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Discussion Starter · #1 ·
I'm looking at having a 421 SBC built for my '56. I'm bored with my 383...it's just not fast enough.

Anyway, I have a good Muncie M20 that I've thought of putting in my car (it currently has a 700R4).

We're ballparking over 500 lb/ft of torque and well over 500 HP out of the new engine. Will an M20 live behind that kind of power?

I don't race the car, but I do like having fun with it. What do you all think?
 

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I have a 383 in my56 with a m-20 I beat on it regularly I am around 475 ft lbs
 

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"Bumb".....M20 is what I´m going to torture behind my 500hp/500tq small block...so I´m interested about this too :)
 

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Depends on how you drive, what rear gear you have and tires.


But the real question is, will it hold up against my little 355 at the light! :sign0020:
 

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Very difficult to say what the real torque limit is. Most transmissions die from bad shifts, bad clutches, other collateral damage, etc. The other half of the deal is do you have traction and how often do you use it? IMO a Muncie will take just about anything a street tire short of a drag radial will deliver.

We never worried about this in the old days until other wimpy transmissions with "ratings" came along.
 

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A lot of factors, my Muncie didn't live long behind my maybe 200hp 283. It's got a bad knock inside....
 

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I flogged the living heck out of an M20 behind a potent 292 small block. I did side step clutch drops at 6500 RPM and it lived fine with street tires.Now stock rear ends? not so much.
 

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Well they were in all the Corvettes of the late 60's, early 70's and they held up fine. Can't imagine what you could buy that would be much stronger.
Heck the Borg Warners were tough as nails. I've had both through the years and never had a trans go out. Blew everything else out, but never a trans.
Like the others said, it depends on how they are driven. A bad shift, etc. eill tear up anything!
 

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I had the same question on the M20, what was it's limits? I talked to a Muncie rebuilder at the spring Jefferson Wi swapmeet, He's out of Milwaukee Wi, For what it's worth he told me 450 hp/ 450 ft/lbs torque was the limits, unless you beef it up for more hp, it's only a matter of time for it to fail pushing more than that. Some of the things I remembered he said that needed to be upgraded was a plate that went between the tailshaft and main body(had to be steel I believe, so it didn't flex) and a bigger main bearing with more ballbearings I think 10. There was more upgrades, I just don't remember all of them. I was planning on putting my M20 behind a blown SBC, but after talking to him and the price for said upgrades, I picked up a TH350 that was freshly rebuild and beefed up. I'd be curious to hear from Muncieman, He'd know the answer, but I'm with the rest of you I've seen some pretty snotty motors both BBC and SBC and a built 455 olds not kill one, seen those cars break driveshafts, U-joints and rearends, but haven't known of anyone losing a Muncie. I'm thankful you posted this, it will be good to see the consensus.
 

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im putting 570 foot pounds down at the tires, and i beat the crap out of my m20. thats with drag radials on the street. i dont think it would live long on the strip.
 

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Discussion Starter · #11 ·
I'm not planning on racing it much, if ever at all. The nearest drag strip is over 2 hours away. I never side step the clutch and won't be doing any power shifting. I do like to bang through the gears though...maybe I'll need be a little gentle.

I do run MT ET Streets...but that might change. I like how they hook up, but I'd probably be riding in a metal coffin if I ever got caught in a rainstorm with them.

My builder is saying it's going to be closer to 600 HP (not 500 as I recently wrote) and 500 lb/ft of torque, but, that's at sea level. I live at 7200 feet so I figure in about a 20% power loss which brings it back down to 480 HP and 400 lb/ft of torque, which I think a Muncie will handle.

I'm still on the fence about the Muncie. I do really like having the overdrive in the 700R4, but I really dislike automatics. But on the other hand, the thought of plopping down $3K for a Tremec TKO 600 doesn't rest well with me right now.

Decisions.
 

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I just ordered a "New" Muncie from Paul at 5Speed.com in FL. Received it last week and will be installing it sometime during the next week or two.

I went with the M22, wide ratio, Drag Special. It has all the good stuff and Paul assures me it will handle anything I want to throw at it on the street.

He also builds some nice five speeds.

This might be an option for you.

http://www.5speeds.com/
 

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M-20 power bands

I'm looking at having a 421 SBC built for my '56. I'm bored with my 383...it's just not fast enough.

Anyway, I have a good Muncie M20 that I've thought of putting in my car (it currently has a 700R4).

We're ballparking over 500 lb/ft of torque and well over 500 HP out of the new engine. Will an M20 live behind that kind of power?

I don't race the car, but I do like having fun with it. What do you all think?
Sooner or later you will shread it. However if you are willing to invest some money to beef it up you can probably make it work
Start with a new Auto Gear case and a master rebuild kit with max duty bearings, then add a cast iron center support. Cross sand the inside of the cast iron center support and cross sand the outer part of the bearing then clean both parts and use the green heavy duty locite to seal the bearing to the center support. You must stop the bearing from any movement so the unit does not flex. Use quality parts throughout and completet the rebuild

Good luck garys Gearbox
 

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I'm not planning on racing it much, if ever at all. The nearest drag strip is over 2 hours away. I never side step the clutch and won't be doing any power shifting. I do like to bang through the gears though...maybe I'll need be a little gentle.

I do run MT ET Streets...but that might change. I like how they hook up, but I'd probably be riding in a metal coffin if I ever got caught in a rainstorm with them.

My builder is saying it's going to be closer to 600 HP (not 500 as I recently wrote) and 500 lb/ft of torque, but, that's at sea level. I live at 7200 feet so I figure in about a 20% power loss which brings it back down to 480 HP and 400 lb/ft of torque, which I think a Muncie will handle.

I'm still on the fence about the Muncie. I do really like having the overdrive in the 700R4, but I really dislike automatics. But on the other hand, the thought of plopping down $3K for a Tremec TKO 600 doesn't rest well with me right now.

Decisions.
I got my Tremec on sale with free shipping 2195.00
 
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