Just had time to check. My original post on this is correct. All the reverse idler and gears are the same. Rebuild kits for the same years are the same weather it is an M20, M21 or M22. Here is a link:http://www.kajunjon.com/Main/Muncie_Parts.htm

 

Thanks for the offer but I already have both an M20 and M21 in my possession so I'm not looking at other options, at this point. And if I did in the future, I'd be looking for another gear or two so I can stop reaching for it every time I drive the car anyways . Thanks for all the info!

 

I did not like the one M/20 trans I had even though it had 4.11s
in the back just diid not like the drop in revs between 3rd
and 4th, took it out installed an M/21, but if acceleration is your
game the M/20 has the advantage for street gearing, I just don't
like the bigger rpm drops between gears that wide ratio transmissions have.
I have always considered a vette a sports car good for going around corners
maybe some high speeds on the freeway, to me there are worlds better
platforms for drag racing and after you get out of first gear the M/21
to me gear spreads are worlds better for a sports car. I have had
the 2.20 low M/21 with 3.36, 3.70, 373, 4.11 gearing.

 

An M21 with a 3.70 rear end has a first gear ratio of 8.14 to 1. Seems pretty like it would be pretty weak out of the hole with a mild 350. I destroyed a whole lot of M20's when I used to race back in the early 70's. Everytime I see a post as to the strength of the muncie transmissions I remember ripping them out repeatedly in a 67 Camaro with a Stock 350, 390 Rear end, Traction bars and N-50-15 street tires. The only real good ones were the M22's

 

I ran an M21 w/ mild 350 and 3.55 gears. It was a real PIG out of the hole. Last winter I swapped in a ZZ4 (another mild, but higher torque 350) and a M20 w/ the same 3.55 gears and its alot better, but still not great. Gotta side step clutch at 3000 rpm if you want to burn/smoke the 255/60 BFG's. The only rpm drop I noticed on the STREET w/M20 vs M21 was the shift from 3rd to 4th. I didnt think it was a big deal. At 5800 rpm I'm going almost 90mph in 3rd, and I'm looking to start backing out of it when I shift to 4th anyways. On a drag strip, I'm sure it would be much more noticable. Could be a real problem if you run a hot 350 w/ a narrow power band/ high rpm only engine. I'm looking into Super T-10 w/2.88 1st gear to get the car out of the hole easier, and/ or swapping rear gears to 4.11's. Personaly, I wouldnt use an M21 UNLESS I had a killer engine (read: big block torque monster) and/or a lot of gear, like 4.11's or 4.30's. Just my .02

 

no question, M20, The m21 will not help you on the street and will burn up clutches. I had a super t1o with a 2.88 first gear and a 3.70, thought it was to much for pure stock drag racing and bought the m20, missed the first gear zing and went to a 4.11 with the 2.52 first gear!! As others have said look for the 10.0 ratio of first and rear end gear, especially with a low torque small block

 

Hell I tore up my M/20 just power shifting into 3rd with street tires,
my opinion of the muncie even the M/22 they were weak as wet
toilet paper the guys at the strip that had more serious stuff then
me had ford 9 inch rear ends and big block steel cased ford top
loaders, when the doug nash five speed showed up they switched
to it. there were some die hard it must be chevy or nothing guys
scatering the muncies or breaking the axle on there 12 bolt watching
the axle and slick come off with the stuppid C clips.

 

Good question!

I just had to kick in my two cents before going out to replace the Lizard's blown clutch...

I don't know what the rear ratio is, since the housing stamping was rubbed off by the previous owner (off-roading on a curve -- thanks for disclosing). It's the "standard" for an automatic K-code, whatever that is, not the M21 that's in it. And don't let "backyard bubba" switch you over to "fo in de flo"!

In real-world terms, there's not much point in shifting with this combo. We've each started in 3rd gear more times than we could count, and the only way to tell is you only shift once (3-4) that way. A full "launch" from a stoplight sounds like "Buuuh-WOW-WING-WING-WEEEEEEEEEEE...", as the engine doesn't seem to care which gear you pick anyway. Second and third wind up so fast, you don't even need one of them (1-3-4 or 1-2-4 is much nicer than 1-2-3-4). We're not threatening the red line at 45 in first, but we're all done by ~130 in 4th. That's RPM-limited, not drag limited. A warmed-over K-code may not get much respect, but it's not a two-stroke! It actually has a power BAND, which is wasted on such a narrow range of gearing choices.

In case it doesn't show, we REALLY don't like street driving with the M21. The best I can do with it is to hold my right foot down until 4th, then adjust speed to match traffic. The wife likes to lift the throttle in shifts, which means when the clutch grabs on the upshift, we get thrown forward like it was a downshift! Yes, it gets up to the speed limit in a hurry after it climbs up on that tall 1st, but it really wants another gear or two for the open road.

