[fugetaboti]

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

 

[JimM]

I would not put an M21 behind a big block, you just don't need the close ratios.
I'm also not into "originality" to the point where I'd spend thousands to lose 60 cubes, just dress out that 454 to look like a stock 396 and be done!

 

[JOE58]

What is your rear gear ratio?

 

[SooperDave]

With a big block you won't need the 2.5x 1st gear of an M20, but then again, depends, maybe you do if your rear is like a 3.08 or less

 

[rogue68]

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.

 

[fugetaboti]

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?

 

[camaromaniac]

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

 

[RamAirDave]

The ratios in the M20 are shorter 1-3, but the M21 ratios are closer together. The big difference is the drop off between 3-4 with the M20.

 

[68CamaroJoe]

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:

 

[JimM]

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.

 

[fugetaboti]

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

 

[JimM]

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.

 

[fugetaboti]

LOL! That's what I was thinking. What should I pay for a 12 bolt in rebuildable condition, and should I buy one from a first gen, or will any 12 bolt bolt up?

 

[IDS_Bill]

Okay we talked about M20 vs. M21 but what about M20 vs. M22 with - say a 427 and a 12 bolt 4:11 in the back???

 

[fugetaboti]

A 4:11 in the rear with a big block would be overkill wouldn't it? I would think that highway driving would be no fun, but I can't say for sure because I've never experienced it.

 

[IDS_Bill]

A 4:11 in the rear with a big block would be overkill wouldn't it? I would think that highway driving would be no fun, but I can't say for sure because I've never experienced it.

Yeah that's affecting my decision on the rear end. Thinking a 3:73 or so might be better.....?

 

[buenymayor]

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!

 

[68 Ragtop]

Okay we talked about M20 vs. M21 but what about M20 vs. M22 with - say a 427 and a 12 bolt 4:11 in the back???

The ratio's are the same for M21 and M22, so all the ratio discussion vs M20 is the same.

 

[Straight-line-69]

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!

 

[67funcar]

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