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Hey evryone, I have a Munice M-20 4 speed and I want to use in my 55' with a smallblock. What kind of hp and torque do think it could take. It has a 27 spline out put shaft. Any opinons or past experiences would be great...AZ
 

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I have a "Rock Crusher" in my 68 Firebird behind a 375HP 400.You should be safe with the M-20 to at least 400HP on the street.
 

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Muncie

If you have a stock tri-5 rear end you will break the carrier bearing journals, spider gears or axles first. Especially if you have any kind of rubber to the ground. Lon
 

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You are right about the strength of the muncie. My small block puts out 400 ft lbs of torque, and I have already twisted my rear axles once with that rock crusher..
 

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Both the M20 and M21 are good for up about 400HP the weak link is the Rear!! I had a stock 57 rear with 488 posi and dutchman axles it lasted. But I broke a 410 and 370. If you leave stock wheels on it and don't mind wheel spin the rear will last. But put any size drag radial and forget it!!
The M20 is the best trans if you have a stock or slightly modified. It has a 2.20 first gear and It will help get you started, the M21 is the close ratio and has a 2.02 first gear. Good for the track but not in traffic. My buddy just got a 57 two door wagon with an M21 it had a 307 rear and we swapped it for a 370 and it drives much nicer in traffic now. You could keep a 355 of 323 use the M20 and still have a car that can cruise at 65MPH. Its some to think about.
 

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From my experience the "rock crusher" will outlive at least 8 stock rears, in all ratios from 3.08 to 3.73, took a 4.11 or a 4.56 posi to hang in there. I used both with no problem, switched them back and forth dependng on street/strip.
 

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After I get some money saved up I'm going to try to pick up a 4 speed to replace the 3 speed in the 55.
Dave
I would suggest a 5 speed if you plan on taking it on the freeway. I dont know my axle ratio, i am sure it is stock but mine does 60 a 3k. I will be looking for a 5 or 6 speed.
 

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rpm's

i have the 355 rear with a 4 speed and it tach's 3200 @65 mph, it's only 40 miles of highway from one end of the state to the other but if i decide to travel id like to hook up with a 5 speed dave
 

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I think CCI has a hydro clutch setup for the tri fives. You mite want to check it out. The rock crusher was the M22, I think it was the only munice with a drain plug on the bottom, the rest came with common drain/ fill plugs on the side. The common trans for all the Hi Pro cars of the 60"S was the M21 close ratio. I had one on my 69 SS Nova, it was the L78 396/375Hp (425Hp)
I think the M22 was a special order!! and a lot of money at that time!
 

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HP is irrelevant....it's torque that matters. My Duramax only has around 300 HP, but I'll bet it would trash a muncie. 605 ft-lb of torque off of an idle. :)
 

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Is it possible to find them in the junkyards anymore? If I could find one that I could mate an overdrive to, I would go for it.
Eldon
 

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I agree it is torque that eats up a gearbox. We used to have a 55 sedan with a 427 and a muncie, just going into third gear accelerated down and that was the end of that one. Currently i have a 55 with a 350, muncie and it takes all the abuse i throw at it
 

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Um. There is no transmission right now. I will run a crate 350 and a manual trans, but not sure which. The rear gears are stock so they are 3.35's (I think that is what they are) I would not mind a 4 speed if I could find one cheap.
 
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