You want to change from a 4-speed manual to a 3-speed manual?? Or wanting to switch to a 3-speed Automatic??

Difference between M-20 and M-21 is gear ratio's...M-20 is wide ratio (2.52:1 low gear) so it has a bigger rpm drop between each gear, M-21 is close ratio (2.20:1 low gear) and has less rpm drop in each gear.

If you are wanting to switch to a 3-speed Auto, the Turbo 400 is your best choice, and the only one really strong enough.

 

Which is stronger M20 or M21

Clarification to my earlier question. Which 4spd is best suited/stronger M20 or M21 ? Veh: 69 SS chevelle, w/ 450+hp mtr, posi 331 rear. Thks

 

I would install the M-20. About the same for stength. The M-20 has a better ratio for the rear gears you have. My 2 cents.

BTW, every STOCK M-20,21 behind a warmed over BB never lasted too long! LOL!

 

Neither is sized appropriate for your application. The M20 is better suited to your rear gears as previously stated.

You want a muncie with the 1" countershaft "pin". Look at the front face of the transmission and there will be a 1" or 7/8" dia hole below the input shaft. You want that hole to be in good shape, without a wad of silicone stuffed in there. Also, people have used a punch to "peen" more metal into that hole, to tighten up a loose fit. This is also bad news because the case is starting to let go on you.

Realistically; you need an aftermarket "Muncie" with the Supercase, Iron midplate and wide ratio M22W gears. You can get by with a Supercase, Midplate and M20 gears maybe.
Also, you'll want to switch from a 10-spline input to the later 26 spline input; otherwise strength of M20 vs 21 is kind of a moot point.
Let me know if you want me to find a local distributor for the case and midplate for you.
Supercase kits come with a larger front ball bearing and a new high-crush gasket set. Case is A356-T6, cast in Sinking Springs, PA and machined in Syracuse, NY

 

I wouldn't use either M20 or M21 for a BBC with 450 hp. If you don't mind the gear whine, I'd go to a M22 (if your budget allows it) or a Super T10 Borg Warner. Either will hold up much better to a BBC with 450 hp. I'm using a Super T10 behind mine, and it's held up to slicks and hard launches well.

 

4Spd

Appreciate the responses & expertise. I too was considering an upgrade/replacement of a stouter nature. Although, I know the Auto trans is most likely more bullet proof, I'd prefer to stick with the manual 4spd / 5spd type of trans. I'd like to keep the replacement unit as close to stock as possible. It's my understanding Tremec units require considerable customizing of the tunnel area. My SS has a console and I prefer to keep the console and old school stock look. Thks

 

Appreciate the responses & expertise. I too was considering an upgrade/replacement of a stouter nature. Although, I know the Auto trans is most likely more bullet proof, I'd prefer to stick with the manual 4spd / 5spd type of trans. I'd like to keep the replacement unit as close to stock as possible. It's my understanding Tremec units require considerable customizing of the tunnel area. My SS has a console and I prefer to keep the console and old school stock look. Thks

I haven't found the tunnel mods that people speak of to install a Tremec, to be "considerable". In my experience, the only modification I did when swapping a T56 six speed into a '69 Nova, was to trim the brace that runs on the bottom side of the tunnel to clear. I simply used a parting wheel to cut about 8" of the brace flush with the floor above top center of the tunnel. No other cutting, or massaging was needed. I also used the stock crossmember by moving it, and welding a mounting plate on it to attach to the T56.
Doesn't sound like your car is still stock, so swapping to a better 4 speed, or a 5 or 6 speed, wouldn't hurt it's originality.

 

1971BB427, it may surprise you to know that the '64-72 Chevelle/A-bodies have a rather small trans/driveshaft tunnel, quite a bit tighter than the Camaro or Nova...doesn't make sense as it is a mid-size car, but that is how it is. :drunk:

 

That is a surprise! Especially the later '68 and newer Chevelles! I'd have thought the earlier '64-'67 might be smaller, but didn't know the later would be. The T56 isn't much bigger than the TH350 or Powerglides that many of those cars had.
Wonder why Chevy would put a larger tunnel in a Nova and Camaro? Maybe the body sat lower on the chassis, and needed more tunnel depth?

 

If you want to keep the console then I'd keep the 27 spline output shaft your car would have been built with. A Muncie 27 spline mainshaft (in good condition) has no problem at your power level. The 27 spline T10 mainshaft? it would be overloaded, but if the midplate and bearings were good stuff...It'd be okay as long as you didnt get into a lot of wheelhop.

So; since Motive Gear is having QC issues with their Super T10; and they arent playing nice with selling parts into the aftermarket, I cant really recommend a T10 until the supply and quality issues are hammered out.

Get a quality Muncie core; if such things exist. It needs to have:
Good 27 spline mainshaft
Good reverse gears
Good sidecover (LOL)
Good tailhousing
Good synchro hubs
Good throwout bearing tube (front bearing retainer).

Now, buy yourself the following:
M22W Rockcrusher gearset with a 26 spline input
Iron midplate
Overhaul kit for 1" countershaft, 27 spline output 1966-1970 Muncie
(2) sliders
1" countershaft Supercase
Whatever else your donor needs

Yes its a bit of an expense (I'll help you find a vendor if you want); but its all new stuff, easily replaced, It'll last a good long time, fits in your car, reuses the shifter you have, reuses your driveshaft (I'd upgrade to 1330 or 1350 Ujoints if not so equipped).
26spline clutch gives you Big Block holding power
27 spline output is the shorter GM 4spd and will fit easily in everything and anything you want.

