Topic: M20 or M21?
in Forum: C3 Driveline Components
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Houston, TX - USA
Joined: 2/7/2008
Posts: 93
Vette(s): 1970 454 4 speed A/C power everything else
I have a 70 big block 4 speed car.............it does not have a transmission in it and I dont know the rear end gear.................no build sheet yet.
Does anyone have an opinion on going with the M20 for the lower first gear or with the M21....................I'm pretty sure the car had a M21 in it originally.
Thanks
Does anyone have an opinion on going with the M20 for the lower first gear or with the M21....................I'm pretty sure the car had a M21 in it originally.
Thanks
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The big block would work fine with the wide ratio trans(M20) and the lower 1st gear ratio. It doesn't really need the extra "help" that the lower ratio would give, but it wouldn't hurt either. Besides, M20s are a little easier to find than an M21.
It depends on what you intend to do with the car once you have it finished.
To find the info on your differential, get the numbers, and post them here, and we'll de-code it for you.
Click image for linkAdams' Apple 2008-02-21 20:48:31
It depends on what you intend to do with the car once you have it finished.
To find the info on your differential, get the numbers, and post them here, and we'll de-code it for you.
Click image for linkJoel Adams
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Former Member
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Houston, TX - USA
Joined: 2/7/2008
Posts: 93
Vette(s): 1970 454 4 speed A/C power everything else
Thanks for the help, i will let you know the code
According to the info that I have in a 454/390
wide Ratio (M20) Wide Ratio (M21)
Rear gaer
Std - 3.08 Std - 3.36
Opitional - 3.36 Opitional - 3.08 3.55 3.70
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Mounds View, MN - USA
Joined: 5/24/2007
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Vette(s): 70 LT1 coupe, 69 350 HP coupe, 69 390HP 427 coupe, 71 LS5 convert, 85 coupe, 93 coupe
Since your car is 38 years old, I would count the rear gear ratio. Jack up the rear end so both tires spin freely. Turn the rear wheels one full turn and count the turns of the drive shaft. To be real accurate, spin the wheels ten turns and divide.
Most big block cars came with the close ratio, since they didn't need the deeper low, and it is easier to drive the close ratio. But Joel is right, you can't go wrong with either. Someone is reproducing complete Muncie transmissions in M20, M21, and M22s with all new parts.
Most big block cars came with the close ratio, since they didn't need the deeper low, and it is easier to drive the close ratio. But Joel is right, you can't go wrong with either. Someone is reproducing complete Muncie transmissions in M20, M21, and M22s with all new parts.
in Forum: C3 Driveline Components
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