Power111

Gimme the pros and cons of each.

w1ctc

I have a 21 and would like to have the lower 1st gear. More fun in normal street operation and easier on the clutch.

C2BOB

How big is the motor and what rearend gear do you have? A 427 tolerates down to about a 3.08/3.36 with a 2.2 first if threre's not too much cam (street mechanical). This gearing with a 30/30 cammed 327 would be a misery.

Denney

M-20 Pro: low numerical rear end (i.e.3.55:1 or less)
M-21 Pro: high numerical rear end (i.e. 3.55:1 or more)
M-20 Con: High numerical rear end (i.e. 3.55:1 or more)
M-21 Con: Low numerical rear end (i.e. 3.55:1 or less)

5speeds

With people installing 5 speeds with 3.35 1st gears or 2.97 1st gears the M20 with a 2.52 1st gear is a better overall choice compared to an M21, unless you're doing the SCCA thing.

There is more to it in regards to rear end gearing. The 2.20 1st gear is horrible even with a 3.73. Most people who contact me and say they have "an M21" usually don't realize the m20 exists a lot more and in fact have an M20.

The M21 and M20 have the EXACT same percentage drop in all gears except the 3 to 4 shift. The M20 has a much wider drop. If you want a good overall street ratio the M20 offers better drivability. If you're looking for a tighter upper end the M21 is best. In fact M21/M22 ratios were the closest ratios ever produced in a street 4 speed.

Read and LEARN about close and wide ratios at this link:

The new M20SS ratio bridges the gap. Compare ratios, Java required...
Read about that HERE

comp

Bob Schaefer

I agree with 5speed. The gear spacing is virtually identical between the M21 and M20 except for between 3rd and 4th.

What it comes down to is trying to satisfy 3 needs at the same time:

1. providing enough torque multiplication in 1st to get the vehicle moving from a standing start and,

2. having a sufficiently 'tall' top gear to meet economy/noise/comfort requirements for highway cruising.

3. Optimizing gear spacing for the engine's torque curve to optimize acceleration.

In addition to how much torque the engine produces at low speeds, vehicle weight, rear end ratio and tire size are also factors in determining how much total multiplication from 1st gear and the rear end is needed for #1, above.

Then, the overall spread between 1st and 4th determines how highway cruising is.

The narrower spacing between gears becomes necessary to optimize acceleration for "peaky" engines that have high peak torque speeds relative to the peak power speed. This is typical of engines with high lift / long duration cams. (You don't want the engine speed to fall below torque peak rpm after an upshift.)

Power111 - If you have a fairly stock engine (displacement, cam, intake, carb, tires, etc) in a midyear:

Multiply the 1st gear ratio x the rear end ratio, the result should be at least 8.1 - 8.5 for a small block, and at least 7.4 for a big block. This will give enough torque multiplication for smooth starting.

Unless you have an engine that makes peak torque aver 4000 rpm (i.e. wild cam, carb. etc), I'd go with the 2.52 1st gear ratio.

Question for 5speed - Is there a reason why the M20SS 3rd gear is 1.37 rather than 1.28? Just seems to me like the spacing would be better that way....

Best Regards,
Bob S.

JohnZ

My '69 Z/28 has an M21, and was built with the optional 4.10 axle - that combo worked fine; I swapped out the 4.10 for a 3.55 (3800 at 70 finally got to me), which calmed it down nicely on the highway (3200 at 70), but it sucks around town from a dead stop (I knew that going in).

On the other hand, my '67 Corvette (327/300hp) has an M20 with a 3.36 axle, and it's MUCH more pleasant to drive with the 2.52:1 1st gear.

Kensmith

John,

What do you think about a m21 close ratio with 4.11 rear end and a 327/350 motor in a 65 Vert?

Thomas66

I have a freshly rebuilt 010 case M-20 ready to go for my '66 L-79. The original M-21 with 3.70 combination is "ok" (note the lower case), but I want to see what difference the M-20 will make. The transmission was acquired just before fall Carlisle but I wasn't eager to make the change so late in the driving season. I'll hope to make the swap by the end of the month.

AZDoug

I used to run an M21 with 4.56 gears with my 350/327 L-79 motor.

Lotsa fun around town, not too great on the highway.

Full throttle was perfect with M21, as I would shift about 6700 RPM and it would drop to around 3800 RPM or so during the shift, so the car was always in the power band.

Doug

Bob Schaefer

KenSmith - I've had quite a bit of experience with the same combos as JohnZ; a '68 Z/28 w/M21 & 4.10 rear and two 327/350 mid-years - one 3.36 / M20 and my current M21 / 3.70.

The M21 / 4.11 will be really easy to start from rest. Since the 327/350 is a still a fairly mild engine rated with a 3600 rpm peak torque, it'd be fine with either the M21 or M20. The M21 is ok, but not really needed for optimum acceleration. (The 302 Z/28 has very little low end torque and weighs about 200 lbs more than a midyear; therefore, it benefits from the M21 for optimum performance and really needs the 4.10 to start from rest.) The 4.11 will probably result in the revs being about 3100-3400 at 60 mph in 4th depending on tire size. One might find that tiresome......

I also fully agree with Thomas66: the M21 / 3.70 is about as little torque multiplication as I'd want in 1st with the L-79 for a smooth start from rest. I also think the resulting 2800 rpm @ 60 mph would be too much for me on a long highway trip......'guess I'm getting old!

Best Regards,
Bob S.

Kensmith

Bob,

Thanks for the information. Too bad we can't get the best of both worlds without swapping out for a late model tranny. Or some kind of overdrive system for the freeway. But I think I would prefer something that is suited more for around town cruises or Sunday drives. Guess you can always stay in the right lane on freeways and keep it around 55 or 60.

5speeds

There is sort of a best of both worlds....

The M20 SS or some people call it the M21 W will yield this ratio.
2.52 1.75 1.37 1.00

Stock M21 is:
2.20 1.64 1.28 1.00

Stock M20 is:
2.52 1.88 1.46 1.00

If you already have the M20, you change out 2nd, 3rd and the countergear.

The M20 Super Street bridges the gap between wide and close ratio.

Autogear designed the set. Various distributers sell them.

Paul

65air_coupe

I'd love to hear your feelings about the swap when it's complete. I also have the M21/3.70 combination with my L79.

SMR 67

Before I got the Keisler, I had an M20 and really liked the 2.52 first gear with my 3.73. It took off like a jack rabbit, though admittedly, you weren't in first gear too long

The high rpms on the highway is the main reason I went with the Keisler, and you also get an even lower first gear, but I think the M20 is very streetable with a higher rear end ratio.

drd1vette

Ken, I'd say good autocross car. Hey, are you going to Vette Magic in June? I'll be up there. Sat is the car show, Sun drags. Hope to see you. Just ask for "Doc"

Kensmith

I've got it marked on my calender. Got a cell # and maybe we can have a beer or two?

61 Silver

Hi!

Is there any way to tell what transmission you have in a 1963 340HP with 3:55 gears? It is an original Muncie with the serial number.

Thanks Ray

Bob Schaefer

Hello -

A way to tell in a car is to drive at the same road speed in both 1st and 4th gears and note the rpm at each. Then simply divide the rpm in 1st gear by that in 4th. If the result is 2.2, you have an M-21, if it's 2.52 it's an M-20.

You can also compare the road speeds at the same rpm in 1st and 4th. It comes down to whether you think there's more error in the speedometer or the tach. (I've no idea which is more accurate.....)

Best Regareds,
Bob S.