Munci gear ratio calculations

[Steve D.]

I have the following Muncie main shaft gear tooth counts: 1st - 36, 2nd - 30, 3rd - 27, 4th - 24. The corresponding cluster gear tooth counts are 17, 19, 22, and 29. Simple division gives the following ratios: 2.12, 1.58, 1.22, .83. Posted ratios that I have seen for the "wide ratio" box are 2.52, 1.91, 1.48, 1.00. If I divide each of the "simple division" ratios by the 4th gear ratio, I can match the posted ratios. Can someone explain, please?

Steve

[Mike Cobine]

Re: Munci gear ratio calculations

Don't count the gears, count the input shaft revolutions for one output shaft revolution.

[Dave McDufford]

Re: Munci gear ratio calculations

Steve,

There is no "4th gear" gear. In 4th gear the slider locks the input shaft and the output shaft together and the other gears on the output shaft free wheel on the counter gear.

For the first three gears, the gear attached to the input shaft (what you called 4th gear) drives the counter shaft and then drives whatever gear is engaged by the slider. The counter gear is driven at a rate of 24/29.

So your math is correct.

Dave

[Duke W.]

Re: Munci gear ratio calculations

Each "gear ratio" is made up of two sets of mating gears. The first set is the constant counter gear ratio. This is determined by the number of teeth on the clutch gear and front gear on the countershaft.

The actual ratio for the first three forward gears is the counter gear ratio times the ratio of the mainshaft and counter gear for each of the first three forward speeds.

Fourth gear is achieved by locking the mainshaft to the clutch gear, and the ratio is 1:1 or direct drive. Power is transferred directly through the mainshaft with the countershaft just idling.

Starting from the front of the box the first set of mating gears is the countershaft constant. The second mating set is third, the third mating set is second, and the fourth mating set is first.

The fifth and last set, located in the tail shaft housing is reverse.

Using your data I get 2.56, 1.72, 1,34 for first through third, which doesn't match any ratio set I know of. First looks like a Muncie, but second and third looks like a Super T-10. You might want to verify the tooth count and I'm assuming the the sequence you list is the front to rear sequence of the gears as installed.

Industrial gear box ratios are usually expressed as output speed ratio, but automotive boxes quote the input speed ratio. I compute the ratio working back from the output (output tooth count divided by input tooth count), then take the reciprocal. The computation should work like this:

Reciprocal: (front counter shaft gear/clutch gear) x (mainshaft gear/counter gear) = gear ratio.

For third gear:

1/(17/36)(30/19) = 1.34

Duke

[Joe L.]

Re: Munci gear ratio calculations

Steve-----

You've counted the teeth on at least some of the gears incorrectly. No Muncie (or T-10) ever used a gearset with ALL of the teeth counts that you've listed.

[Steve D.]

Re: Munci gear ratio calculations

Joe

I marked the gears as I counted, and counted them twice.
Below is an excerpt from the Muncie parts pricelist at 5Speeds.com,
which lists parts matching those tooth counts.
Could someone have mixed items in a previous rebuild of this box ?

1st Speed Gear 36 Tooth M20 M21 $85.00 View Part

2nd Speed Gear 30 Tooth M20 M21 $85.00 View Part

3rd Speed Gear 27 Tooth M20 M21 $85.00 View Part

Input Shaft M20 24 Tooth 10 Spline $145.00 View Part

Countergear Early M20 29-22-19-17T $185.00 View Part

[Steve D.]

PS

The car is '63, but the drivetrain is very un-original. The transmission case casting # is 3851325.

[Mike Cobine]

Re: Munci gear ratio calculations

Page 359, Model Year 1965, Drive Units - Manual Transmission
4 speed
first - 2.56
second - 1.91
third - 1.48
fourth - 1.00
reverse - 2.64

Sound like a match.

Same is true for Model 1964, page 321.

[Mike Cobine]

Forgot - Corvettes: Technically Speaking *NM*

[Joe L.]

Re: Munci gear ratio calculations

Steve-----

Based upon the main case casting number, the transmission is a 64-65 Muncie with wide ratio (2.56:1). Also, I believe that I was incorrect in my previous posting. The gear tooth counts that you have listed are correct for the wide ratio 63-65 Muncie.

[Duke W.]

Re: Munci gear ratio calculations

Countershaft ratio = 29/24 = 1.21

First: (29/24)(36/17) = 2.56

Second: (29/24)(30/19) = 1.91

Third: (29/24)(27/22) = 1.48

The CR box uses the same mainshaft gears (first through third) and the same tooth count on the mating countershaft gears. The only difference is the constant countershaft ratio, which is determined by the clutchgear (what is referred to in your post as the "input gear")and front countershaft gear that mates with the clutch gear.

The clutch gear and countershaft determine whether the box is CR or WR. Everything else is the same, so swapping these two parts can turn a CR into a WR and vice versa.

Duke

[David Dawdy]

Muncie 4 Spd

Go to muncie4speed.com for a good parts list and nice exploded view, which
should help with the orientation of the Muncie.

[George C.]

Re: Muncie 4 Spd

Steve,

I can't add much to the gear ratio discussion, Duke and others have covered that very well. In your messages you posted the car is a 63, but has non original drive train components. From the case number you have a late 65 and up transmision. You can further confirm this by checking the diameter of the counter gear shaft, should be 1 inch. Also the speedometer gear is on the right side of the transmission.

For what it is worth,

George #36908