OTOH, the M20 still has the same 1:1 top gear, as you've noted. The lower three should let us use more of that K-code "power band" and we'll get through the parking lots more gracefully, but we're left with the ~3000RPM screaming at relatively-sedate (~80mph) cruise speeds... (RPM numbers are approximate as the (well lubed) tach cable broke after only a little testing, a couple of weeks after we got the car on the road again; and MPH is approximate because you shouldn't let "backyard bubba" switch you over to "fo in de flo"!)

Bottom line, I can see where the M21 could be useful on a track, especially if the track is consistent. If you were racing on a fast, open track you'd switch to a tall rear gear, or use a lower (numerically higher) gear for a tight, twisty track; then work the trans a lot to stay right at the peak of the power band most of the time. I don't even care about "floor it, turn left", but I'd bet the M21 is the perfect tranny for that. For "civilian use", an M20 is still not enough choices in the modern world (why else do you think they make those 6 & 8 speed wonders?), but it will at least more-fully utilize that big American V8 instead of acting like a Japanese 2-stroke dirt bike (which probably has at least 6 gear choices).

The best advice I can offer is, if you think about your power band first, try to work out your overall (tranny * diff * tire diameter) gearing so that when you upshift you're going from the same point on the top of the RPM curve to the same HP point on the uphill side, and you cruise at traffic speeds at the low end of that power band. This will give you (IMO) the best performance to fuel economy ratio, plus the optimum top end for your power plant. IMO and YMMV, of course.

Ballpark guessing here, but I'd estimate in a K-code with a 3.70 rear, an M20 would make your life quite pleasant around the neighborhood and "daily driver" work with 2nd - 4th for once you get rolling -- you'll work the throttle (RPMs) more; and if you don't frequent "the line", the M21 would serve you well in 1st through 4th (you'll work the clutch more) once you get rolling. Your engine and the lightweight car make it hard to pick the "wrong" gear. By 4th, you're at 1:1 with either one, so...?

We're shopping for a taller (numerically lower) posi diff and an M20 to make both the low-and-slow and the rush (Did you notice how "WOT" and "WOW" only differ by one letter?) more mellow. That's our "real world".

And saving up for a 5-speed WR someday. Expanding the envelope...

 

It all has to do with mathematics...

You have more than 1 PAIR of gears in contact to obtain a ratio...

The ratios are:

driven / drive x driven /drive = final ratio.

The ONLY difference of the M20 and M21 boxes is the tooth count of the input shaft and matching section of the countergear.

Physical 1st, 2nd and 3rd and reverse gears are THE SAME.

That is why the actual percentage drop of 1 to 2 and 2 to 3 is identical.

Paul

 

With the engines we build today, with their long flat torque curves, there's little reason for a close ratio tranny, which is designed to keep an engine within a narrow torque band. So, when you shift an M-20 you'll reduce the RPM's more than with the 21, but since you have the torque from A-Z, that's a good thing.

Plus, a "wide ratio" will get you off the line quicker, and could allow for a higher rear gear for the highway (lower numerically).

Given the choice of M-21 with 3.70's vs. an M-20 with 3.50's, give me the latter any day.

But, with the engines we build today (strokers, aluminum heads, rollers, etc.), your old Muncie won't take more than 4-500 HP before it pukes. So, here's who'd I call:

www.autogear.net

Great guys who can sell you a better, stronger mainbox and hardened nickel gears. They can sell their own version of the Muncie which is muuuch stronger than the original that has a host of upgrades that will allow it to live to 500 HP and beyond.

Good luck!

 

For street driving I would say having more first gear multiplication helps a little so the M-20 is better than a M-21/M-22 in that respect. You can have more gear in the tranny and less in the differential for a better street combo.

After that, it is a question of your power band. The close ratio was designed to be used with tighter power band, higer RPM engines. Maximizing a given displacement generaly favors higher RPM and a more narrow power band where the close ratios are supposed to be a benefit. A torquey stroker engine doesn't benefit from the close ratios as much because they have torque over a wider range. Wider ratios are probably a better choice for them.



-Mark.

 

Well put brother Mark. Can you do the same chart for the M-20?

 

I could make a chart BUT I did not make that one. I stole it from another page many moons ago. I have no idea exactly where. The gear drop concept has a lot of importance so I saved it and just reposted.

-Mark.

 

I would do the M20. I have a 69 that came with a 350/350 and 3.70's and had an M21. My motor has a lot of bolt ons and a cam and a few other things, but it's mostly stock. I switched to the M20 a few years ago because I got a good deal on one. I find it way more street friendly. Easier takeoff from a stop (also harder lauch if that's what you are after) at the stoplight, way less shifting on our local 30-50 mph roads, and the same 4th gear on the highway. Just easier and more fun to drive.

That said, I am throwing a Tremec in this Vette as soon as possible. I can't stand the 70-80 mph highway trips anymore...time for a change.