly different gear ratios in a significantly different case. In addition to the M20, a close-ratio 4-speed was sold as M21, and a Heavy-Duty close-ratio 4-speed was sold (at a much higher price and much more limited application) as M22. The buyer had no choice as to the transmission manufacturer; use of the aluminum-case Muncie or the cast-iron-case Saginaw was determined for you by the factory as a function of your model. M21 and M22 transmissions were even more limited to specific models, with details of the limitation depending upon the model year.
The Camaro 4-speed manual transmissions ratios for both Muncie and Saginaw, with other Muncie data, are as follows:
Maincase Gear Ratios Input Grooves Cluster Output
RPO Years Casting 1st 2nd 3rd 4th Spline (Input) Pin SplineMuncie
M20 63-65 3851325 2.56 1.91 1.48 1.00 10 None 7/8-inch 27
M20 66-67 3885010 2.52 1.88 1.46 1.00 10 2 1-inch 27
M20 68-69 3925660 2.52 1.88 1.46 1.00 10 2 1-inch 27
M20 70 3925661 2.52 1.88 1.46 1.00 10 2 1-inch 27
M20 71-74 3925661 2.52 1.88 1.46 1.00 26 2 1-inch 32
M21 63-65 3851325 2.20 1.64 1.28 1.00 10 1 7/8-inch 27
M21 66-67 3885010 2.20 1.64 1.28 1.00 10 1 1-inch 27
M21 68-69 3925660 2.20 1.64 1.28 1.00 10 1 1-inch 27
M21 70 3925661 2.20 1.64 1.28 1.00 10 1 1-inch 27
M21 71-74 3925661 2.20 1.64 1.28 1.00 26 1 1-inch 32
M22 65 removed 2.20 1.64 1.28 1.00 10 None 1-inch 27
M22 66-67 3885010 2.20 1.64 1.28 1.00 10 None 1-inch 27
M22 68-69 3925660 2.20 1.64 1.28 1.00 10 None 1-inch 27
M22 70* 3925661 2.20 1.64 1.28 1.00 10 None 1-inch 27
M22 71-74 3925661 2.20 1.64 1.28 1.00 26 None 1-inch 32
*Colvin reports that 454 Chevelle M22 in 1970 used the 1971-74 splines
Saginaw
M20 6-cyl 3.11 2.20 1.47 1.00
M20 V8 2.54 1.80 1.44 1.00
Sell them both for a premium and buy a T56 w/bellhousing adapter?
I had the M22 rock crusher in my old GTO. You couldn’t hurt it. Find one of those.
+1431 for the Rock Crusher. It’s a beast.
I remember this case. It came through my office.
You cant beat the gear whine of an M22 rock crusher- its music to the ears. the gears are at a different angle making the noise and being stronger
For the rear gear ratios he’s talking about you don’t want a M21 or M22. The 2.20 first gear is too tall. Close ratio trans generally only came from the factory with 3.73 or lower rear gears. Longer rear gears came with a wide ratio trans.
My firebird has the factory 2.43 trans with 3.23 rear. It’s kind of tough at low speeds even though it only has 27 inch tires. Fortunately the Pontiac engine has lots of torque down low.
If you 350 is going to have a lot of cam, I wouldn’t recommend a close ratio trans unless you are going 3.73 or 4.11. Otherwise you’ll have to slip the shit out of the clutch to get it going. Not to mention parking lots will be a pain in the ass.
so besides the ratio difference, the m-20 is not really weaker than the m-21 so there is no real advantage strength wise. if someone has a good deal on an m-22 i’d go for it, even if i have to put 3.73’s in. as for my engine setup, the engine is 30 over, the cam is a speed pro sealed power CS-179R)-valve lift .447, .050 dur:222. hydrolic lifters(flat tappet), right now it has a set of 76cc heads on, but if i can get the cash together i have a set of 1969 041 heads (64cc i think) that just need some freshening up. edelbrock performer with a 4 barrel holley. unless i can get a really good deal on a 12 bolt, i will be going with a 10 bolt for awhile also.
Pretty sure the only real differences in the -20 and -21 was the gear ratio, but the pre-'71 boxes weren’t particularly strong. They’ll work fine for a mild street application, but a '94-up T56 out of an F-body might be worth looking for if you’re not restoring the car to stock.
The T56 isn’t the be-all-end-all, and neither is the Muncie.
Muncie 4 speeds, as with many geared systems, often fail when the separating forces disrupt the gear mesh excessively. A somewhat weak case causes this. A Supercase and iron mid plate will greatly improve the strength.
I myself am a 4 speed fan. The shift feel of a six speed is nothing like it. The 4 speed is lighter and an easier install. The six speed is more economical if you are doing a lot of highway cruising. T56 is stronger bone stock, but both boxes can be built quite strong. And both can be broken, especially with slicks.
If you want to spend $600 or so on new gears you can make whatever you want. You can buy an M22 gearset. You could also buy what they call a M22 wide which is M20 ratios with ~20 degree helix angle like an M22.
As for your original question, they are probably pretty close. I don’t have the gear geometry handy to take a close look at the gears. A smaller input gear could be a detriment to the M20 over the M21, but afaik the input/main drive gear aren’t a common failure mode. Since they are on the low torque side of the box and so close to the bearings one wouldn’t expect them to be.
Probably for what you intend to do with this car any of the Muncie four speeds mentioned will be fine. If you are going to beat the crap out of it and launch on slicks I’d recommend M22 gears, supercase, and iron midplate. Or spend ~$7g on a Jerico 
Damn, he know’s his shit^:)