M20 or M21
 

ok...so i have a M21 which is 2.20 1st gear....This is a 455 with a big ass bottom end cam and I have NO experience with BOTTOM END....always a 400 shifting at 6000+ ....
...Butler/Comp SP Street Performance Cam, Hyd FT, 274/286, 230/236, .488/.491, 112 Hyd CCA-BP6014SP

1800-5400

Shift point is 53--5400 rpm....
? is with a 3600 lb turd 2nd gen would a 3.42 be a but deep

OR

A M20 with 2.54 1st gear and a 3.23? or is that a bit log legged to get it rolling?

I realize a 3.42 is a GOLDEN number ...but I do not have that on the shelf. I have 3.23, 3.73. and 4.56...

M20 or M21
 

GM was thinking of avoiding Warranty issues. M21 is stronger then an M20 due to internal stresses of the trans. The gears are pushing each other away trying to split the case.

I don’t believe Pontiac ever put an M20 behind a 455. When they switched back to T10’s in 75, they always used 2.43 first gear on the 400’s. I’m guessing for the same reason.




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Be aware that there are spline differences between the early and late Muncie’s .
The early had a 10 spline input and small output, and the late one used a 26 spline input and the big TB400 output.

My Nash 5 speed is a 26 spline. Just order a 11" 26 spline clutch disc and you are good to go. Same deal with the output shaft yoke. I had a custom heavy duty driveshaft made with the good joints that matched up to the Ford 9" and the 400 Yoke on the trans output.

Only difference will be the rear axle he plans to run and the u-joint there.

Tom V.

For all around driving, if I really have to go with a 4 speed, I'm an M-20 guy and then gear the car mild. I really don't like M-21's unless I'm running a 3.73 minimum.

But sort of sounds like your mind is made up for you if you already have an M-21 sitting there.

Personally if it were my choice I'd just run a TKX and ditch the Muncie stuff.

This.

I’m in the process of scoping out a stroker engine for my ‘70 TA, which has a 400/M21/3.55 from factory. I’m pretty sure that I am going to add the TKX to my build and stock the original M21 under the bench with the original engine.


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this is a throw together job to get going down the street..It is based off of what I HAVE or can trade out. :)...I HAVE a M21 (fully rebuilt) that i drove before with my 400 and it SUCKED....even with the 3.73's...ended up putting in 4.56's. I defense of that statement....It was a good combo for point straight and SEND IT. Just not a very civil driving car.

If you have it, why not? Should be fun. First gear is a little tall but a 455 with a cam like that should be fine.

Unless you think you can sell it or trade it and get an M20 instead without too much cost or hassle, in which case get the wide ratio for sure.

ok so after putting all the numbers together I was surprised to learn how close the M20 and M21 are. I do the short story...so we know 1st gear is different from 2.56 vs 2.20 BUT...the astonishing part is the shift recovery from 1-2 = SAME. The shift recovery from 2-3 = SAME. Now the shift recovery from 3-4 in the M21 is same as 1-2, 2-3, while the 3-4 shift in the M20 is a little bit larger step at .675 (m20) vs the .781 for the M21.

Now all of this is just numbers, NOTE with the M21 the recovery may be the same BUT you will be going abut 6-9 MPH faster as 1st gear has longer legs. Looks like it will be the M21 (have complete) and the 3.73 gear (have and has been ran in THIS 10 bolt before). While this combo puts me cruising @ 60 mph @ 2650 ish RPM I don't expect to be doing any cross country trips anyway (in this configuration)

Again all of this comes from MY previous experience of my 400, M21, 3.73 combo that was NOT the FUN driver I had hoped. I believe the issue then was the 400 with it's RAIV cam and STUFF...really did not turn ON till 3200 rpm. Anybody who has had one will understand the ON part because it's like a SWITCH turning on the POWER and then pull up to the 6400 shift point. (numbers indicate about 425 max hp and 425 ish torque around 4000rpm) My 455 current build SHOULD be closer to 425 torque @ 2000 rpm and 500 from 3000-5000 rpm so a DIFFERENT driving experience for sure?

Given the dramatic difference in builds....anybody else been down this road

The bigger the engine, the smaller the cam, the less important the 1st gear choice.
67-- your torque should be closer to 540 lbs.

