Muncie M20 or M21?
Thread Starter
Instructor
Joined: Apr 2006
Posts: 243
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From: Canton MI
Muncie M20 or M21?
I know there have been threads on this but the last one I read was 2002 so I wanted to ask the opinions of the newer crowd.
Question is, what's the difference between M20 and M21? Is it just the ratios for the first three gears (obviously the "OD gear"...I put that in quotes in lieu of our 7 and 8 speed trannys that are out there today...is still 1:1).
Question #2: Which one would YOU put in a mild 350 with a 3.70 rear end? More importantly, why? Real pluses and minuses. My gut tells me not to mess with the combinations the GM engineers used but the devil on the left shoulder tells me that it could be a way to gain some performance and not really hurt anything.
Thanks!
Question is, what's the difference between M20 and M21? Is it just the ratios for the first three gears (obviously the "OD gear"...I put that in quotes in lieu of our 7 and 8 speed trannys that are out there today...is still 1:1).
Question #2: Which one would YOU put in a mild 350 with a 3.70 rear end? More importantly, why? Real pluses and minuses. My gut tells me not to mess with the combinations the GM engineers used but the devil on the left shoulder tells me that it could be a way to gain some performance and not really hurt anything.
Thanks!
Thread Starter
Instructor
Joined: Apr 2006
Posts: 243
Likes: 0
From: Canton MI
Ok...
First, thanks for the responses. But, I'm talking only about M20 and M21 (not M22) and was hoping to get a little more technical information and maybe some first hand experience with how these would feel on the street with my rear-end ratio.
Anyone else?
Anyone else?
Team Owner
Joined: Oct 2004
Posts: 23,831
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From: Springfield MO
St. Jude Donor '07
The close-ratio transmission was intended more for racing than the street. Depending on your engine and rear-end gearing, it might not be particularly street-friendly. Other than the gear ratios, I don't think there are major internal or external differences.
Safety Car
Joined: Aug 2006
Posts: 4,102
Likes: 1
From: Forsyth Illinois
The 21 would be the more desireable,because of the gearing w/your mild 350,and 3.70s out back,but I have had several w/20s,and they were just fine.
What year trans?The earlier 20s were weaker w/a 7/8s shaft(63-69),compared to the 1 inch shaft of the 21s.All were 10 spline,even the 22 in 68/69.In late 70 all three went to 26 spine until the end of the run in late 74.
I would get a 21,if it's a 60s,or 70 model due to the shaft size,but I may be mistaken,but I believe the 65 396/425 horse vett had the m20,and they held.Don't overlook a Surep T10,either,just to have a muncie.They are good trans,too.
I have a 75 Super T10 that I would let go,and a friend w/a trans shop has a rebuilt 69 21,and a blank casing 68 m20 forsale,rebuilt.He want's a grand each,and I'd probably let the T10 go for 450.00,plus shipping.A M20 would hold what you are running,tho.Used ones are around 2-500.00,unless someone is doing the numbers game,and then they get you.
What year trans?The earlier 20s were weaker w/a 7/8s shaft(63-69),compared to the 1 inch shaft of the 21s.All were 10 spline,even the 22 in 68/69.In late 70 all three went to 26 spine until the end of the run in late 74.
I would get a 21,if it's a 60s,or 70 model due to the shaft size,but I may be mistaken,but I believe the 65 396/425 horse vett had the m20,and they held.Don't overlook a Surep T10,either,just to have a muncie.They are good trans,too.
I have a 75 Super T10 that I would let go,and a friend w/a trans shop has a rebuilt 69 21,and a blank casing 68 m20 forsale,rebuilt.He want's a grand each,and I'd probably let the T10 go for 450.00,plus shipping.A M20 would hold what you are running,tho.Used ones are around 2-500.00,unless someone is doing the numbers game,and then they get you.
Le Mans Master
Joined: Jan 2006
Posts: 6,506
Likes: 139
From: Butler Pa
Which is which? Is the M21 the close ratio?
