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Old 02-25-2006, 02:52 PM
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How to bullet proof a m21?

I am building a 72 Camaro and have a very stout 355 putting out over 550 hp. I was talking to some great guys on here and they said I would need to launch my car at 6000rpm or so to keep car from falling on its face lol. My question is I have a m21 and they also were telling me that it wont hold up? What can I do to make it hold up? Thanks for all your help in advance.
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Old 02-26-2006, 06:32 PM
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ain't no way. It will live pretty good behind 400 horse in a street enviornment. Start banging big shifts and it will bite you, probably sooner than later.
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Old 02-26-2006, 07:00 PM
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The thing that will kill an M21 is if you start running huge slicks and really start hooking up real good. If you want to make the M21 a little stronger replace the aluminum mid plate with a steel one from 4speeds.com.

I agree the M21 will hold up real well against 400HP.


Vince
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Old 02-26-2006, 07:25 PM
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nissan

I had a 67 comaro with a 427 built to over 750 hp.and a ford 9" rear.scattered the doug nash when I down shifted, went to a junk yard with $200 got a m-21 took out the dogs and stand on it hard,touch the clutch and power hard an quick into the next gear, but never down shift till you stop.the car ran 2 years an the tranny never let go.
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Old 02-27-2006, 07:34 AM
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trans

Not to sound stupid but are you sure of those hp numbers??? Is that 550 on the jug???? Thats 1.57 hp per CI

The fact of the matter is that no factory standard transmission is going to hold up to that hp and 6000 rpm launches,,, it's just not going to happen... no matter what mod's you do to it...

The only chance you might have would be with the new super case muncie M-22 with all the aftermarket up-grades.... They start at $1850.00 and no-body in there right mind will guarantee them in that application..

So IMO you need to look into a aftermarket transmission like the Jerico,or maybe a used doug nash.....

Keith
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Old 02-27-2006, 08:32 AM
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No I am not positive about the hp. I am going off of what I was told by fellow mwmbers on this forum. The question is not how much power I am makeing it is the strenght of a m21. I thank you all for your input. I guess I will use it for now and just baby it till I can get an aftermarket trans. Thanks again for all the help. Don
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Old 03-07-2006, 02:18 AM
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i was in the same situation as you

i have a 67 chevelle with a 383 thats been dyno at 650 and i put a 125 nitrous shot on top of that, i had a new m22 not even a 21 and it blew up in 1 day, i put a keisler built tremec tko600 in it and no problems launching it at those rpms, it was a bit pricey $4000 but well, well worth it.if your gonna do it do it right!
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Old 03-07-2006, 10:31 AM
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The 6000 rpm launch will break it and it also suggests shift points that are considerably higher then a syncronizer tranny can shift at. The syncro is nothing more then a brake to slow down a gear for an upshift or, speed up a gear for a downshift and at some point in the rpm range the rotational mass of the syncro becomes to heavy to reliably allow the tranny to be shifted.

If you have ever "missed a gear" at 8500 rpm with a muncie what you probably experienced was getting the lever to the gate and having the syncro not allow the slider onto the next gear and in your haste wound up engaging the clutch under full throttle while in neutral. In reality, you woulda hit the gear if it coulda been selected.

In the old days we would grind every other tooth off of the syncro because we thought that fewer teeth would allow the high rpm shift at least more reliably. The fact is however, we were simply lightening up the rotational mass and therefore allowing the syncro to work at a little higher rpm I suspect.

If you go with a Jerico or G Force make sure you get a "clutch assisted" tranny for the street. The Jerico road race version would be the right choice for the street I believe. The "clutchless" 4/5 speeds will be more difficult on the street because they will knock theirselves out of gear (neutral) everytime you lift your right foot (coast) unless you hold them in gear. Lenco also makes a street tranny.

Your existing clutch/bellhousing program will likely go out the window also. Hays and the tranny manufacturer will have suggestions for the correct set up for your combo.

This all may seem a little pricey but if you want to launch it and shift it reliably (above 7000-7500) this will be the cheapest, most reliable route over the long run.
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