C2- Muncie rebuild question

[Robert M.]

A friend has a 66 427/390 hp cpe and is in the process of rebuilding his transmission. The car has a 3.70 posi and originally came with the M21 close ratio transmission. Upon disassembly,we noticed the imput shaft has two rings which I believe is indicative of a M20 wide ratio. His question is which is the more drivable ratio for the street with the 3.70 rear? It is not the original transmission.

[Joe L.]

Re: C2- Muncie rebuild question

Rob-----

With a 3.70:1 rear gear and L-36, I think that either an M-20 or M-21 would be ok. However, I think that I'd probably favor the M-21 for this combination.

[Robert M.]

Re: C2- Muncie rebuild question

Joe, what is your rationale for the choice of the M21? Just so I understand your reasoning.

[Joe L.]

Re: C2- Muncie rebuild question

Rob-----

With an L-36 torque and a 3.70:1 rear gear you don't really need the low first gear. Since you don't need the low first gear, then you might as well have the closer gear spacing that the M-21 provides.

The M-20 is really designed to be used for applications in which you have a lower numerical rear gear ratio (for better highway cruising) and lower torque engines. In these applications, the lower first gear ratio provides much better 1st gear performance. What you lose with it is the closer transmission gear spacing, but the trade-off favors the lower first gear ratio in these applications.

The relatively high torque of the L-36 combined with a higher numerical ratio like 3.70:1 doesn't need the lower 1st gear transmission ratio of the M-20.

[Robert M.]

Re: C2- Muncie rebuild question

JOe

Thanks for the in depth explanation.

[William C.]

Re: C2- Muncie rebuild question

On the other hand, a M-20 in a car with a 3.70 rear feels just like a car with a 4.11 and a close ratio thru the first three gears. I used to ask my friends, "If you aren't ahead of the other guy by the end of third gear, isn't the race over?"

[Robert M.]

Re: C2- Muncie rebuild question

Interesting point, and wouldn't the M20 be better for cruising with the 3.70 rear?

[William C.]

Re: C2- Muncie rebuild question

4th gear in the M-20 and M-21 are the same. The M-20 gives better aceleration thru the gears at the expense of a greater rpm drop between 3rd and 4th. Performance in 4th with either trans is identical.

[Joe L.]

Re: C2- Muncie rebuild question

Rob-----

No, both the M-20 and M-21 have a 1:1 4th gear ratio. So, cruising is the same for both variants.

What Bill described is definitely the way it is. Basically, that's the reason that I mentioned that either one would be ok. With the M-20 and the rest of the drivetrain described, you get a better shot from a dead stop. With the M-21 you get closer gear spacing. Overall, even though I'm a BIG fan of M-20's, I think that the M-21 would be the best for this combination. GM agreed with me since they never offered an M-20 with a 3.70 axle ratio and ANY engine.

[Robert M.]

Re: C2- Muncie rebuild question

Joe and Bill, thanks. That settles it. I had it straight when I explained it to him,but he somehow managed to confuse the issue to where I wasn't sure anymore. I told him exactly what you said Joe in your last post,that a M20 WR ratio was not available with any 3.70 rear to the best of my knowledge. Thanks again to both of you.

[mike cobine]

Re: C2- Muncie rebuild question

The closer and even gear spacing of the close ratio transmission has forever been flaunted as the transmission to have for racing. This way you rev to 6500, shift and it drops to a given point. Rev to 6500 and shift and it drops to about the same point. This keeps you on the same part of the power curve for each shift.

Works fine at the drags and road racing.

HOWEVER, the torque curve of a 390 hp BBC is such that you are not shifting at 6500 and the power band is so wide that a drop to 4000 or 3800 or 4200 between shifts using a wide ratio is virtually irrelevant.

It will give you great stop light take off power like having a 4.11 rear.

On the street, most action requires a very low take-off gear (first and rear combined) and tons of torque. The first 100 ft are usually enough from a stop light to put the other guy away.

And then again, who is really street racing these old Corvettes anymore anyway?

[William C.]

Re: C2- Muncie rebuild question

I hate to disagree, but the M-20 was available with a broad range of rear end ratios, my favorite was a '69 Z-28 I ordered for a friend of mine with a 4.10 and a wide ratio box. It ran thru the first three just like a close box with a 4.88 gear, and still got a respectable (for then) 15-16 mpg on Business trips to Detroit. Also, unless my memory is playing tricks with me, one of my roomies had a '65 Chevelle SS, 327/350 hp with a 3.73 and an M-20, used to go GTO "hunting" in it!

[Michael H.]

Re: C2- Muncie rebuild question

I didn't know a wide ratio 4-speed was available with any diff ratio on a Z28. Thought they all came with 2.20's.

I always disliked the ratios between 1st and 2nd in a wide ratio. Seemed like there was almost no change in RPM and was almost a waste of time even using 2nd gear on the street.

[William C.]

Re: C2- Muncie rebuild question

Hate to tell you michael, but the 1-2 drop in an M-20 and a M-21 are Exactly the same. The same gears are used in the trans. The only difference in the boxes in is the 3/4 drop. that allowed GM to commonixe on most of the parts in the trans, with only the input and the cluster changing.

[Joe L.]

Re: C2- Muncie rebuild question

Bill-----

They may well have offered an M-20 transmission with higher numerical gear ratios in other Chevrolet cars. However, they never did for any Corvette big block. For Corvettes, the M-20 was never offered with any ratio numerically higher than 3.70:1. Even the 3.55:1 and 3.70:1 were available only for some SHP small blocks (66-72 with L-79, L-46 or LT-1) with M-20. No ratio numerically higher than 3.36:1 was ever offered for a Corvette big block with M-20.