M21 Strength
Race Director
Joined: Dec 1999
Posts: 19,610
Likes: 778
From: Forked River NJ
Which one. The early ones are a little weaker than the later ones due to larger counter shafts and out put shaft. Traction is what breaks things not HP. 400 HP can wipe out a Muncie with slicks and I have seen 1000 HP put through a Muncie and live with street tires. What is the weakest link?
Race Director
Joined: May 2005
Posts: 10,674
Likes: 122
From: Millington Illinois
St. Jude Donor '08-'09
I used to tear out 1 a month with my 67 camaro with a stock 1970 350 4 bbl and 390 gear with N-50-15 street tires drag racing. It was the weak link and I had a 10 bolt rear end. If you want a strong one check Autogear.com
Last edited by 63mako; Jun 29, 2009 at 06:37 PM.
Thread Starter
Safety Car
Joined: Mar 2009
Posts: 3,558
Likes: 4
From: Oakland California
Which one. The early ones are a little weaker than the later ones due to larger counter shafts and out put shaft. Traction is what breaks things not HP. 400 HP can wipe out a Muncie with slicks and I have seen 1000 HP put through a Muncie and live with street tires. What is the weakest link?
Drifting
Joined: Jul 2005
Posts: 1,562
Likes: 2
From: Spring Texas
Burning Brakes
Joined: Jun 2003
Posts: 1,022
Likes: 6
From: CT
I broke off a few teeth on my input shaft and cracked the tranny housing on mine while doing a max effort 1st to 2nd shift. I'm only pushing in 350-375hp range. i have bigger 275/60 street tires but they usually spin anyway. I got a good used case and rebuilt the tranny but hopefully i can upgrade in the future.
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Race Director
Joined: May 2005
Posts: 10,674
Likes: 122
From: Millington Illinois
St. Jude Donor '08-'09
An iron midplate from Autogear will bump up the potential a lot. I know I used to blow mine from case flex on powershifts. It was always the cluster gear that went. Iron midplate reduces case flex signifigantly and it is a pretty easy swap.
Race Director
Joined: May 2005
Posts: 10,674
Likes: 122
From: Millington Illinois
St. Jude Donor '08-'09
Melting Slicks
Joined: Aug 2006
Posts: 3,429
Likes: 20
From: Kingston Ontario
I might get kicked for this opinion but here it goes. I think the ST-10 is stronger than the 21 and 20. It has a larger output shaft and larger counter shaft. Both similar to the 22 and you can get an iron case from say a 78 Z-28 which is a lot stronger. They are also easy to rebuild. I beat on(draggged) an aluminum case ST-10 with over 400hp in my old 81 and it always worked fine. These are a lot cheaper than the muncies.
Drifting
Joined: May 2006
Posts: 1,544
Likes: 0
From: Youngstown Ohio
A lot of variables to consider...We and the original owner ran the original M21 short throw with special cut gears since 1970 with over 650 hp and 600 torque from a 468ci bb and 18" wide sticky tires, with regular maintenance and rebuilds we have had no problems. We only weight 2650# and run hard but aren't launching and in roadracing smoothness counts. Your applications may have more to do with dictating what fits...what is your intent?
I'm sold on the M21 but maybe we have been lucky...
I'm sold on the M21 but maybe we have been lucky...
Last edited by 73-84 IMSA Widebody; Jul 1, 2009 at 01:13 PM.
