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Thread: 3.91 Differential with a Getrag 242 Transmission?

  1. #1
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    3.91 Differential with a Getrag 242 Transmission?

    I pulled a 3.91 differential off of a 1981 donor car. Did not grab the 5 speed transmission. It had many miles, I didn't have the ability or will to grab the transmission. Figured I could find a lesser worn transmission when it came time for the upgrade.

    I've toyed with the idea of throwing the diff in my stock Getrag 242 transmission on my 78 or upgrading to the 5-speed but never pulled the trigger.


    Thinking of doing that project this summer now that I have the time and ability. Would it be worth throwing the 3.91 in the stock 242 transmission, or finding a late model 1980 5-speed transmission first? I know the gear ratios are different, but I haven't seen too many threads with this setup but I know they are there (search is failing me)

    I don't plan on doing a lot of performance modifications and it could be likely that the difference is lost to someone like me. But if Im going to do the job, I want to at least consider it.. If anyone has this setup, please let me know! I would love any information.

    edit: The target would be an 83 transmission, Getrag 245/10 close ratio/”dogleg”. There's one in Houston I could get a hold of, would need to pull it myself.
    Last edited by jaredmac11; 04-04-2016 at 10:25 AM.

  2. #2
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    Cool

    Here is info on Getrag transmission and picture of the sport model-close ratio. Close ratio is 1:1 in 5th gear whereas the economy is .805

    http://www.bmwe21.net/?page_id=124

    As to the 3rd member the higher the numbers the faster you wind up your motor, Lower numbers are better for top end, higher number better for short end. There is also drive shaft length and flange considerations to factor in.

    Randy

  3. #3
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    Quote Originally Posted by 320iAman View Post
    Here is info on Getrag transmission and picture of the sport model-close ratio. Close ratio is 1:1 in 5th gear whereas the economy is .805

    http://www.bmwe21.net/?page_id=124

    As to the 3rd member the higher the numbers the faster you wind up your motor, Lower numbers are better for top end, higher number better for short end. There is also drive shaft length and flange considerations to factor in.

    Randy
    Thank you very much, that's a good resource and great information.

  4. #4
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    ear plugs may be needed.

  5. #5
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    Quote Originally Posted by gsumzzz View Post
    ear plugs may be needed.
    from the high rpms I hope and not the ill-fitted diff...

  6. #6
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    going from 3.64 to 3.91 will increase rpms on freeway cruising by about 7 percent. 3.91/3.64 = 1.0742. So if you were highway cruising at 4000 rpm, you would now be up to 4,300 rpm, given the same miles per hour speed.

  7. #7
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    I didnt know that. I think it would be wise to get the 5 speed then since it already cruises at a pretty high RPM.

  8. #8
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    Quote Originally Posted by jaredmac11 View Post
    ...it already cruises at a pretty high RPM.
    That's ok for a small 4 cylinder, should be that way. It's not meant to be like a Buick with a big V-8

  9. #9
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    Quote Originally Posted by jaredmac11 View Post
    edit: The target would be an 83 transmission, Getrag 245/10 close ratio/”dogleg”. There's one in Houston I could get a hold of, would need to pull it myself.
    Are you saying that you believe US model 83 cars have a dogleg? In that case you case you are mistaken. No US E21 ever had a dogleg. They're very hard to find here and worth $2000 easily. Plus the final gear ratio would be 1:1, identical to your 4 speed. So that does nothing for your high RPM problem.

    The 83 transmission is a 5 speed overdrive with nearly identical ratios to 80-82. It is a different transmission, but without any real difference. It just has a shorter 2 piece case instead of the 80-82 3 piece case. And so it uses a different driveshaft and shifter linkage that can be harder to find. It's generally less of a pain in the butt to stick with the more common transmission, but once it's up and running you'd never know the difference from inside the car.

