Is there an easy way to find out what transmission you have when the tranny is STILL in the car? (M20 four speed or a M21 four speed with "Close Ratio") I know the cases and numbers are the same. Its easy if one can look at the end shafts, but what about if its still installed? Can I turn the motor "One" rotation and count the turns of the tail shaft? Will this be different between the two trannys? Thanks
-
-
Re: M20 vs. M21 (close ratio) How can you tell.
Ken-----
Yes, you can do as you suggest. That's a positive and definitive way to determine which Muncie variant you have (M-20 or 21). To do this, you would need to jack up the car and put the car in first gear. Turn the engine 1 complete revolution and count the revolutions of the driveshaft (or, output shaft). The problem is that the difference bwteen an M-20 and M-21 will be slight in terms of output shaft revolution difference. But, if you're careful, you should be able to tell the difference. You can also "magnify" the relationship by turning the engine over multiple, complete revolutions while counting the driveshaft revolutions.In Appreciation of John Hinckley -
Re: M20 vs. M21 (close ratio) How can you tell.
You can drive the car and see how much the RPM's drop from 3rd into 4th at a constant speed. CR will drop about 600 and WR will drop closer to 1K.Comment
-
Re: M20 vs. M21 (close ratio) How can you tell.
The easiest way is just to drive the car and note the following:
1. speed at 3000 in third gear
2. if at the same speed in fourth gear revs are about 2300-2400 it's a CR; 2000-2100 it's a WR.
The 1-2, 2-3 intergear spacing for both are the same. The difference is the much wider 3-4 spacing on the WR version, and you can usually "feel" the big 3-4 gap in normal driving.
DukeComment
-
Re: M20 vs. M21 (close ratio) How can you tell.
Ken what year of Corvette do you have? Another way of telling a Muncie's gear ratio if it has not been changed is the suffix letter stamped on 69-74 Muncies. If it is an A=2.52:1 M20 wide ratio and a B=2.20:1 M21 close ratio. If you have a 69-74 Muncie with a C=2.20:1 M22 close ratio, you would have the heavy duty Muncie, sometimes referred to as the "rock crusher".Comment
-
Re: M20 vs. M21 (close ratio) How can you tell.
Seems that the WR would be a better overall transmission than the CR. When I am cruising at 60 mph the engine just seem wound up too much. Part of that is the 3:73 gears. I have the CR.
Does the CR have any other benefits?Comment
-
Re: M20 vs. M21 (close ratio) How can you tell.
Jim,
In my opinion a CR is better for a car that produces most of its power within a narrow rpm band. The smaller drop in rpm between gears allows the engine to stay within this power band.
WayneComment
-
Re: M20 vs. M21 (close ratio) How can you tell.
The T-10/Muncie gear spacing, 1-2 and 2-3, is the same for both WR and CR. The only gap difference is 3-4.
DukeComment
-
Re: M20 vs. M21 (close ratio) How can you tell.
I am a little confused. My understanding is that the M21 was numerically "different" in 1-2, 2-3 and 3-4 spacing from the M20.
M20 1st 2.52:1, 2nd 1.88:1, 3rd 1.46:1 4th 1:1
M21 1st 2.20:1, 2nd 1.64:1, 3rd 1.28:1 4th 1:1
Seat of the pants with the CR unit is a longer 1st gear requiring a little more rpm or using some clutch finesse on some cars necessary.Comment
-
Re: M20 vs. M21 (close ratio) How can you tell.
Ratio spacing or "intergear ratio" is the "ratio of the ratios".
For example the 1-2 WR intergear ratio is 2.52/1.88 = 1.34
The CR 1-2 intergear ratio is 2.20/1.64 = 1.34.
If you carry out the calculation to more decimal places, there is a slight difference, and this is because the ratios are rounded off to two decimal places, so the calculation is only valid to two significant decimal places, but if you use the actual tooth counts to compute the intergear ratios, you will get the same result to an infinite number of decimal places.
You can run the numbers for the 2-3 spead.
It is obvious from inspection that the 3-4 WR intergear ratio is 1.46 and 1.28 for the CR.
At any shift speed the revs that pick up in the next higher gear are the shift speed divided by the intergear ratio, so obviously, the larger the intergear ratio, the greater the rev drop off, and the difference in WR and CR is only the 3-4 intergear ratio.
The reason why the intergear ratios in the lower gears are EXACTLY the same is because the tooth count on the mainshaft gears and their mating countershaft gears are the same for both versions. The ONLY difference is the countergear drive ratio.
The overall transmission gear ratio is the product to two gear set ratios - the countershaft drive ratio which is constant, and the ratio of each countershaft/mainshaft set. Of course, fourth is direct, and torque goes straight through the mainshaft, not through the countershaft as in all other gears.
The CR/WR ratio sets are the result of only a different countershaft drive ratio, which consists of the clutch gear and driven gear on the countershaft. Other than the driven gear the countershaft, the other countershaft gears are the same for both versions. All other parts inside contemporaneous CR and WR (except M-22) are the same, and a CR can be converted to a WR and vice versa merely be swapping the clutch gear and countershaft.
The overall "feel" of a gear is the product of the transmission ratio times axle ratio. This is the "overall drivetrain ratio". If you run the numbers for a WR with a 3.36 and a CR with a 3.70, you will find that the overall 1, 2, and 3 drivetrain ratios are close to the same - actually the WR/3.36 is about five percent shorter (larger number), but it's not signficant. So with the same engine the cars would have about equal acceleration, however, if the "drag race" went beyond the top end of third gear, the WR/3.36 conbination would fall behind in fourth because the revs would drop off much further than the CR/3.70 combination.
Being as how the shift to fourth would not occur until the range of 85-100 MPH depending on engine redline, it's not an issue in most street driving, but if you had a WR gearbox in road racing you would be creamed on the main straight after you shifted to fourth.
For the above reasons, the WR/3.36 combination is probably a better overall combination for the street than the CR/3.70 combination. The WR/3.36 yields the same acceleration to any legal speed, and gives you a taller cruise gear.
DukeComment
-
Re: M20 vs. M21 (close ratio) How can you tell.
Duke,
Thanks for the explanation.
I am still a bit confused because I thought the 65 WR unit had 24 teeth with the counter gears being 17, 19, 22, 29 vs the CR unit with 26 teeth 17, 19, 22, 27 ?
In any case my 65 has a CR with a 3.55 rear which, from your explanation, appears to be a good compromise setup although when I first got it the previous owner had installed a WR unit. Since the CR was the matching "correct" tranny I put it back in for flight judging. The WR seems more streetable for my taste.
Thanks again.Comment
-
Re: M20 vs. M21 (close ratio) How can you tell.
I don't know the tooth counts off the top of my head, but the only difference in counter gear tooth count is the driven gear off the clutch gear.
Of course, if you replace a CR with a WR while leaving the axle ratio unchanged. the first three gears will be shorter, and the car may seen peppier, but the tradeoff is the big 3-4 gap.
DukeComment
Comment