The most expensive option would be to buy a crate muncie; if you go that route, Id opt for the Italian gears, the 18 spline lightweight sliders, the aftermarket bronze bushing sidecover...but Id still keep it 26 splines at the front and 27 splines at the back.

The cheap answer is to take your M20 or 21 core; and convert it to a 26 spline M20. Either change the input if you start with a 1" c/shaft M20; or change the input and cluster to the 26 spline M20 stuff if you're using an M21 donor.

Expect to need 2 sliders, 2nd gear and an overhaul kit at least.

FYI we build more of these 26/27 spline wide ratio gearboxes than any other version. Something like 60 of them this year alone. If it was all the Italian gears, shafts and synchros with American castings; Id have no problem with it in your car even close to 600hp NA

 

If you want to keep the console then I'd keep the 27 spline output shaft your car would have been built with. A Muncie 27 spline mainshaft (in good condition) has no problem at your power level. The 27 spline T10 mainshaft? it would be overloaded, but if the midplate and bearings were good stuff...It'd be okay as long as you didnt get into a lot of wheelhop.

So; since Motive Gear is having QC issues with their Super T10; and they arent playing nice with selling parts into the aftermarket, I cant really recommend a T10 until the supply and quality issues are hammered out.

Get a quality Muncie core; if such things exist. It needs to have:
Good 27 spline mainshaft
Good reverse gears
Good sidecover (LOL)
Good tailhousing
Good synchro hubs
Good throwout bearing tube (front bearing retainer).

Now, buy yourself the following:
M22W Rockcrusher gearset with a 26 spline input
Iron midplate
Overhaul kit for 1" countershaft, 27 spline output 1966-1970 Muncie
(2) sliders
1" countershaft Supercase
Whatever else your donor needs

Yes its a bit of an expense (I'll help you find a vendor if you want); but its all new stuff, easily replaced, It'll last a good long time, fits in your car, reuses the shifter you have, reuses your driveshaft (I'd upgrade to 1330 or 1350 Ujoints if not so equipped).
26spline clutch gives you Big Block holding power
27 spline output is the shorter GM 4spd and will fit easily in everything and anything you want.

The most expensive option would be to buy a crate muncie; if you go that route, Id opt for the Italian gears, the 18 spline lightweight sliders, the aftermarket bronze bushing sidecover...but Id still keep it 26 splines at the front and 27 splines at the back.

The cheap answer is to take your M20 or 21 core; and convert it to a 26 spline M20. Either change the input if you start with a 1" c/shaft M20; or change the input and cluster to the 26 spline M20 stuff if you're using an M21 donor.

Expect to need 2 sliders, 2nd gear and an overhaul kit at least.

FYI we build more of these 26/27 spline wide ratio gearboxes than any other version. Something like 60 of them this year alone. If it was all the Italian gears, shafts and synchros with American castings; Id have no problem with it in your car even close to 600hp NA

just for giggles, what would something like that cost?? you can pm. me if you don't want to post in a thread..
over something like a jerco or liberty trans.

 

M20 vs M21

Thanks for the detail response. Based on your response, I would appreciate some leads on locating a suitable replacement. Do you have an estimated cost for a replacement unit as described? Thks

 

I think prices for various 4 speed transmissions can vary quite a bit. Depending on whether you're starting with all brand new parts, or building up your existing trans to handle higher HP. When I was building my last car, I was trying to find a 4 speed that would handle the torque and HP I planned, plus a few trips to the strip on occasion. My first choice was a M22, but the prices around my area were $2200-$2600, depending on who had them. Next was the Super T10, and prices ran in the $1300-$1700 range. I had time to be patient, and ended up finding a freshly rebuilt Super T10 from a '71 Camaro with a 402, and got the transmission, with shifter for $550. I opened it up, just to confirm it really was all new, and then bolted it into the car. It's been a great trans for the last 18 months, with 3 trips to the drags.

 

Check your PMs Gearhead; but 1971BB is correct. There are some really tricked out M23s that can end up close to $3k but the lions share is under $2200 depending on the source and what they use in their assembly. The Asian gearsets are a couple hundred under the Italian, cast rings are half the cost of forged ones; cheap hubs and sliders are out there...and those gearboxes are functional, and okay for MOST people. It just hurts when a guy thinks he is getting the premium stuff because he paid over $2000 (or worse, they TOLD him he was); and he got the cheap rebuilder stuff in a new case.

If you find a good deal, feel free to ask if they're an approved A.G. distributor. I can usually find you a shop that can do what you need, in your region

 

M20 vs M21

I'm not in a "must have now" mode. Ive been looking with the intent to buy the "right" replacement unit. If you've got some leads please let me know. Thks

 

Where are you located?

 

Chip; feel free to send me a private message. I can say that rebuilding a core around here is usually $700-1000 labor and tax incl; but I don't know what your labor rates are. You'd be a bit higher if you have added an M22 based gearset.

Our biggest distributors are:
Jodys Transmissions (PA)
Medatronics/5speeds.com (Florida)
Jeff Collins/Midwest Muncies (Wisconsin)

But there are plenty of smaller boutique builders out there.