Street Driving an M21 462ci
 

Hey guys, I’m in the process of building my 74 TA and wanted a 4 speed. I picked up a fine spline M21 but I hear they need a ton of gear to make it fun to drive. Should I just look for an m20 to replace it with. I think the rear gears are 3.08’s. Thanks guys

Both the M21 and 22 have a 2.20 first gear .

How is your 455 Cam’ed?

How tall are your rear tires you have or will be running ?

Since a 455 has such a wide and flat torque band when Fitted with the right Cam a wide ratio trans is not a handicap.

2.54:1 posi gear is fine. All that matters is your throwout bearing be Hydraulic.


If you use the stick mech fork trow deal, thenthe 455 TQ will readjust you pedal setting on every fly.

RA iv HFT, 231/240. Tires probably 27” tall

I absolutely would not run a 2.20 1st gear with a 3.08 rear lol

If you're dead set on a 4-speed you'll be happier and better off with an m20 with a 2.52 1st gear or change the rear gear to something better suited for the 2.20

i have a 505 with a 3.55 gear and an m23 super case with a 2.98 first and i still want more first gear. sorry but i just love letting the clutch out and the engine barely notices. makes it so much easier to drive up a steep gravel driveway or inch along in traffic.

big pontiac tractor motors with automatic transmissions do really well with tall gears. but for a stick shift, i still say get that thing moving with as much gear as you can!!!

Definitely get the M20.

Might think about getting a 3.23 rear to go with it, like the factory did with every ‘75-‘76 455 Trans Am of which all were wide ratio 4-speed equipped.

That seal it, M20 or ST10.

I have now a tkx 2.87 first gear and 3.31 rear,,,its much more pleasurable on take offs compared to the m21 but even though that sounds like a big jump in 1st gear it actually feels normal. I wouldnt want it any taller.Not geared too low. So i would imagine a m20 compared to a m21 would feel nicer on takeoffs but its not going to feel like a big change but with a 3,08 rear i would definitely go with the m20 at the least. Its still going to feel a little tall in 1st gear. I think the tkx comes in a 3.25 first gear,,wonder how that would be.

Wasn’t the Borg Warner ST10 used during those years? Probably used the 2.43 first gear on those cars with highway gears. Anything less will not be enjoyable.

I agree, even a 2.52 with a 3.08 isnt ideal but the op isn't giving any other options. I typically set them up here with a decent amount of first gear multiplication, it's just much more pleasurable to drive as already mentioned. Especially driving them as much as I do, 10,000 plus miles a year. And in most cases these days many don't even mess with the 4 speeds anymore and go right to a 5 speed to enjoy highway cruising as well. This opens up both ends of the spectrum. Now you can have your first gear, and even better gear multiplication through the other gears if you setup a proper rear gear, and still go to sleep on the highway. It's a win win.

The BW super T 10s used in Pontiacs from 75
Are good for handling a solid 450 hp motor being bolted to them.

Here’s the ratios they could be had in.

All Pontiacs from 75 on used the standard wide ratio model, however late 70s Chevy Z28s used the special wide ratio model .

Personally, I like the 2.64:1 first gear. That is a nice compromise for a heavy street car with a low numerical rear axle. They also put a 2.88:1 first gear in a super T-10. I wouldn't be afraid to try that either. I like the super T-10. I feel it has a better shift feel and is more positive with a superior side cover design and more robust shift arms and forks than a Muncie. Most on the forum would disagree with me though.

The 3.27 first gear with a 3.08 rear sounds like a good combo but yes not what he’s asking about. If someone can afford it though the tkx would be my choice. What are you going to spend on a nice m20?

Here’s my experience with higher geared rear ends.

I have 3.08’s/M-20 (2.52 1st) 4-speed in my ’70 Lemans 4-dr. that weighs 4000 lbs. with me in it. The engine is a .040” over 400/462 stroker with a RAIV 041 ‘clone’ cam with 1.50 rockers from Bullet (circa 2009), 6X-8 heads with SD CNC porting (255/210 cfm) with 9.5 compression, factory iron intake/Q-jet, and repro RA manifolds. It has no problem pulling away from a stop without excessive clutch slipping with the 7.76 1st gear multiplication, but a steeper 1st gear would be welcome…somewhere around 2.80 would be nice. I like the 3.08 gear for cruising.

I also have a ’71 455HO GT-37 with a BW-ST10 (substituted for the original M-13 3-speed) with the 2.64 1st gear (from Jim Mino) with the original 3.31 rear axle (8.74 1st gear multiplication). The 455HO is .040” over (464 cu. in.) and all stock except for a Summit 2802 cam. The stock heads 197 HO heads flowed 230/175 cfm and compression is only 8.61. It loves the 13% steeper 1st gear to overcome the full point less compression compared to the 462 in the Lemans.

It's interesting that 455HO ’71 A-bodies would come with M-22/3.31 gears when equipped with A/C. That’s only 7.28 1st gear multiplication! The ’73 SD-455 Firebirds were factory equipped with the M-20 wide box instead of the close ratio M-22 to overcome the even softer low-end torque with the ‘Y’ cam (8.62 1st gear multiplication).

Dennis

I can give you a couple price points from the last 2-3 years. I have built 4 Muncies for friends/customers. Swap meet wide ratio boxes in usable condition seem to run $700.00-$1000.00. That doesn't mean they are good necessarily. Just usable. Of the 4 brought to me for overhauls, 2 were able to be rebuilt with small parts kits, 1 or 2 synchronizers replaced and 1 or 2 gears replaced. New brass of course and gaskets. Parts average cost around $400.00-500.00. One just needed a small parts kit, brass, speedo output gear and gaskets. About $200.00 for parts. The last one was an M22 from a big block Chevelle. Almost every piece in it was destroyed. Only parts I could reuse were the case, side cover, and main shaft. Even the tail housing and bearing support needed some welding and repair! He wanted to keep it numbers matching so I fixed it. Had $1700.00 in parts. New gear set was made in Taiwan and was very nice I am happy to report. I generally charge around $500.00 labor for a basic complete overhaul of a Muncie. Not including welding and other strange problems that sometimes come up.
So if you have a Muncie that is all torn up, a TKX does enter the conversation IMO.

I have the original M21(2.20) and 3.42 rear in my TA, while it is useable, it is not a great, street friendly combination. I have a rebuilt M20 that will be going in this winter. I would prefer to do a 5 or 6 speed swap but they are so pricey. Plus if I’m honest this car does not see many highway runs, certainly not long ones and probably never will.

I don’t think even a ST10 has a low enough first gear for a 3.08 rear gear. If I had 3.08 rear gears I’d look for a aftermarket M23 with a 2.99 first gear or get a M21 /20 rebuilt with the new extra wide ratios swapped in it.

Then you would have the best of both worlds great first gear and reasonable highway cruise rpm.

The ST-10 I got from a 1980 Z28 I think has a 3.42 first gear. Stout 383 SBC and a 3.73 autocrossed the crap out of it with road race slicks!

But I know Larry Navarro who posts here sometimes grenaded a ST10 in his 79 TA with a medium 455., not even power shifting when it went.

Until the 454/455 came along wasn’t it recommended /required from Gm that it was 3.90 and up for the m21 trans?