I have a 1953 chevy pickup that I am putting a 383 stroker (450-500hp) and a 10 bolt with a 3.73:1 posi. I am wanting to put a manual in it, but I don't really care about the overdrive. I was thinking a muncie would be a good choice. The truck is going to be a daily driver with some occassional bracket racing. I have read lots of post on here and on the internet, but I still cannot decide which would be better. The muncie m20 or m21? So, here I am asking to see what you guys think. Thanks, Jeff
No real difference in strength, just in the ratios. In general, street driving is better with the wide ratio trans unless you've got rear gears in the 4.11 and up range.
they are pretty much the same trans..only diffrence is the gearing .one is close ratio and the other is wide ratio...
M20 wide ratio, like link states works well with 3 series diff. Also the first gear 2.52 X 3.73 (rear gear) gives you 9.3996 make for a nice launch at your day at the drags.
IIRC the m-20 has a 7/8" countershaft while the m-21 has a 1", possibly a little strength advantage to the m-21. the case / countershaft fit is the weakspot on these tranny's my .02 dave
The Muncie is a decent trans and is pretty strong for its weight. Although it can be broken with less power than you will have, the driver is a major factor in how durable any trans is. You could get by with either, but..... The M20 has a fairly high first gear of 2.5:1. The M21 only has a 2.2 first gear! For a strert car, especially one with a 3.73 axle ratio, the M20 would be a better compromise. All things being equal, the higher the first gear, the higher the max torque capacity of the trans. That means the M21 is slightly stronger than the M20.
I may be remembering incorrectly, but I don't recall the countershaft diameter being different on the M20 vs M21. I believe GM increased the countershaft diameter on both versions of the trans at some point in time?
500hp to a 10 bolt rearend? Honestly, it's not the transmission I'd be worried about. I have an M-20 for one of my cars and that's what I wanted, better for highway driving I figure.
This link has a little bit more info about the shafts, and identification, but I would probably get a unit with a 1" shaft, that way you could always throw some M22 gears in it at a later date, if you wanted. http://www.5speeds.com/muncie2.htm
in 1966, GM went to a 1" countershaft for the Muncie transmission. Technically, the M21 is a stronger unit since the input shaft is slightly larger and can take slightly more abuse. I would go with a 1966 or newer M20 for your setup and be done with it.
Thanks everyone for the input. I know they are different transmissions, one is wide ratio and the other is close ratio. I was leaning more toward the M20 from all of my research I've done, but I am still pretty new to these muncies so I wanted to get some professional opinions. I think I will go with the M20 due to the ratios and the fact that this will be more of a street truck (daily driver). Everyone says the 10 bolt is going to be the weak point, but I have had good luck with them. I have completely gone through this rearend and put all good strong stuff in it. So far this rear has been amazing to me. Maybe one day it will fail me, but for now it will get me by. Thanks Everyone, Jeff
About the 10-bolt; Things like the vehicle's weight, how it is driven, how much traction it has, how much torque the engine has, the parts the axle was built with, all have a big influence on durability. As to the driver party off the equation, I have seen race drivers consistently faster and winning with parts another driver would break or use up during the race.
The Jalopy Journal
