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Post Info TOPIC: M20 or M21 ?


Addicted!

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M20 or M21 ?


I just finished rebuilding an M21 transmission and got to thinking maybe I should swap out the M20 for the M21. But first maybe I should determine which trans will be a better all around trans for my car. The car has a 3.08 gear Posi the tires are 25.6 inch diameter. The M20 currently in my convertible is an early year wide ratio trans with the 2.56 first gear. The close ratio M21 has the 2.20 first gear. My question is, which transmission would give me the best drivability?



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Canadian Poncho Superstar!

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I'm thinking stick with the M20 for the lower (numerically higher) 1st gear, mainly because of your 3.08 rear gears. But I haven't given your tire diameter any consideration.

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Yes, for sure M20. Even if you had the lowest 14" tire there is (short of that weird low rider stuff!) you wouldn't like an M21 with 3.08. An M20 is a great trans for all around driving. An M21 is for a high revving musclecar with steep gears. An M21 with some steeper gears just begs you to full throttle shift it!

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1966 Strato Chief 2 door, 427 4 speed, 44,000 original miles



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I would think the 21 with the 308 would be an easier driving off the line choice. Not that hard to swap out just to try.



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70 SS ACADIAN wrote:

I would think the 21 with the 308 would be an easier driving off the line choice. Not that hard to swap out just to try.


 It's a 2:20 first gear vs the 2.56 Kevin. Unless you use extra rpm it's tougher to launchconfuse

But you're right, that's the nice thing about a Muncie, the swap is simple.



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1966 Strato Chief 2 door, 427 4 speed, 44,000 original miles



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4SPEED427 wrote:
70 SS ACADIAN wrote:

I would think the 21 with the 308 would be an easier driving off the line choice. Not that hard to swap out just to try.


 It's a 2:20 first gear vs the 2.56 Kevin. Unless you use extra rpm it's tougher to launchconfuse

But you're right, that's the nice thing about a Muncie, the swap is simple.


 There you go, I learned something. I was thinking the 21 had the lower 1st gear. I guess an old guy like me should stick to a power glidebleh



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A Poncho Legend!

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LOL, nah, at least a TH400!!!

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1966 Strato Chief 2 door, 427 4 speed, 44,000 original miles



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If I step on the gas too hard it spins the tires with ease and seems to pull well in every gear. But I am curious to find out the difference between the two.
You are right Kevin, it is an easy swap, if I can change the clutch in less than three hours the trans swap should be quicker. Since concrete does not set well in these freezing temperatures and I am getting another day off tomorrow, maybe I will change it. And I a week or two the roads will be dry enough to test it out.

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Poncho Master!

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My '65 Chevelle had a 3:08 ring gear(now has 3:73) and an M21 and was a lot of of "clutch riding on takeoff". I would stick to the M20

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Addicted!

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torque is key.

my knee-jerk reaction after reading 3.08:1 was that I'd stay away from a 2.20 first gear, because it will be far from ideal.

I am unaware of any division ever putting less than a 3.42:1 behind a M21/M22 ever.
Even with 3.42:1 & 2.20 first, you're looking at a far from ideal 7.524 effective first gear ratio.
Your current (with a 2.56 1st?; most have a 2.52) effective first gear ratio is 7.88, but swapping in the M21 will put your effective first gear at a paltry 6.776.

I used to have a 3.55:1 behind my M22, and I knew it was less than ideal, and for the most part it was fine to drive.
My brother currently has a 3.08:1 behind my old M22 in his car (it's only there temporarily till he can setup another rear with 3.42:1 or better), you definitely have to feather the clutch and gas when starting off...

If you have Buick realm torque, I'd say go for it;
If you feel like swapping transmissions back and forth to see if you like it, go ahead.
I'm guessing you won't like it.




-- Edited by unruhjonny on Tuesday 21st of January 2020 02:18:54 PM

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A Poncho Legend!

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I do have to say though I thought I'd hate the 3.07 gears in my 4200 lb Strato Chief with the M20 and it's fine in 1st gear. However my tire is only 26.4" so that likely helps a little bit.

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Addicted!

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After considering all who have given there advice on the site and a few local car guys I am not going to change transmissions. Aaarg, I need to get back work and leave the car alone, its working fine. Maybe I should get back to taking the dents and scratches out of the extra spinner caps laying around. Thanks to all who chimed in.

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Maybe time to build a third car with all those extra parts wink



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