simple (for someone) rear end question
Thread Starter
Safety Car
Joined: Sep 2005
Posts: 4,062
Likes: 103
From: Southeast, Tennessee
St. Jude Donor '09 thru '26
simple (for someone) rear end question
i did the usual google search and searched the tech forum here, but can't really decide if i got my questions answered regarding the original options for rear end ratios and whether there was a difference between the standard rear end ratio for the automatic trans versus the 4 speed (closed ratio M20) trans without opting for the high performance option. it appears there was a "high performance" rear gear option, but i can;t find information as to whether that was an option available to both or just the 4 speed and what the various "standard" and "optional" rear end ratios were.
can someone provide a link to the proper information or perhaps a cut and paste of it here??
the reason for my inquiry is that i am converting my 78 automatic to an OEM 4 speed (M20) and would like to know if i can use my current posi rear or if there was a standard 4 speed rear that was used at a bit lower ratio other than the optional one??
many thanks
can someone provide a link to the proper information or perhaps a cut and paste of it here??
the reason for my inquiry is that i am converting my 78 automatic to an OEM 4 speed (M20) and would like to know if i can use my current posi rear or if there was a standard 4 speed rear that was used at a bit lower ratio other than the optional one??
many thanks
Melting Slicks
Joined: Sep 2004
Posts: 3,227
Likes: 670
From: Northeast
M21= Closed ratio not the m20
If you are looking for the standard choice then:
L48 4spd (2.85 low) Std 3.36
L48 auto = 3.08 & 3.55(above 4000 feet altitude)
L82 4spd (2.64 low) Std 3.70 & 3.36 (below 4000 feet)
L82 4spd (2.43 low) Std 3.70
L82 auto Std 3.55.
With that said these are the standards. Someone on this forum has a closed ratio 78 M21 and he opted for a different rear end than 3.70 and got it.
You should write to Hunt4cleanair and ask him to send you the write up he did about the 4spds. Very informative.
I got my info from the 78 Restoration packet (specifications).
If you are looking for the standard choice then:
L48 4spd (2.85 low) Std 3.36
L48 auto = 3.08 & 3.55(above 4000 feet altitude)
L82 4spd (2.64 low) Std 3.70 & 3.36 (below 4000 feet)
L82 4spd (2.43 low) Std 3.70
L82 auto Std 3.55.
With that said these are the standards. Someone on this forum has a closed ratio 78 M21 and he opted for a different rear end than 3.70 and got it.
You should write to Hunt4cleanair and ask him to send you the write up he did about the 4spds. Very informative.
I got my info from the 78 Restoration packet (specifications).
Thread Starter
Safety Car
Joined: Sep 2005
Posts: 4,062
Likes: 103
From: Southeast, Tennessee
St. Jude Donor '09 thru '26
thank you both for the very informative input. i greatly appreciate your help. i guess my problem now will be what it will be like using the 3:08 rear on a 383 stroker (.30 over with a mild cam and approx 9.5:1 compression). i assume the low end will be pretty weak, but the cruising speed will be at lower RPM and will produce better MPG. opinions regarding the trade off???
thank you again
steve
thank you again
steve
Safety Car
Joined: Aug 2000
Posts: 3,711
Likes: 43
From: San Antonio TX
A true close-ratio Muncie (M21 or M22) and a 3.08 rear gear gives you a 6.75 ratio at the axle. Unless this car has some phenomenal low-rpm torque and you are using a 35-40lb flywheel, it will not be an easy car to get off the mark. Generally recommended target for the axle is 9:1 for a street-driven manual trans car. A wide-ratio Muncie (M20) with it's 2.64 1st gear gets you acceptably close a 8.1 and the car will be fine if the engine doesn't sacrafice much low-rpm torque and you use at least a 30lb flywheel.
Unfortunately, an engine build is more than just pistons pushing on rods and rotating the crank. You also have to consider what the crank is turning and this would include an appropriate transmission and final drive. Others who have made a mistake in the rest of the drivetrain can relate how little fun driving such a mismatched colletion of parts can be.
It's hard to gauge your outcome since a "mild cam" covers a lot of ground.
