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Thread: M20 or M21

  1. #1
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    Question 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 ?
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  2. #2
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    Default Re: M20 or M21

    Quote Originally Posted by StratoBlue72 View Post
    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.
    Greg -
    - - -
    1970 GS Stage 1 (owned August 1980) 462 Edelbrock heads, TA212, 4 wheel disc, Turbo 200-4R, close ratio steering, tubular front arms, 3.73 TrueTrac
    1971 GS 350 (8/16/14) 4 speed, 91k stock, converted to front discs and tubular uppers

  3. #3
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    Default Re: M20 or M21

    Quote Originally Posted by knucklebusted View Post
    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.

    Yes, 71 and up came with the T-400 output.

    As fas as which one- M20 or 21, it all depends on the rear gearing. You wouldn't want a 21 with a 3.08 gear
    Jason Cook
    BPG #1675
    "Expert in obsolete technology"

    1968 Riviera GS- some assembly required
    1969 Riviera GS- my first car
    1969 GS-350
    1970 Stage 1 4 speed Aquamist
    1970 GS-455 Bamboo Cream beater
    1971 GS 455 11.93@113.36
    1972 GS-350
    1970 Stellar Industries vintage go kart

    Deo duce, ferro comitante

  4. #4
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    Default Re: M20 or M21

    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
    1969 Special Deluxe L-77 high compression 350 13.12@102 Pure Stock Drag Car with Iron intake and exhaust manifolds.
    New low E.T. 13.02@102 Fall Martin F.A.S.T. 10-15-16
    Just because it hasn't been done, doesn't mean that it CAN'T be done!
    *NEW* low E.T. 13.01@101 Osceola Dragway 10-23-16


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  5. #5
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    Default Re: M20 or M21

    What was the gear option that got you an m21 over the m20?
    1970 Buick GS.....under resurrection

  6. #6
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    Default Re: M20 or M21

    Quote Originally Posted by Tom Miller View Post
    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
    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.
    Greg -
    - - -
    1970 GS Stage 1 (owned August 1980) 462 Edelbrock heads, TA212, 4 wheel disc, Turbo 200-4R, close ratio steering, tubular front arms, 3.73 TrueTrac
    1971 GS 350 (8/16/14) 4 speed, 91k stock, converted to front discs and tubular uppers

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    Default Re: M20 or M21

    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.
    Shawn
    "I drive too fast to worry about my cholesterol..."
    -72 Skylark- Project "Rusty Hulk"- 462ci 4 speed street bruiser
    Build Thread: http://www.v8buick.com/showthread.php?293656-Build-Thread-Project-Rusty-Hulk
    -72 Skylark- The wifes ride, " Elsa " - bone stock and classy cruiser
    BPG #2288 (RIP)
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  8. #8
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    Default Re: M20 or M21

    Quote Originally Posted by buick64203 View Post
    Yes, 71 and up came with the T-400 output.

    As fas as which one- M20 or 21, it all depends on the rear gearing. You wouldn't want a 21 with a 3.08 gear
    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 ?
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  9. #9
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    Default Re: M20 or M21

    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 ?
    REMEMBER TO SAVE YOUR ALUMINUM CANS FOR RECYCLING (Ford is counting on you to help build more F-Series pickups)

  10. #10
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    Default Re: M20 or M21

    Quote Originally Posted by StratoBlue72 View Post
    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.
    Greg -
    - - -
    1970 GS Stage 1 (owned August 1980) 462 Edelbrock heads, TA212, 4 wheel disc, Turbo 200-4R, close ratio steering, tubular front arms, 3.73 TrueTrac
    1971 GS 350 (8/16/14) 4 speed, 91k stock, converted to front discs and tubular uppers

  11. #11
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    Default Re: M20 or 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.

  12. #12
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    Default Re: M20 or M21

    Quote Originally Posted by StratoBlue72 View Post
    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 ?
    Yes.
    Jason Cook
    BPG #1675
    "Expert in obsolete technology"

    1968 Riviera GS- some assembly required
    1969 Riviera GS- my first car
    1969 GS-350
    1970 Stage 1 4 speed Aquamist
    1970 GS-455 Bamboo Cream beater
    1971 GS 455 11.93@113.36
    1972 GS-350
    1970 Stellar Industries vintage go kart

    Deo duce, ferro comitante

  13. Default Re: M20 or M21

    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.

  14. #14
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    Default Re: M20 or M21

    Quote Originally Posted by philbquick View Post
    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.
    REMEMBER TO SAVE YOUR ALUMINUM CANS FOR RECYCLING (Ford is counting on you to help build more F-Series pickups)

  15. #15
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    Default Re: M20 or M21

    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.

  16. #16
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    Default Re: M20 or M21

    Quote Originally Posted by N360LL View Post
    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.
    While I generally agree that deviating from factory generally debases the car, there are a select few modifications or deviations from factory that make the car more valuable. On the top of the list is converting a bench seat, column shift GS to a 4 speed. Aside from making a hole in the trans tunnel. the rest of it all bolts in so the OP isn't hacking it up so to speak. Im sure the OE/ numbers matching trans will be sold with the car if it ever gets sold. And if the trans isn't original to the car, even more of a reason to do it.

    Everyone wants a 4 speed car, even if its not original.
    Jason Cook
    BPG #1675
    "Expert in obsolete technology"

    1968 Riviera GS- some assembly required
    1969 Riviera GS- my first car
    1969 GS-350
    1970 Stage 1 4 speed Aquamist
    1970 GS-455 Bamboo Cream beater
    1971 GS 455 11.93@113.36
    1972 GS-350
    1970 Stellar Industries vintage go kart

    Deo duce, ferro comitante

  17. #17
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    Default Re: M20 or M21

    Quote Originally Posted by knucklebusted View Post
    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.
    .
    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.
    My name is Jeff, and I am in the running for "longest time to complete a restoration" (started around 1980).
    '69 GS400, 4-speed, a decent amount of Kenne-Bell in the engine.

 

 

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