Quick question on telling the difference between M-20 & M21 installed in car?

[Larry E.]

Let's take a 1966 Corvette with an L79 that is on a lift and not available for a drive. Is there a way to tell what transmission the car
has by looking at it? If not I would think the code on the differential might give a clue assuming
the rear end has never been changed out. In most cases the ratio would be higher numerically-eg.>
3.55 vs. 3.36. Remember the question is "just my looking at the car". Thanks in advance-Larry

[Jack H.]

Re: Quick question on telling the difference between M-20 & M21 installed in car?

No way by just looking that I'm aware of Larry. I don't think the differential ratio has much reliability either as there were many combinations and not much correlation to transmission ratio option in my opinion. Maybe if the differential ratio was one extreme or the other, it *might* help statistically to some degree. BTW, there were more M21's than M20's but not by a significant number, so that doesn't help much either.

All I know to do is by putting the car in 1st gear, rear wheels elevated (or driveshaft disconnected) and rotate the engine one revolution and count the rotations of the transmission output. Alternately, removing the transmission side cover and count gear teeth. Both of those options obviously violate your "just looking" caveat though.

[Gene M.]

Re: Quick question on telling the difference between M-20 & M21 installed in car?

How about looking at the tag number hanging off the side cover bolt.

[Joel M.]

Re: Quick question on telling the difference between M-20 & M21 installed in car?

Tom Parsons told me how to tell the difference visually from the outside a number of years ago. I don't remember the details but seems there was a "tell". Suggest checking with Tom.

[Duke W.]

Re: Quick question on telling the difference between M-20 & M21 installed in car?

The WR was only available with 3.36 and 3.55 axle ratios, close ratio was 3.70 and 4.11, only. So the speedometer driven gear will tell you which one as long as the transmission, axle, and driven gear are OE equivalent. The gear is easy to remove. Remove the cable, use a wide blade tool to remove the adapter, document the color and tooth count, and someone can tell you what axle ratio that gear was used for.

Duke

[Jim D.]

Re: Quick question on telling the difference between M-20 & M21 installed in car?

The WR was only available with 3.36 and 3.55 axle ratios, close ratio was 3.70 and 4.11, only.
Duke

So why does my 65 L78 with its original (all numbers match) trans. & rear end have a close ratio with 3:55 rear gears?

[Joe L.]

Re: Quick question on telling the difference between M-20 & M21 installed in car?

So why does my 65 L78 with its original (all numbers match) trans. & rear end have a close ratio with 3:55 rear gears?

Jim------


For 1965 L-78 the only transmission available was the close ratio Muncie. However, a 3.55:1 was an optionally available ratio. 3.08:1 was also optionally available. 3.36:1 was the standard ratio.

I think Duke was referring to small blocks. For big blocks, I expect due to the higher torque, GM did pair the close ratio transmission with numerically lower rear gear ratios than they generally did for small blocks.

[Jim D.]

Re: Quick question on telling the difference between M-20 & M21 installed in car?

Jim------


For 1965 L-78 the only transmission available was the close ratio Muncie. However, a 3.55:1 was an optionally available ratio. 3.08:1 was also optionally available. 3.36:1 was the standard ratio.

I think Duke was referring to small blocks. For big blocks, I expect due to the higher torque, GM did pair the close ratio transmission with numerically lower rear gear ratios than they generally did for small blocks.

I agree Joe. My point was that blanket statements don't always apply. I realize the massive torque of the BB came into play compared to the wimpy SB.

[Duke W.]

Re: Quick question on telling the difference between M-20 & M21 installed in car?

So why does my 65 L78 with its original (all numbers match) trans. & rear end have a close ratio with 3:55 rear gears?

I was ONLY referring to the OP's question that was SPECIFICALLY for the L-79 engine option, which was available with both CR and WR four-speeds. L-78 was ONLY available with the CR four-speed. Apples and oranges. Drivetrain availability charts for most years are listed in the NCRS Spec. Guide.

Duke