C2- Muncie rebuild question

[Joe L.]

Re: C2- Muncie rebuild question

Bill-----

It is true that the only difference in actual parts between an M-20 and M-21 are the input gear and the cluster gear. However, I believe that there are different "drops" between all the gears in either transmission.

[William C.]

Re: C2- Muncie rebuild question

Negatory my friend, the percentage drops are the same with the exception of the 3/4 shift.1-2 is 25% for M-20, 21, 22. 2-3 is 22% for all and 3-4 is 32% on the 2.52 low and 22% on the 2.20. The numbers for the '63-65 wide boxes are slightly different on the 2-3 shift.

[Michael H.]

Re: C2- Muncie rebuild question

Bill,

That's not exactly true. There is a difference in ratio drop, or spread, between 1st and 2nd for the 2.20 CL and the 2.54 WW transmissions. Nearly 20% difference actually. The fact that the main drive gear and counter gear ratios are different between the M20 and M21 means that there also has to be a change in ratio spread, if all else is the same.

The 1st to 2nd gear shift in a 2.20 produces a ratio change of roughly .65. The same 1st to 2nd shift in a wide ratio 2.54 is a ratio change of roughly .55. That's a big difference.

The other problem with a wide ratio, as far as I'm concerned, is the wide drop in ratio, and RPM, from 3rd speed to 4th. The WW 2.54 has a .51 ratio change and the CR 2.20 has only a .31 ratio change. This is exactly in reverse from what the ratios should be. The big drop, or change in ratios should occur in a low vehicle speed area and the close 3rd to 4th change should occur at higher speed.

Borg Warner correct much of this years ago when they introduced their close ratio units with much lower 1st speed gearing. The big drop was between 1st and 2nd and the rest felt much like a typical 2.20 Muncie. Much improved gear selection.

I dislike 2.54 wide ratio Muncie transmissions. I agree with Joe Lucia and Mike Cobine. I love the rhythmic sound of the engine RPM increasing in some well spaced pattern instead of ratios that scatter the RPM range all over the map.

Michael

[William C.]

Re: C2- Muncie rebuild question

The drop in ratio from 1-2is 2.56-1.91 in the eqrly M-20, or 25%. The Later m-20 IS 2.52 TO 1.88 OR 25%. the M-21 in all series is 2.20 to 1.64, OR 25%. The difference in the input and cluster is all accounted for in the 3-4 shift.

[Michael H.]

Re: C2- Muncie rebuild question

That's correct Bill. However, it's 25% of a greater number. It's 25% of the 1st gear ratio, 2.52, which is roughly .65. It's relative to the initial ratio, not the difference in actual ratio from one ratio to another. The difference between 2.56 1st gear and 1.91 2nd gear is .65 no matter how you do the math. The .65 is the number that will be the difference in ratio between these two speeds.

The 2.20 would be the same 25% but it would be 25% of a smaller number, 2.20. So 2.20 minus 25% is 1.64. The difference, numerically, would be different than that of the 2.56 wide ratio. It would be roughly .55 instead of .65.

You can definitely feel the difference between the CR and WR transmissions just driving the car. If you multiply out the overall gear ratio through the transmission and diff., it's a substantial difference and quite obvious.

[William C.]

Re: C2- Muncie rebuild question

To make it a simple as I can in trans talk, if you shift from first to second in any of these boxes at 4000 rpm, the same speed road will be maintained at 3000rpm in second gear. The torque delived to the wheels will be different, but it will also be a 25% change from where it was in first gear to where it is in second gear.

[mike cobine]

a whole lot more complicated than numbers

I think if we knew all the answers to this, we'd find that reasoning was much more complex than we imagine. Possible lines of thought are:

- The high horse engines were touted as the race cars, the low horse and the sporty car for the masses. One the street, a wide ratio, low rear hydraulic cam car could quite possibly blow away the dual quad/FI for the first hundred feet or so and wouldn't that be embarrassing to the guy who paid a 25% premium in price. Kind of like the base C5 blowing away a Z06 C5 from the light. How many would buy a Z06 then? Chevy would NEVER let that happen.

- The lower the rear, the more accentuated the effect of the wide ratio becomes. Thus it would not be "comfortable" to the casual buyer, who would be the more likely purchaser of the low hp/wide ratio car.

