I have a factory 72 GS 455 four speed car. It does not have matching numbers drivetrain. I just blew my Muncie trans fourth gear is fried and other problems as well as a cracked front case. I brought to a friend who will rebuild it for me. I thought I had an M21 but he tells me it is an M20 wide ratio I will be buying a new gear set so my question is what gear set should I buy, the M20 or M21? The car has the 3.42 rear end gears. What gear set came with my car from the factory? Thanks Tim
I'd guess it came with an M21 close ratio from the factory. My 71 350 had an M21 with 3.08 gears. I've recently swapped it for an M20 with the lower first gear and love it. With 3.42 gears and a 455, you could probably get by with an M21 but I'd prefer an M20 because with the torque of a 455, you won't need a close ratio transmission. Do you know if it was a fine spline input shaft? If so, that is a later model transmission, generally 71 or later.
Orig. would've been an M-21. This may help. https://www.5speeds.com/muncie3.htm The big difference looks like the 3-4 spread. Other than that, the M-20 definitely looks better esp. for numerically lower rears. I had 3.73 in my old 350 4-sp. w/ an M-21 and still felt like a taller 1st would've helped. I'd have loved to bought your car when it was for sale btw. I'm building another 350 4-sp. GS to replace my old one and had too many cars already.
I'm considering 4 wheel discs next. Would love to stumble upon a tilt wheel. I would have loved an even wider ratio Super T-10 but the M20 is great.
I'm not saying 4 wheel disk isn't superior, but I had manual drum on mine (pretty much a stripper incl. manual steering - new one even more so) w/BB alum. drums and had redone the entire system when the car got restored and that car stopped extremely well. I think where the disk really shine is in repeated hard stops.
Drums are more prone to lockup than discs. They aren't as good in wet either. Never been a fan of drums and their Rub Goldberg design. All those springs, brackets and shoes.
All of the 71-74 Muncies had the fine-spline input with the larger TH400 style output. Do you still have that style in your car now?
Had an M21 in my Vette and hated it, almost no difference between 2nd and 3rd gear. I'd go with the M20.
I have a Super T10 with the 2.64 1st gear in my big block El Camino and it's a great street transmission. Just throwing it out there. If your Muncie is a `71-`74 trans with the larger tail shaft and fine spline input, a Super T10 should bolt right in the car but you'll need different shifter arms.