    Personally I'd rather not combine the 3.91 with a non-overdrive. I like decent gas mileage and lower noise. I would install a 5 speed in your car and keep the 3.64. That's what I plan to do with my 79. With a slightly upgraded engine I think it would out-accelerate the same car with a stock engine and 3.91. Of course you could do both and get even more acceleration, but it's not a drag car anyway. The only real downside is that E21 LSD's in 3.64 are super hard to find and expensive. For that reason, an overdrive plus 3.91 is a very acceptable setup. A lower diff is a bit of a band-aid for an undesirable power to weight ratio anyway. If you had a 300hp motor in there, you would not want a 3.91 at all. You'd be going the other direction. That's why you find a 4.10 in an E21 315, and a 4.44 in an E36 318ti. I've been hoarding parts for a future 300-ish HP build and I've selected a dogleg 1:1 transmission and a 3.25 LSD (heavy duty parts, not intended for E21's). That's a starting point though, you can't precisely calculate the ideal diff ratio.

  10. #10
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    Quote Originally Posted by Layne View Post
    Are you saying that you believe US model 83 cars have a dogleg? In that case you case you are mistaken. No US E21 ever had a dogleg. They're very hard to find here and worth $2000 easily. Plus the final gear ratio would be 1:1, identical to your 4 speed. So that does nothing for your high RPM problem.

    The 83 transmission is a 5 speed overdrive with nearly identical ratios to 80-82. It is a different transmission, but without any real difference. It just has a shorter 2 piece case instead of the 80-82 3 piece case. And so it uses a different driveshaft and shifter linkage that can be harder to find. It's generally less of a pain in the butt to stick with the more common transmission, but once it's up and running you'd never know the difference from inside the car.

    Personally I'd rather not combine the 3.91 with a non-overdrive. I like decent gas mileage and lower noise. I would install a 5 speed in your car and keep the 3.64. That's what I plan to do with my 79. With a slightly upgraded engine I think it would out-accelerate the same car with a stock engine and 3.91. Of course you could do both and get even more acceleration, but it's not a drag car anyway. The only real downside is that E21 LSD's in 3.64 are super hard to find and expensive. For that reason, an overdrive plus 3.91 is a very acceptable setup. A lower diff is a bit of a band-aid for an undesirable power to weight ratio anyway. If you had a 300hp motor in there, you would not want a 3.91 at all. You'd be going the other direction. That's why you find a 4.10 in an E21 315, and a 4.44 in an E36 318ti. I've been hoarding parts for a future 300-ish HP build and I've selected a dogleg 1:1 transmission and a 3.25 LSD (heavy duty parts, not intended for E21's). That's a starting point though, you can't precisely calculate the ideal diff ratio.
    I must have been mistaken because I did think it was exclusive to all 83 models. Also, I visited the garage I dumped all this stuff in several years back and I must have been sleeping, but I did retain the transmission and driveshaft with the LSD from the 81. Couldn't find it at first.

    Given this... I will likely just plug all of that in to my 78. Despite having nearly 250k miles on it, I think having the LSD would be more fun ride. Despite a higher ratio, the LSD is all the fun anyways, right?

    Ive never had interest in a drag car. It all comes down to road feel, which I love in my e21. I assumed the LSD would give a more spirited drive. As long as I'm not losing that in this transition from a 3.64 to a 3.91. Really educational information though. This type of stuff was always a bit ahead of me..

  11. #11
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    5 speed came into Austin today. But, Layne is probably already been there.


  12. #12
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    Quote Originally Posted by dups View Post
    5 speed came into Austin today. But, Layne is probably already been there.
    I should. They're getting to be worth a buttload.

  13. #13
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    Layne, so what type of diff do most do with a swap, for instance the m42? I'm content with my m10 but I've always wondered. Is it a grab bag?

  14. #14
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    the only factory diff worth installing is a 3.91:1 however if you have some spare change i would suggest building a custom 4.10

    i just finished building a 4.44 for my m42 swap, but haven't installed it yet. actually i built it for track duty.
    Tom D

    77 e21 - m42
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  15. #15
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    Just out of curiousity, what do custom diffs run, about? And yes, I have my stock 3.64 diff and then the 3.91 LSD. Think the LSD would make the most sense for now. I wonder how the car will feel after the transmission and differential swap..

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