Year 1963-1965 Type: M20 Rings: None Ratio: 2.56 / 1.91 / 1.48 / 1.00 / 3.16
Year 1966-1974 Type: M20 Rings: Two Ratio: 2.52 / 1.88 / 1.46 / 1.00 / 3.11
Year 1963-1974 Type: M21 Rings: One Ratio: 2.20 / 1.64 / 1.28 / 1.00 / 2.27
Year 1967-1974 Type: M22 Rings: None Ratio: 2.20 / 1.64 / 1.28 / 1.00 / 2.27
Year 1963-1965 Type: M20 Rings: None Ratio: 2.56 / 1.91 / 1.48 / 1.00 / 3.16
Year 1966-1974 Type: M20 Rings: Two Ratio: 2.52 / 1.88 / 1.46 / 1.00 / 3.11
Year 1963-1974 Type: M21 Rings: One Ratio: 2.20 / 1.64 / 1.28 / 1.00 / 2.27
Year 1967-1974 Type: M22 Rings: None Ratio: 2.20 / 1.64 / 1.28 / 1.00 / 2.27
Drifting
Joined: Jan 2006
Posts: 1,944
Likes: 22
From: Yooperville MI
Other than some minor engineering differences the major difference is the percentage of ratio drop through the gear range from the previous gear. An M20 with it's 2.52 first gear is easier off the line than a M21 with it's 2.20 first gear.
Below would be the percentage of drop from one gear to the next:
M20 2.52-1.88-1.46-1.00 Drop= 25%-22%-31%
M21 2.20-1.64-1.28-1.00 Drop=25%-21%-21%
As you make the ratios closer you end up having to change the first gear ratio to end up with 1:1. An M21 should have a rear end ratio of 3.70 or greater for best drivability due to the tall first gear. In other words, the M21 will be harder to get rolling than a M20. The M21 was available only on high performance engines where the M20 was standard on engines up to the basic 4 barrel models but was also available on most high performance applications.
Below would be the percentage of drop from one gear to the next:
M20 2.52-1.88-1.46-1.00 Drop= 25%-22%-31%
M21 2.20-1.64-1.28-1.00 Drop=25%-21%-21%
As you make the ratios closer you end up having to change the first gear ratio to end up with 1:1. An M21 should have a rear end ratio of 3.70 or greater for best drivability due to the tall first gear. In other words, the M21 will be harder to get rolling than a M20. The M21 was available only on high performance engines where the M20 was standard on engines up to the basic 4 barrel models but was also available on most high performance applications.
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Melting Slicks
Joined: Jan 2004
Posts: 3,247
Likes: 210
From: Detroit MI
M20 is wide, M21 close. For street use you may find the M20 more suitable for better stoplight to stoplight acceleration; the M21 will feel more doggy. The close allows higher mph in each of the first three gears, at the cost of acceleration, and the tighter spread from first to fourth keeps the engine in it's powerband better. Obviously fourth is the same between the two.
For racing, I use both transmissions. With my 4.11 gear, I like the wide for autocrosses so that I can get a good launch and then go to second quickly and leave it there, and have about 70 mph top speed. For road courses, the close enables me to wind out third a little more where if I had the wide I'd have to shift to fourth.
For racing, I use both transmissions. With my 4.11 gear, I like the wide for autocrosses so that I can get a good launch and then go to second quickly and leave it there, and have about 70 mph top speed. For road courses, the close enables me to wind out third a little more where if I had the wide I'd have to shift to fourth.
Race Director
Joined: May 2005
Posts: 10,674
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From: Millington Illinois
St. Jude Donor '08-'09
M20 is wide, M21 close. For street use you may find the M20 more suitable for better stoplight to stoplight acceleration; the M21 will feel more doggy. The close allows higher mph in each of the first three gears, at the cost of acceleration, and the tighter spread from first to fourth keeps the engine in it's powerband better. Obviously fourth is the same between the two.
For racing, I use both transmissions. With my 4.11 gear, I like the wide for autocrosses so that I can get a good launch and then go to second quickly and leave it there, and have about 70 mph top speed. For road courses, the close enables me to wind out third a little more where if I had the wide I'd have to shift to fourth.
For racing, I use both transmissions. With my 4.11 gear, I like the wide for autocrosses so that I can get a good launch and then go to second quickly and leave it there, and have about 70 mph top speed. For road courses, the close enables me to wind out third a little more where if I had the wide I'd have to shift to fourth.
M20 for the street with a mild 350 and 3.70 gears. Ideal launch ratio in 1st gear is 10 to 1.The M20 and a 3.70 is 9.32 to 1.
Le Mans Master
Joined: Jan 2006
Posts: 6,506
Likes: 139
From: Butler Pa
Now I'm confused. According to this, I should have an M20. My car is a '69 originally with the base 300hp/350cid. The trans is the original, numbers match. Last year I had the tranny rebuilt. It needed the reverse gear. I ordered and installed a gear for an M21. This should not have worked.