Unfortunately, an engine build is more than just pistons pushing on rods and rotating the crank. You also have to consider what the crank is turning and this would include an appropriate transmission and final drive. Others who have made a mistake in the rest of the drivetrain can relate how little fun driving such a mismatched colletion of parts can be.
It's hard to gauge your outcome since a "mild cam" covers a lot of ground.
Le Mans Master
Joined: Apr 2007
Posts: 7,353
Likes: 72
From: Graceland in a Not Correctly Restored Stingray
Handy link below lists available power teams, but there's no rule which says you have to adhere to it...
http://carburetor-manual.com/libro/1...tion-Guide.pdf
http://carburetor-manual.com/libro/1...tion-Guide.pdf
Thread Starter
Safety Car
Joined: Sep 2005
Posts: 4,062
Likes: 103
From: Southeast, Tennessee
St. Jude Donor '09 thru '26
A true close-ratio Muncie (M21 or M22) and a 3.08 rear gear gives you a 6.75 ratio at the axle. Unless this car has some phenomenal low-rpm torque and you are using a 35-40lb flywheel, it will not be an easy car to get off the mark. Generally recommended target for the axle is 9:1 for a street-driven manual trans car. A wide-ratio Muncie (M20) with it's 2.64 1st gear gets you acceptably close a 8.1 and the car will be fine if the engine doesn't sacrafice much low-rpm torque and you use at least a 30lb flywheel.
Unfortunately, an engine build is more than just pistons pushing on rods and rotating the crank. You also have to consider what the crank is turning and this would include an appropriate transmission and final drive. Others who have made a mistake in the rest of the drivetrain can relate how little fun driving such a mismatched colletion of parts can be.
It's hard to gauge your outcome since a "mild cam" covers a lot of ground.
Unfortunately, an engine build is more than just pistons pushing on rods and rotating the crank. You also have to consider what the crank is turning and this would include an appropriate transmission and final drive. Others who have made a mistake in the rest of the drivetrain can relate how little fun driving such a mismatched colletion of parts can be.
It's hard to gauge your outcome since a "mild cam" covers a lot of ground.
thank you for the help and insight.
Thread Starter
Safety Car
Joined: Sep 2005
Posts: 4,062
Likes: 103
From: Southeast, Tennessee
St. Jude Donor '09 thru '26
Handy link below lists available power teams, but there's no rule which says you have to adhere to it...
http://carburetor-manual.com/libro/1...tion-Guide.pdf
http://carburetor-manual.com/libro/1...tion-Guide.pdf
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Burning Brakes
Joined: Sep 2005
Posts: 769
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From: Goose Creek SC
Now I am curious about my combo. My '68 came with a 327/300, a four speed and it has the original rear end marked as a 3.70. Is that an unusual combo? What are the chances my 4 speed may be an M21 vs. an M20 (given that rear end)? My tranny is original but I dont know if it is M20 or M21. This is a great thread, thanks for starting it.
Racer
Joined: Jan 2010
Posts: 428
Likes: 1
From: San Antonio Texas
Thread Starter
Safety Car
Joined: Sep 2005
Posts: 4,062
Likes: 103
From: Southeast, Tennessee
St. Jude Donor '09 thru '26
Now I am curious about my combo. My '68 came with a 327/300, a four speed and it has the original rear end marked as a 3.70. Is that an unusual combo? What are the chances my 4 speed may be an M21 vs. an M20 (given that rear end)? My tranny is original but I dont know if it is M20 or M21. This is a great thread, thanks for starting it.
http://www.hunt4cleanair.net/
Race Director
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From: Mustang OK
2026 Corvette of the Year Finalist - Modified
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Once upon a time, long, long ago, I thoroughly enjoyed 3.70, 4.11, 4.56 gears in my cars (and gas was $.23-.25cents/gal). But now I've switched to rears with 2.41, 2.73, 3.08 ratios. Of course, my cars have 4-5spds with a low 1st gear and engines with plenty of torque, which results in virtually no feathering of the clutch------------------------------NO, none of my cars have auto trannys! so the combinations work just fine. The big plus is much lower rpm on the Interstate. Back in the days of .23-.25/gal gas, the term Interstate hiway did not even exist!
Last edited by DZAUTO; Mar 31, 2020 at 02:06 PM.
