Some rear differential questions
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Racer
Joined: Jan 2006
Posts: 265
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From: Bowie MD
Some rear differential questions
Well, my knowledge of rear differentials is approximately nil. I know that I will need a different ratio, but I figured I'd get there when I got there. However, the machinist working on my engine is interested in the rear differential ratio and transmission ratios. The transmission is easy, as I will use an M21. I'm going to be putting about 500hp or so out of the motor when all is said and done. My questions are:
How do I determine what I have in there currently?
What ratio do I want for what I'm going to be running (I am not going to race, etc., but be a weekend driver). What kind of RPMs will I expect at say, 70/80mph?
Someday, down the road, I may think about a switch to a 5-speed. Will this cause me any problems other than a "simple" transmission swap with respect to the rear differential?
How do I determine what I have in there currently?
What ratio do I want for what I'm going to be running (I am not going to race, etc., but be a weekend driver). What kind of RPMs will I expect at say, 70/80mph?
Someday, down the road, I may think about a switch to a 5-speed. Will this cause me any problems other than a "simple" transmission swap with respect to the rear differential?
Le Mans Master
Joined: Jan 2000
Posts: 8,918
Likes: 27
From: New Hill NC
The rear will have a code stamped on the under side flange near the spring mount. This would give you the ratio assuming it hasnt been changed. To verify, jack up the car and turn the wheel while counting the number of rotations of the drive shaft. In gerneral at 60 MPH these numbers will be about right.
3:08 - 2300 RPM
3:36 - 2500 RPM
3:55 - 2600 RPM
3:70 - 2750 RPM
4:11 - 3100 RPM
3:08 - 2300 RPM
3:36 - 2500 RPM
3:55 - 2600 RPM
3:70 - 2750 RPM
4:11 - 3100 RPM
Tech Contributor
Joined: Aug 1999
Posts: 15,175
Likes: 3,990
From: Connecticut, USA
The M-21 is a close ratio (2.20 1st)and probably came with 370 gears.
Depending on the cam, heads, intake I would probably use new 373 Tom's gears in it. They will work great with the M-21 or 5 speed option. Problem is finding them now. I have 1 set left but it's reserved for a current rebuild I'm working on. The 373 set is stronger and better then the 370 sets that are available now so depending on how quickly you need gears will determine what you end up with. Talk with your shop and see where the RPM range of the engine is really going to be and you'll be able to go from there.
If you need help rebuilding the diff let me know and I'll walk through it with you.
Good luck
Gary
Depending on the cam, heads, intake I would probably use new 373 Tom's gears in it. They will work great with the M-21 or 5 speed option. Problem is finding them now. I have 1 set left but it's reserved for a current rebuild I'm working on. The 373 set is stronger and better then the 370 sets that are available now so depending on how quickly you need gears will determine what you end up with. Talk with your shop and see where the RPM range of the engine is really going to be and you'll be able to go from there.
If you need help rebuilding the diff let me know and I'll walk through it with you.
Good luck
Gary
Thread Starter
Racer
Joined: Jan 2006
Posts: 265
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From: Bowie MD
Thanks for the information. I will take a look this weekend to see what differential is in the car. I'm not sure anymore if the car was originally manual or automatic (the current transmission is an auto, but I also have a '74 M-21 with the shifter still attached 
).
I will be putting out gobs of low-end torque with the finished motor. It'll be a weekend driver, but I may spend a decent amount of highway time with it. For the sake of comparison, how much low-end punch will I lose if went with, say 3.08 over a 3.73?
).I will be putting out gobs of low-end torque with the finished motor. It'll be a weekend driver, but I may spend a decent amount of highway time with it. For the sake of comparison, how much low-end punch will I lose if went with, say 3.08 over a 3.73?
Race Director
Joined: May 2005
Posts: 10,674
Likes: 122
From: Millington Illinois
St. Jude Donor '08-'09
Originally Posted by nihil
Thanks for the information. I will take a look this weekend to see what differential is in the car. I'm not sure anymore if the car was originally manual or automatic (the current transmission is an auto, but I also have a '74 M-21 with the shifter still attached ).
I will be putting out gobs of low-end torque with the finished motor. It'll be a weekend driver, but I may spend a decent amount of highway time with it. For the sake of comparison, how much low-end punch will I lose if went with, say 3.08 over a 3.73?
).I will be putting out gobs of low-end torque with the finished motor. It'll be a weekend driver, but I may spend a decent amount of highway time with it. For the sake of comparison, how much low-end punch will I lose if went with, say 3.08 over a 3.73?
Last edited by 63mako; Jul 7, 2006 at 10:16 AM.
Thread Starter
Racer
Joined: Jan 2006
Posts: 265
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From: Bowie MD
Originally Posted by 63mako
Lots with a M21. An M20 would be much better if you want to keep stock 3:08. The M21 with 3:08 will fall on it's face taking off from a stop sign. It feels like taking off in second gear. The best option with a 3:08 ia a TKO 500
Thanks