- Torque breaks things, and while you could get the same overall effective gearing/torque multiplication with a 3.70 & wr as a 4.11 & cr, the location of that torque may be placed at a point in the driveline which could cause a lot more problems, i.e, warranty costs.

- Marketing: The wide ratio may have been marketed as the cruising or rallying 4 speed and the close ratio as the racing four speed. Setting up the equipment to counter that would not be good business.

- Power band on various engines. If you have driven a low RPM engine with a really low rear, you know it hits top end far too soon. So to keep acceptable top end speeds (hey, this is a CORVETTE, not a Nova), you couldn't let some engines have the really low rear gears.

- Likewise, the high revving engines really aren't happy in the low RPMs so a wide ratio would tend to get them out of the power band at times, which would again make owners question why they spent a 25% premium for a car that falls flat on its face at some points.

And there may be reasons we haven't even gotten close to. It could be as simple as Zora Duntov insisted on the close ratio, or Ed Cole needed to sell a given number to keep them available for the SCCA/NHRA certifications (GM justification, not SCCA/NHRA).

[Michael H.]

Re: C2- Muncie rebuild question

Bill,

I don't know how to post a math formula on this board so I'll have to write it out the long way. It goes like this. RPM, divided by ratio, (overall ratio, including transmission and differential) times tire circ in inches, divided by 63,360, times 60. If you use a common diff ratio (3.70) and the two different transmission ratios from both a wide and close ratio transmission, you will see that change in vehicle speed after the gear change from 1st to 2nd in each calculation will be different. If you go backwards through the formula, you can calculate the difference in engine RPM between the two different ratios for each transmission. I know this is confusing but I don't know how to explain it any better. It all has to do with the overall ratio, not the percentage of gear reduction from 1st to 2nd. As you mentioned, the percentage remains the same at .25 for each ratio but that has nothing to do with the actual difference in output shaft speed or related engine speed.

[William C.]

Re: C2- Muncie rebuild question

Michael I think we are saying the same thing. In low gear, more torque is delivered to the tires with a wide ratio box than a close ratio box. That means, for a given rear end ratio, and a given engine torque curve, without allowing for tire slippage, the car with the wide ratio box will accelerate more agressively than the close box until the 1-2 shift.

[Michael H.]

Re: C2- Muncie rebuild question

Yes, I think we agree on that. It would be a simple matter of overall gear ratio. As the numerical value increases, so too does performance, at least in that gear. (to a point) It's the price that has to be paid in 2nd and 3rd that I have a problem with. As stated in my previous post, it's the newer close ratio Borg Warners with a 2.36 or even a 2.64 1st gear that I would be after if I wanted a lower 1st gear. Best of both worlds with a 3.55 or 3.70 differential. I have a 3.70 with a 2.20 in my 66 425 HP car and I think it's the absolute ultimate combination for a big block.

[mike cobine]

Re: C2- Muncie rebuild question

Instead of writing out an equation, why not just go here and plug in numbers to get the results?




RPM and MPH calculator

[mike cobine]

But the original question was a 390 hp

and tops out around 115 with the 3.70.

[Michael H.]

Re: C2- Muncie rebuild question

Thanks Mike. That's exactly what we needed. Love the numbers for the 2.43 1st gear Borg Warner. Great 1st gear and the rest are nearly exactly the same as a 2.20 Muncie. Would be perfect for a small block or even a big block with 3.55 or less (numerical)diff ratio.

[mike cobine]

Re: C2- Muncie rebuild question

Interesting note on this if you plug in 2.54 for the tranny and then plug in 2.20, you will see that the 3.08 with the 2.54 is nearly identical to the 3.55 with the 2.20.

The 3.36 and 2.54 are slightly below the 3.70 and 2.20, nearing the 3.90 often used but never offered.

I guess Chevy decided more take-off performance was not needed especially since the 4.11 and 4.56 were available for real racing and the combo of the wide ratio and 3.70 would have been roughly the same as a 2.20 and 4.30. The use of such a low gear to get the high performance off the line would have prevented most from high speed top end and would have slowed potential buyers who wanted to buy a drag car for the street AND still have respectible top speed to outrun the cops when caught street racing.

To anyone racing at a track, such was not a problem.

[John H.]

Re: C2- Muncie rebuild question

1st-Gen Z/28's were "buyer's choice" for transmissions - M20, 21, or 22; whatever you wanted.