Yes only difference is the input gear so the proportions stay the same. 21 is best with a small high winding motor with a lotta cam so stays near the torque peak and a short axle. Best for speed shifts. 20 is better for a larger slower winding engine with a long axle.

I have a -20 with a warmed over 400 and 3.55 4 pinion. I works for me.

let's make this real simple. are there better transmission options? sure. are you gonna have plenty of fun with that m21 behind a 455? dayum straight you are.

I have a plain 455, 3:23, and M21.........plenty of fun.

George

Way back before calculators it was much more of a chore to compute out gear ratios, but I was getting ready to order my 67 and wanted acceleration but was aware of the long commutes that were necessary so I trudged through pages of multiplying things out to find the best compromise.

First three gears with a 3.36 rear with a wide ratio Muncie are the same as the first three gears with a 3.90 rear and the close ratio. So I had three gears to stay beside the close ratios that were out there, and to top it off I had the first three gears lower than the guys that order the 3.55 and close ratio. The good was basically was a good freeway friendly 4th gear -- but I better be out on the 3.90 equipped car because there was a massive drop between my 3rd and 4th gears.

Fast forward 50 plus years and first gear feels too high in traffic and high gear just isn't cutting it and I'm stuck in the slower lanes. Did an upgrade to the TKO-600 which gave a lower low and a higher high and brought the car into 2021. Now it feels like I have 4.56's in low and 2.56's in high.

Lust4speed is correct.

Having both M-20's and M-21's I can tell you, for an M-21 to be as quick you need to have a pretty good gear in the back. The M-20's are just quicker and you don't need as much rear gear to do it. By the time you're rolling 80+ mph for 4th gear, the rpm drop in the M-20 is less of an issue anyway with momentum.
I've run that setup with SBC's and even with less torque, the M-20 going into high gear was never an issue. And the nice thing about it, since I didn't need as much rear gear with it, it was a better highway cruiser than the M-21.


But like I mentioned earlier and Lust4speed has touched on, the TKO is a much better choice that covers both ends of the spectrum and will completely change the car and the driving experience.

I understand you'll have to do what your budget allows.

My vote is a 3.23 rear gear and an M-20. Pontiac sold 326 powered cars with 3.08 gears and the same trans. Your 455 should be able to tough it out. I yanked the 3.36 gears out of my '67 GTO about 15 years ago and swapped in a 2.56 posi......I love cruising 80 mph at 2450 rpm and getting 21 mpg while doing it. My days of screaming along in the slow lane getting passed by Hondas and Subarus getting 10mpg and running hot are far behind me and I have no desire to return to them.

"M20, 3.23 for all around best driving. 2.20 first? what the he!! was GM thinking?"

Back in the day drag racing was king 2.20 first gear also provided less of a gear spread RPM drop between gears and the 2.20 first ratio also required a taller rear gear which was desireable for 4th gear acelleration

It was a different time thats why 2.20 first was mainly in your flagship GM go fast cars

Gas mileage and weaker low torque 4 cylinders brought in the 5 + speeds just to make those cars work then the 5 + speeds were used in more powerful cars, evolution.

Now this is news to me! The M21 with its close ratio gears is FASTER than the M20. I've had two cars. a M20 with a 3.55 rear and a M21 with a 3.90 gear. The shift from 3-4 really dropped the rpms' in the M20 where it was not nearly as bad with the M21. Don't let anyone kid you, the M21 with the right rear gear will keep the rpm's up in the HP range better than a M20 and is faster. Much more desirable Muncie. If you get a chance to read the 4-part series on Project Judge in the 1969 Popular Hot-rodding magazine, they used both Muncie trans. The M21 was faster and they go into reasons why as they were building up to a RAIV engine and drag car from originally a RAIII 3 spd car. Generally speaking, M21 trans are more expensive from collectors than a M20 for obvious reasons.

Like we talked about, first three gears are pretty equal and that takes you up to about 84 MPH. No argument that the close ratio has the advantage on the dragstrip, but even today most stoplight to stoplight races are over before finding 4th. Besides, if the race is long enough the 3.90 car is going to run out of gear and the 3.36 car is going to drive right on by.

M20 with 3.36 rear and wide ratio Muncie
9.07 1st gear total multiplication
6.32 2nd gear
4.91 3rd gear
3.36 4th gear

M21 with 3.90 rear and close ratio Muncie
8.58 1st gear total multiplication
6.40 2nd gear
4.99 3rd gear
3.90 4th gear