Race Director
Joined: May 2005
Posts: 10,674
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From: Millington Illinois
St. Jude Donor '08-'09
Now I'm confused. According to this, I should have an M20. My car is a '69 originally with the base 300hp/350cid. The trans is the original, numbers match. Last year I had the tranny rebuilt. It needed the reverse gear. I ordered and installed a gear for an M21. This should not have worked.
Pro
Joined: Aug 2000
Posts: 713
Likes: 1
From: plymouth mass
I am pleased with the street performance of my mild 350 in front of a M-20 and 3:36 gears. The low 1st gear provides for a good launch with the 3:36 gears while still enjoying decent highway performance.
Melting Slicks
Joined: May 2007
Posts: 3,246
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From: Bay Area CA
I just got done driving my 'stock '73 L48 with a M21 close ratio, 3.55 rear. The M21's first and second really kinda tach the motor, the ratios are really close, way too much overlap for good street driving, third is a step higher in difference then the diff between first and second, there is kind of a small dead space where you don't want to be- too high for the low ratio second and to low for the higher third, at about 30 - 40 mph, the tourque is good though being it's a 350. RPMs drop from about 3000 to 2000 on shifting into third. 4th is of course pretty high, but am still running at about 3000 rpm at 70 mph, and that's with a 3.55, with a 3.70 the taching around town would not be to my liking, I would prefer the M20, better to pull through the gears. C. Probably going to chang the rear end to a 3.08. be at about 2500 rpms at 70 mph.
Last edited by RunningMan373; Oct 18, 2007 at 09:35 AM.
Drifting
Joined: Sep 2006
Posts: 1,259
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From: MS
Runningman and Bashcraft, the M21 was not available with the base engine in any year Corvette, so if original, you will both have the wide ratio M20. Can't explain the reverse gear situation but no way was a base engine car built with an M21.
In the post by ahoover, he shows that the for the first 3 gears, the ratio steps between the M20 and the M21 are about the same and it is only the M20's big drop between 3rd and 4th where you can notice a difference, so that is why the M20 is preferred for the street as you normally would not be in a situation where you are making a 3 to 4 upshift at max rpm and the lower 2.52 first gear gets you rolling easier.
In the post by ahoover, he shows that the for the first 3 gears, the ratio steps between the M20 and the M21 are about the same and it is only the M20's big drop between 3rd and 4th where you can notice a difference, so that is why the M20 is preferred for the street as you normally would not be in a situation where you are making a 3 to 4 upshift at max rpm and the lower 2.52 first gear gets you rolling easier.
Race Director
Joined: May 2005
Posts: 10,674
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From: Millington Illinois
St. Jude Donor '08-'09
Kajonjon.com has good info on parts and what is interchangable.
Tech Contributor
Joined: Aug 1999
Posts: 15,176
Likes: 3,991
From: Connecticut, USA
I have a 69 that had a M-20. They originally came with 336 gears as this is a good combination setup for street and highway. I installed 373(not 370) gears and it was fine as well. Certainly took off faster but highway RPM was up about 400 RPM.
The M20 could be had with 308 as well. So to answer your question it will work just where to you want to run RPM wise at 65 mph?
A side note. A buddy with a 76 and ST-10 (264 1st) had me build him a diff with all the mods. He had me install 273 gears. I thought those would be a problem starting off but he loves it. The speedo is off about 8 mph but he loves the gear.
The M20 could be had with 308 as well. So to answer your question it will work just where to you want to run RPM wise at 65 mph?
A side note. A buddy with a 76 and ST-10 (264 1st) had me build him a diff with all the mods. He had me install 273 gears. I thought those would be a problem starting off but he loves it. The speedo is off about 8 mph but he loves the gear.
Race Director
Joined: Jan 2000
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From: Corsicana, Tx
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On the street I'd definitely use the M20. The lower gears of 1-3 will help driveability a lot. If you were even drag racing, you would barely be getting close to needing 4th gear anyway in the 1/4 mile...so all your acceleration is in the first 3 gears.
When I raced with M-20's...I just made sure I wound 3rd gear a little higher to make up for the bigger drop into 4th. The other spreads are essentially the same as noted above.
JIM
When I raced with M-20's...I just made sure I wound 3rd gear a little higher to make up for the bigger drop into 4th. The other spreads are essentially the same as noted above.
JIM