Team Camaro Tech banner

M20 or M21?

5K views 30 replies 16 participants last post by  pdq67 
#1 ·
I have a chance to have a guy rebuild my M20 for $175 plus parts, but he also has many rebuilt M21's and 22's available. I've heard nothing but good things about his trannies. The question is should I have him re-build my M20, or should I try to trade him for a rebuilt 21 and attempt to get a deal on it? I plan to put it behind a big block, either 396, 427, or 454. I have the 454 now, but I can't get over how incorrect it would be for a 68. What do you guys think?

Rick
 
#5 ·
One other factor to consider besides ratios is how quiet the transmission is. If yours is working well you could wind up with a transmission that is noisier if you trade it.
 
#6 ·
Thanks for the input guys. I'm not exactly sure what the rear end ratio is, but I can tell you this, it is a 10 bolt on a RS non SS which would have come with a 327. Not sure which 327 though. Anyone know if their were different gear ratios available on these cars?
 
#7 ·
Personally, I do not care for the close ratio M21. I like to wind my gears out a little further and run lower rpm's on the freeway. Recommend that you check the sticky here on tc to spec out your drivetrain numbers to get the driving experience you are looking for--- no sence in guessing. Charlie
 
#9 ·
Ive heard from many guys that the M21 is definately the way to go if you're running a Big Block. Im in the same scenrario as you, except Im comparing bewteen the M21 and the TH400.:)

Cool name by the way man...definately mafia italian style. LOL.

Perfect to say to an Import ricebuner guy... "You think youll beat my american muscle car"...?
"Fugettttttaboudddddit!!!":thumbsup:
 
#10 ·
ekkk!

The M20 is the "wide ratio" trans, low gear is 2.52

M21 is "close ratio" 2.20 low.

Your "3.27" rear is probably 3.08, copuld be 2.73 or 3.23 tho.

ALL muncies are 1:1 high, freeway rpm's will be the same!

Might also look into a tko 600 5 speed.

Again, with a big block and tall geared rearend, I'd use the M20 if I didn't get a 5 speed, and leave the tall rear alone.
 
#11 ·
Thanks again guys, you guys are awesome. Thanks for the compliment CamaroJoe...most people don't get it! I came up with that name years ago and it wasn't the way I wanted to spell it, but I was limited on how many letters I could use. I will more than likely pull the rear end cover off and count the gears to be sure. Not to go off the subject, but does anyone here think that a 10 bolt will hold up to a BBC? Because I'm really wanting a 12 bolt, but I'd rather spend the money elsewhere on my build. One guy told me to run the 10 bolt untill it grenades and then switch it out. Not too sure I wanna have a rear end give out while driving.

Thanks again,
Rick
 
#12 ·
Not too sure I wanna have a rear end give out while driving.

Thanks again,
Rick
It's no fun. I lost mine last fall on a 3500rpm launch, running borrowed ET Streets behind a 383 and tko. Thank goodness the guy who loaned me the tires also loaned me his trailer.

Some have had the 8.2's last pretty well (behind automatics) even in high 10 sec cars. Behind a stick, if you have traction, fugetabotit.
 
#17 ·
I had a 4:10 gear and M22 behind a 396. 4:10 is too low to be running around on the highway. I scattered the 4:10 one day and put it back together with a 3:55 ratio. Much better than the 4:10. Best of luck!
 
#19 ·
With the wide torque curve of a 454, no need for a close ratio. It's the opposite to what a 302-Z would need since it's powerband is much more narrow. Plus, the M-20 would be better for blast-off. You'll be ok with street tires, a Muncie, and healthy 454, but add some traction and the ol' Muncie (any GM Muncie) will likely grenade.

If you haven't already, investigate the Autogear Muncies,..probably 25% stronger (guessing). Has a more sturdy box, bigger front bearing, cast-iron mid-plate, and stronger high-nickel gears.

www.autogrear.com

Good luck!
 
