M20 vs M21
Thread Starter
Instructor
Joined: Nov 2009
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M20 vs M21
I am still going through the #'s on my 69 BB coupe. It has a #'s matching M21 and a 3.08 rear axle. I had someone tell me they only put the M21 in cars with 3.36 or higher gears. The rear has the proper BB end cap straps, but that doesn't mean it wasn't replaced. Is this an uncommon combination?
Rich
Rich
Drifting
Joined: Oct 2005
Posts: 1,615
Likes: 150
From: Forth Worth TX
Not sure how common it was, but it was an available option.
Page 90, https://www.gmheritagecenter.com/doc...t-Corvette.pdf
Page 90, https://www.gmheritagecenter.com/doc...t-Corvette.pdf
Team Owner
Joined: Jun 2000
Posts: 38,923
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From: Southbound
Cruise-In II Veteran
Race Director
Joined: Dec 1999
Posts: 19,610
Likes: 778
From: Forked River NJ
If that is truely the combo you have it has to be a dog off the line. Your overall gear ratio in 1st gear would be 6.77. You should be at least in the 9s or 10s. You can probably do about 80 mph in 1st gear. While not very desireable for performance I guess someone could special order it. Most 2nd gear over all ratios are in the high 6s or low 7s so its like you would be starting off in 2nd gear. Lots of clutch slipping to get rolling.
IMO I think the M21 is a terrible street transmission. The close ratio was designed for road racing to keep the motors in peak HP The M20 wide box was a much more street friendly transmission. Fire suit on for this
IMO I think the M21 is a terrible street transmission. The close ratio was designed for road racing to keep the motors in peak HP The M20 wide box was a much more street friendly transmission. Fire suit on for this
Instructor
Joined: Jul 2016
Posts: 226
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From: Crescent Beach, FL
https://www.hagerty.com/apps/valuati...vette?id=47218
Last edited by JohnnyQuik; Jun 21, 2017 at 06:16 PM.
Melting Slicks
Joined: Dec 2016
Posts: 3,193
Likes: 184
From: Central Bucks County, Pennsylvania
If that is truely the combo you have it has to be a dog off the line. Your overall gear ratio in 1st gear would be 6.77. You should be at least in the 9s or 10s. You can probably do about 80 mph in 1st gear. While not very desireable for performance I guess someone could special order it. Most 2nd gear over all ratios are in the high 6s or low 7s so its like you would be starting off in 2nd gear. Lots of clutch slipping to get rolling.
IMO I think the M21 is a terrible street transmission. The close ratio was designed for road racing to keep the motors in peak HP The M20 wide box was a much more street friendly transmission. Fire suit on for this
IMO I think the M21 is a terrible street transmission. The close ratio was designed for road racing to keep the motors in peak HP The M20 wide box was a much more street friendly transmission. Fire suit on for this
Advanced
Joined: Nov 2015
Posts: 78
Likes: 16
From: Concord California
When I had the m-21 rebuilt for my 68 L68 (427) my guy recommended the m-20 2.52:1 first gear, he said the regular m21 first gear could be a pain to use around town with my 3:36 rear diff. So that's what I did, made sense to me, we'll see for sure at some point .
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Instructor
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Thread Starter
Instructor
Joined: Nov 2009
Posts: 242
Likes: 10
If that is truely the combo you have it has to be a dog off the line. Your overall gear ratio in 1st gear would be 6.77. You should be at least in the 9s or 10s. You can probably do about 80 mph in 1st gear. While not very desireable for performance I guess someone could special order it. Most 2nd gear over all ratios are in the high 6s or low 7s so its like you would be starting off in 2nd gear. Lots of clutch slipping to get rolling.
IMO I think the M21 is a terrible street transmission. The close ratio was designed for road racing to keep the motors in peak HP The M20 wide box was a much more street friendly transmission. Fire suit on for this
IMO I think the M21 is a terrible street transmission. The close ratio was designed for road racing to keep the motors in peak HP The M20 wide box was a much more street friendly transmission. Fire suit on for this
Le Mans Master
Joined: Nov 2004
Posts: 5,253
Likes: 768
From: Waterford WI
Well Hagerty thinks the M21 adds $1800 to the value of my '70 big block, so I guess I'll keep it. With 500 lb-ft of torque on tap, the first gear ratio doesn't matter so much.
https://www.hagerty.com/apps/valuati...vette?id=47218
https://www.hagerty.com/apps/valuati...vette?id=47218
Drifting
Joined: Jun 2017
Posts: 1,988
Likes: 101
Lot's of off the wall numbers there. 119K for a number 1 car! Maybe the restoration cost but not market value. Value guides for the most part are worthless. Maybe this is a gimmick to over insure cars for higher premiums.
Value Adjustments
+$1,800 for close ratio 4-spd. -8% for auto trans. +$200 for pwr wndws. +$2,000 for a/c. +$1,850 for hard top. +$900 for custom interior (leather). +$2,500 for pwr strg. +$1,500 for tilt/tele strng whl
Value Adjustments
+$1,800 for close ratio 4-spd. -8% for auto trans. +$200 for pwr wndws. +$2,000 for a/c. +$1,850 for hard top. +$900 for custom interior (leather). +$2,500 for pwr strg. +$1,500 for tilt/tele strng whl
