comments on this drivetrain combo?
Thread Starter
Burning Brakes
Joined: Feb 2003
Posts: 764
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From: Harvard MA
comments on this drivetrain combo?
I have a 66 L-79 with M-21 and 3.7 posi. I'm about to rebuild the tranny and wonder if I'll enjoy the car more if I replace the M-21 with an M-20. Anyone care to comment on the pros and cons of this swap?
Racer
Joined: Feb 2003
Posts: 445
Likes: 47
From: Reading Pa
When I bought my 66 it had 4:11 rear gears and M21 trans. Great fun around town but tiring on highway trips. I swapped the rear to 3:70 and it took a lot of the fun out. So I swapped the M21 for a M20 and it feels pretty close to when it had 4:11's. The lower 1st gear in the M20 makes a difference.
Brian
Brian
Melting Slicks
Joined: Apr 2002
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From: 20 forward gears to shift through in MD
Originally Posted by brucep
I have a 66 L-79 with M-21 and 3.7 posi. I'm about to rebuild the tranny and wonder if I'll enjoy the car more if I replace the M-21 with an M-20. Anyone care to comment on the pros and cons of this swap?
Personally, I wouldn't do it. But, if you decide to do so, you can expect to suffer most (as in, have a noticible drop out of the power band) from 3rd to 4th. But then again, to each there own... 
Race Director
Joined: May 2000
Posts: 12,712
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Originally Posted by brucep
I have a 66 L-79 with M-21 and 3.7 posi. I'm about to rebuild the tranny and wonder if I'll enjoy the car more if I replace the M-21 with an M-20. Anyone care to comment on the pros and cons of this swap?
If you convert to a WR and install a 3.36 axle, the first three overall ratios will be about the same, so acceleration up to about 110 MPH will be about the same. There would be a big 3-4 gap, but you would have a more relaxed highway cruise gear - ten percent less revs at any speed.
If you are considering installing a WR and keeping the 3.70, I don't see the point unless to get off on burnouts.
Duke
Drifting
Joined: Apr 2004
Posts: 1,303
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From: Allen Park Mi
I personally would do it and am considering it in my car.
that WR makes a huge accelleration difference, and 3-4 becomes like overdrive.
i've always said, if you are worried about loosing too many rpms from 3-4th gear... you've either lost the race, or on your way to getting pulled over for doing 75 over the speedlimit LOL
also... i'm not sure how many of us drive around alot in 1st gear LOL
but you'll notice a nice improvment in acceleration and will love it.
Aaron
that WR makes a huge accelleration difference, and 3-4 becomes like overdrive.
i've always said, if you are worried about loosing too many rpms from 3-4th gear... you've either lost the race, or on your way to getting pulled over for doing 75 over the speedlimit LOL
also... i'm not sure how many of us drive around alot in 1st gear LOL
but you'll notice a nice improvment in acceleration and will love it.
Aaron
Melting Slicks
Joined: Apr 2002
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From: 20 forward gears to shift through in MD
Originally Posted by aaronz28
...that WR makes a huge accelleration difference, and 3-4 becomes like overdrive...
4th gear in both the M-20 and M-21 are 1:1, how can that be considered "like overdrive"??????
Race Director
Joined: Jan 2003
Posts: 17,696
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From: Central Arkansas
I have an M-20 with 4.11:1 rear and 427 in my car and with cruising around town the M-21 would probably be more of a pain for stop and go. At one time I planned to replace my M-20 with a M-21 to get back to original equipment, but now I've decided not to.
Dan
Dan
Burning Brakes
Joined: Sep 2002
Posts: 1,003
Likes: 38
From: Scottsdale Arizona
I have a L79, 3.70 rear and M21 - My friend has a M20 - there is a difference, but for my money, I am going to change the rear to a 3.36 or 3.55 for the highway. Good luck with your decision! Ron 
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Le Mans Master
Joined: Feb 2005
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From: New Jersey
having had a m-20 and a m-21 in a old sreet rod I had the 20 was more pleasant to drive but the main shaft on a 20 is only 3/4 inch and is its weak link and will never stand up to the beating a 21 can take, but I still like the 20 better for everyday driving
Team Owner
Joined: Oct 2000
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From: Washington Michigan
Originally Posted by Mr Sidepipes
having had a m-20 and a m-21 in a old sreet rod I had the 20 was more pleasant to drive but the main shaft on a 20 is only 3/4 inch and is its weak link and will never stand up to the beating a 21 can take, but I still like the 20 better for everyday driving
Le Mans Master
Joined: Feb 2005
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From: New Jersey
Originally Posted by JohnZ
Nope - no difference in mainshaft or countershaft sizes between M20/M21 for any given year; all countershafts were 7/8" from '63-'65, and they were 1" from '66-up regardless of the gearset ratio; the only difference between an M20 and an M21 is the ratios.
Race Director
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Mainshaft torque load is a function of torque multiplication by the gearing. Since the WR has ten percent shorter gearing than the CR, the WR mainshaft will see a correspondingly higher mainshaft torque load for the same input torque.
Transmissions are rated by peak input torque and maximum revs, not maximum horsepower. For a family that has different ratio sets, the one with the shortest (highest numerical) first gear will have the lowest rated peak input torque.
Bottom line, it's easier to break a WR than a CR with a high torque engine.
Duke
Transmissions are rated by peak input torque and maximum revs, not maximum horsepower. For a family that has different ratio sets, the one with the shortest (highest numerical) first gear will have the lowest rated peak input torque.
Bottom line, it's easier to break a WR than a CR with a high torque engine.
Duke
Last edited by SWCDuke; Mar 8, 2005 at 04:01 PM.
Team Owner
Joined: Oct 2000
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From: Washington Michigan
Originally Posted by Mr Sidepipes
John, I did some research and your right, why was the m-20 always considered the Muncie that couldn't take a beaten? I remember people always shying away from them? what size mainshaft did a m-22 have? I've had one sitting in my basment for 10 years waiting for a project to put it in