#20 ·
I run a 427 (medium high perf) and M20 with 3.31 rear. I like this combo for all around use. Not the best launch for racing but reasonable RPM on the highway. It will still smoke the tires just fine and I don't have to put up with 4.11's on the highway. I would stay away from low geared rearends unless you are racing or are running an overdrive for highway cruising. I like 3.55 or 3.31 with 3.73 as a maximum for big blocks.
Only my opinion. Have fun with your BB car!
Grant
 
#21 ·
The ratios of 2nd:1st (.75) and 3rd:2nd (.78) are the same for all three 4-spd Muncies. The numeric dropoff from 3-4 is more dramatic (or wider) in the M20 (.68) than the other two (.78). So it really boils down to how much torque multiplication you need off the line, and with a big block you don't need too much like you do with a 283 or 302.
 
#30 ·
Hey fugetaboti,

It seems like you and I are almost in the same exact scenario.

My 68 camaro was an original 307 small block car with a 2 speed powergilde and a 10 bolt.

Finally I have the 396 Big Block I installed in my camaro last week. However,Im still looking for a 12 bolt for my Camaro.

With that 396 making a mild 425 horsepower for a BB, everyone is telling me to run a 3:73 gear ratio out back in order to be an all around good street car.
They also say that if im going to run a big block, I definately need to consider a 12 bolt rear end out back (still searching for one).

Now that I installed my 396, im in the dilema of a Munice or turbo 400.
By the looks of it, I may just on fact convert the car over to a Munice 4-speed car...
It seems to be a really popluar choice and decision by most the guys on this site.

If I go with the Muncie, Im also in need of deciding between an M20 or M21. Although Ive heard between the M20 and M21, the M21 is stronger, im still not sure in which direction to go?
...
 
#26 ·
Rockcrushers (M-22) are indeed stronger. The straighter cut gears prevent the stretching of components, such as the maincase, caused by the "thrust" of angle-cut gears...(angle cut gears are much quieter though). Also, the straigter helix angles meant a cooler running tranny. The M-22 gear sets used a stronger alloy steel. Plus the magnetic drain plug was a nice feature.

There's a good write-up in Colvin's book (Chevrolet by the Numbers) regarding the M-22.
 
#28 ·
So I took out my Father-in-Law's 68 RS/SS this weekend. It has a 383/450HP with an M-21 and 3:73 12 bolt. I now know I will get an M-20. The 21 had such close gear rations that it was almost like you weren't shifting. The 3:73s in the rear put it at 4000RPM at 70MPH. Whew!

But it sure was fun to drive!!!!!
 
#29 ·
So I took out my Father-in-Law's 68 RS/SS this weekend. It has a 383/450HP with an M-21 and 3:73 12 bolt. I now know I will get an M-20. The 21 had such close gear rations that it was almost like you weren't shifting. The 3:73s in the rear put it at 4000RPM at 70MPH. Whew! But it sure was fun to drive!!!!!
With an M20 you'll be shifting at a lower MPH in all three gears but the drop off in rpm from 1-2 and 2-3 is EXACTLY THE SAME in an M20 as an M21/M22.

Assuming a 3.73 rear your effective gear ratios would be:

gear wide close
1 9.55 8.21
2 7.12 6.12
3 5.52 4.77
4 3.73 3.73

The wide ratio (I suppose) is called that not because the ratios are wider (they're actually the same) but because the absolute numerical difference from one gear to the next are wider. Look at the above.

For the wide "ratio", 1st gear is 2.56*3.73=9.55 and 2nd is 1.91*3.73=7.12. An absolute diff of 2.42 or a 7.12/9.55=25% drop off in gear (which translates to 75% of the rpms at the moment you shift). So, if you shift at 6000 rpm in 1st you'll be at 4500 at the moment you get into 2nd.

For the close "ratio", 1st gear is 8.21 and 2nd is 6.12. An absolute diff of 2.09 and still a 25% drop off in gear. So, if you shift at 6000 rpm in 1st you'll be at 4500 at the moment you get into 2nd. No difference there.

However, with the M20 you'll be going slower at each shift point.
 
#31 ·
I dearly love my M-20 and 12-bolt's 3.31's so go from there!!

Regardless of what engine is in my car through the years!!

Big engine, torque it out of the hole at "git-go" on motor vs a little motor by rolling her out easy until about "2,600 to 3,000 rpm" and NAIL her and HOLD-ON!!

He, He!!

pdq67
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top