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DanBoyd 09-11-2011 04:25 PM

Muncie 4 speed
I need advise badly. I have been stressing over an automatic 67 Camaro to a 4 speed conversion. I know I want the 4 speed. I have the choice of M20 or M21, 2 guys are standing by waiting on the phone to ring. Unsure which one to purchase and use.

My setup is 1989 355 ci with one piece seal. Rear end is a 2:73 at best (airplane gears). Going to change the rear end to 3:43 mostly or 3:73 being second choice. The car is not a daily driver, nor an interstate trip car. Will be driven for fun and sunday drives. I like take off red light to red light. Son llooks hot shots. so do I.

Which transmission should I choose and why?

Which type of flywheel and clutch and pressure plate? Will 1989 350 flywheel work because I don't think 1967 flywheel bolts on the same unsure.

poncho62 09-11-2011 06:02 PM

Yes, you will need a later flywheel.....snout on the 1 piece sael crank is different than the 67 one....

As for the tranny, if you are going with the steeper gears, go with the M21.....If staying with a highway gear, the M20 would be better as it has a wider gear ratio in the gears.

DanBoyd 09-11-2011 06:54 PM

Thank you very much

willowbilly3 09-11-2011 09:07 PM

I'd go with the M21, it's a little stouter.

WDCreech 09-11-2011 09:36 PM


Originally Posted by willowbilly3
I'd go with the M21, it's a little stouter.

willowbilly, What do you base your opinion on? The M20 and M21 are the same transmission except for gear ratio. The M22 was stouter than either, but I don't think there is any difference between the M21 close ratio and the M20 wide ratio.

willowbilly3 09-11-2011 09:43 PM

What I base it on is some really old memorys of rebuilding an M21 I had in my 64 Cutlass. My classmate at auto tech college was in transmission class and doing the work. He also had an M20 apart and the cluster shaft in it was smaller when we were comparing parts. But then a 40 year old memory might not be accurate.

solidaxel 09-11-2011 09:55 PM

Save your money on the rear end and get the best of both worlds, go for a Tremac TKO 500 and it will work great with your 2:73's, just right 1st and super 5th OD

AutoGear 09-12-2011 12:24 PM

All things being equal (and they never are) an M21 will be marginally stronger than the M20 and we're talking percentage here. The farther 'away' from 1:1 you get, you lose some strength.

Total Gear Reduction is essentially the mechanical advantage of the geartrain to move the car (first gear times the axle ratio).
The reality is you will have a total gear reduction of '8.2' with the M21 and 3.73s (2.2 x 3.73) Thats a pretty soft 1st gear. Use the M20 and 3.73s (2.5 x 3.73) This will give you a TGR of 9.3. The vaunted M22 and 4.11s (2.2x4.11) was 9.04.
This is all assuming both available units are in good shape. I would also prefer a 1" countershaft over the 7/8" countershaft for strength and for parts availability.

Also note that if your car was originally 32 spline, a 27 spline setup (or vice versa) may cause shifter-to-console interference. AND pre 1969 units use a stud mounted linkage on the sidecover and reverse levers; 70 and later units use a bolt-on type. These do not mix and match well.
None of these problems are insurmountable as long as the unit is in good repair.

DanBoyd 09-14-2011 04:38 PM

Huge thanks to everyone. I went with the M21. You all have some great points, plus M21 is rebuild and trust the gentlemen that had it. Unsure of clutch and pressure plate to go with. Also flywheel. I have 89 5.7 engine. I live in a tight curve hilly terrain, do you say 3:43 or 3:73? Mileage at these gas prices means a lot but it is not going to be driven on the interstate. The car is just my stress relief car. I love a 4 speed and curves. The powerglide just seems lame with the current setup.

solidaxel 09-14-2011 04:44 PM

Go with the 3:73, it will relieve more "STRESS" !!

2old2fast 09-14-2011 05:54 PM

I'm probably wrong about this ,but weren't both the m21 & m22 available w/ either the 2.56 low or the2.20 low [wide ratio-close ratio] , other than that their pretty much identical except foe countershaft size !?!?


TimoFinn 09-14-2011 11:26 PM

Here`s a good site to look for the differences in muncies

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