M20 or M21

I'm thinking about making my '72 Stage 1 conv. a 4 spd., and want to make it as authentic of a conversion as possible.
Which trans. would absolutely be the correct one for this application ? Any difference between a/c and non a/c as to which trans. is correct ?

 

I believe it would have come with a large output shaft M21. My 1971 350 GS is an M21 with the large output shaft and fine spline input.

 

Weight plays as much a part as gearing, and camshaft engine rpm range.
My 73 Stage 1 4 speed had 3.42 gears, but I ran an M20. The low 1st gear in a 4200+lbs car was fantastic. Ran the car at the Pure Stock Drags, ran 13.89 with it. Car was much more responsive on the street around town at lower speeds(where the car spends the most time). Personally, I would run the M20, I think you will have a much better driving car that will be responsive and fun to drive.
I wouldn't run a M21 with anything less than a 3.73 gear. Hi

 

What was the gear option that got you an m21 over the m20?

 

Yes, my 1971 GS 350 has a 3.08 rear with an M21 and it sucks from a dead stop plus you can skip any gear without consequences. I go from 1st to 3rd lots of times. If I gear down to 2nd, I go straight to 4th on the up-shift.

If I could find a reasonable, decent priced M20 fine spline input/large output, I'd buy it.

 

I've got a wide ratio with a 3.42 rear and I often times feel 1st is a bit on the short side to be honest. But when you wanna take off and go to red line from a dig, you'd never guess it was a mild lil 3.42 out back! All the balls you need in first, for sure. Heck, when I'm feeling mellow I take off in second.

 

Mine is a 3.42, so did all the 71-2's with a 3.42 and all the 70's with 3.42-3.64 come with M-21's ?

 

I never did pay any attention to what transmission is in our '72 GS 350 4spd. It has a 3.08 rear, so would it have an M20 ?

 

Was it a little weak off the line? Did it go 40MPH in 1st before you hit redline? Could you skip second gear and go to third? If so, it was an M21.

 

To the best of my knowledge if a Muncie transmission was installed in a Buick at the factory it was the M-21.

Chevy and Pontiac used the M-20 in their lower hp & tq cars mainly with the small cube engines and some lower hp big cube engines.
The M-20 internally is not as strong as the M-21 & M-22 due to the gear sizing to create the 2.52 1st gear ratio.
The replacement gears that are available today are stronger due to 40 years of improvement in metallurgy.

The M-20 has a 1st gear ratio of 2.52 which makes it a nice choice for the street especially when the rear axle has less than a 3.42 gear.
The M-21 has a 1st gear ratio of 2.22.

To determine the effective 1st gear final drive ratio multiply the 1st gear ratio with the rear axle ratio.

2.22 x 3.08 = 6.84 final drive ratio
2.22 x 3.42 = 7.59
2.52 x 3.08 = 7.76
2.52 x 3.42 = 8.62

As you can see the M-20 with it's low 1st gear helps compensate for the numerically low rear axle/highway gears to help get the car moving from a dead stop.

I have used the M-20, M-21 & M-22 with rear axle ratios from 2.56 - 4.11 in my 455 powered GS.
The M-20 does not like mass doses of tq. I have broken more than one M-20 due to 3000 rpm power shifts from 1st to 2nd.

Also it is very important to use the proper GL-4 gear lube. I suggest using the Brag Penn or equivalent specialty gear lube.
The GL-4/GL-5 gear lube on the shelf of your local parts store is not the same. It DOES NOT have the additives included that protects the brass blocking rings.
I WILL NOT use fully synthetic gear lube in my Muncie's. I have broken 2 different Muncie's that had the synthetic gear lube within 1 week of installation.

 

My 70 Vette had a 3.70 rear and an M21 and I still didn't have enough 1st gear. There was almost no difference between 2nd and 3rd.

Converting an M20 to an M21 only requires changing the input and cluster (only the front gear is different) gears, and yes, 1st, 2nd and 3rd ratios all change since they all go through those 2 gears. There is no difference between the strength of an M20 and M21 since they use the same gears. The M22 is stronger since it has straighter cut gears and all have the 400 output shaft.

Close ratio trans' are really for drag racing with a lot of rear gear, 4.11+, and, in my opinion, are not suited for street use.

As far as authenticity is concerned, they both look the same, YOU need to drive it.

 

I've got a '70 Chevelle LS5 with an M22, geared 3.31 and I don't really care for the tall first and reverse in it. And like you say, almost no difference between 2nd and 3rd.

 

I'm going to throw cold water on this. IMHO, converting a rare car from factory specification to something else is not vise, either now or in the future. The loss in value moving forward is significant. Yes, I know they built a few 4-speed Stage 1 Convertibles in '72 and they are almost impossible to find, but they exist. The same goes for '71 and to a lesser degree the '70 and '69 cars as well.

From a practical standpoint, there is a lot of part gathering that needs to be done and then modification of the car to make it all work and look correct. Some of the parts are reproduced some not.

In the end it is your choice about what you do with your car. And if you choose to go this route why limit your choices to M20, M21 and M22. Why not a Richmond 5 or 6-speed transmission? After all they made a few 3-speed GS 455 and Stage 1 convertibles in '70.

 

Once I was 3 hours from home in my 69 GS400 when something catastrophic happened inside my M21. I had NOTHING but 4th gear. It wouldn't even go into reverse or neutral. I had 4th, or I could depress the clutch pedal, that's all.

I made it home because with a 3.73 rear and a torquey engine, I could still take off from a standstill when the lights turned green.

I just needed to make sure that I didn't get to a place where I needed to back up.