Question: M20 or M21 4 Speed?

[Mixdnuts]

Hello, Newbie posting. I recently purchased a 1977, L-82 with a 4 spd. Doing limited research, I just figured that it had the M21 close ratio transmission, which was the "only" option you had if purchasing the L-82 and a standard transmission. Well, I just got the dealer invoice (below) and it lists the transmission as "M20". What gives? Any thoughts/ideas would be greatly appreciated.
thanks in advance for your help!
-Matt

 

[Vettehead Mikey]

Ordering a M21 was optional on L82 as was M40, neither at extra cost. Standard trans was the M20.

 

[73shark]

IMO unless you are going road racing, the M20 wide ratio is better for the street. 1st gear is lower and will get you of the line quicker.

BTW it's a nice looking car. Have fun with it.

 

[Mixdnuts]

Thanks guys. I'm very happy with the car and the 4 spd (I spent ALOT of time searching for an L-82 with a 4 spd), just trying to determine which trans it actually has. If you read the second paragraph under "Transmissions" below, you can see why I'm confused. According to multiple sources, the M21 was the "only" option available if you purchased the 77 with an L-82. I think I'll get under the car tonight and see if I can decipher the codes. I'll post an update.

 

[sstearns2]

$550 for A/C!! In 1977, wow!

Scott

 

[Mixdnuts]

No kidding! A/C cost more than the upgraded engine! $281 for the radio is no cheap eats either.

 

[Vettehead Mikey]

I think they've got it backwards. To get M21, you had to order L82. In other words, no M21 with L48.

 

[Mixdnuts]

OK, I took some pictures last night. The good news is that it appears to be the original with VIN derivative stamped on it. Bad news is that I'm still not sure if it's a M21 or M20. The code stamped before the VIN is "WC 97" with the last number (?) only partially stamped. I believe that this "translates" to W= Borg Warner, C=March, 9= the 9th of March, and the 7= 1977. I think the third partially stamped number is either a "2" or "3" which would indicate the shift that produced it at the factory.

Also included below is the forged (?) casting (?) number, which I have no clue on. The only thing I could decipher is that "903" indicates that it is made from aluminum. Is there anything else that you guys can decipher from this?

Is there another way to "crack the code" on this (without cracking open the tranny and counting teeth)?

thanks!

-Matt

 

[73shark]

You could drive at 2,000 rpm in 1st gear and see what the mph is. Then with the tire diameter and rearend ratio, calculate the driveshaft rpm. From that you should be able to get the 1st gear ratio.

 

[JohnZ]

Clean the rest of the goop off the driver's side of the main case below the side cover, and you'll find a painted stamp; "HS" is a 2.64:1 1st gear (wide ratio), "HW" is 2.43:1 (close ratio).

 

[Mixdnuts]

Thanks, John. I cleaned up the goop on the driver’s side this morning and I couldn’t find the stamp. Am I looking in the right place (pics below)? What are the chances of the paint stamp lasting 32 years?

p.s. 73Shark, you have got to be sh*ting me?!! I guess you haven't seen my math SAT scores...

 

[Vettehead Mikey]

I came across another reference that more or less inferred that all L82s came with M21 in 1977, but made no mention why or what the source of info was. Seems like a strange thing to do, but........

Here's a quote from another website:

"You can also find out if it's an M20 or M21 by the drop in rpm going from 3rd to 4th.
Go on the road at exactly 2000 rpm in 4th and maintaining the same mph, downshift to 3rd.
If the revs go to about 2900, it is an M20, if they go to about 25-2600 it is an M21."

No idea if this is accurate or not.

 

[JohnZ]

Thanks, John. I cleaned up the goop on the driver’s side this morning and I couldn’t find the stamp. Am I looking in the right place (pics below)? What are the chances of the paint stamp lasting 32 years?

That's the right location, but the paint stamp has disappeared over the years.

 

[Mixdnuts]

Here's a quote from another website:

"You can also find out if it's an M20 or M21 by the drop in rpm going from 3rd to 4th.
Go on the road at exactly 2000 rpm in 4th and maintaining the same mph, downshift to 3rd.
If the revs go to about 2900, it is an M20, if they go to about 25-2600 it is an M21."

No idea if this is accurate or not.

Thanks, Vettehead. I gave it a try a few minutes ago. when I dropped it into 3rd, the RPMs were consistently around 24-2500. So, depending on the source, it looks like M21. I have also read other postings indicating that there is a noticeable drop in RPMs (kinda like shifting into overdrive) when shifting into 4th with the M20. When I shift into 4th, I'm wishing there was a 5th... I think I average about 3500 at highway speeds. She runs cooler stuck in bumper-to-bumper traffic.

Thanks, again for your help!

-Matt

 

[Vettehead Mikey]

All C3 4 speeds (M20, M21, M22) have a 1:1 ratio (direct drive) in 4th gear so there's no clue there, one way or the other.

 

[73shark]

But since the 3rd gear ratio is different on the M20 & M21, there would be a slightly different rpm increase when downshifting.

 

[6880 Mike]

...I came across another reference that more or less inferred that all L82s came with M21 in 1977, but made no mention why or what the source of info was...

The numbers aren't there. The Black Book shows 2060 M21s for '77 and 6148 L-82s. Looks like about 4000 L-82s got the TH400.

 

[Vettehead Mikey]

Sorry, I meant that all L82 that had 4 speeds were M21, not that all L82s had a 4 speed. Still learning to talk the language good even after all these years.

If this story is true, I still wonder why M21 was mandatory on L82 and M20 was not available? Close ratio helps emissions? :confused

 

[73shark]

According to the '77 sales brochure (I realize these aren't always accurate), you could get the M20, M21, or M40 w/ the L-82 while the base engine only got the M20 or M40.

 

[6880 Mike]

According to the '77 sales brochure (I realize these aren't always accurate), you could get the M20, M21, or M40 w/ the L-82 while the base engine only got the M20 or M40.

I'm guessing in the spring of 76 when they were preparing the info for the 77 brochures, they intended to have the M20 available with the L-82. Somewhere between the early planning stages and beginning of 77 production, things changed.

The same thing occurred for the 1980 models. Early literature had the 4 speed available with the L-82, but that got nixed in production due to emissions standards.