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Thread: Install Differential 3.91

  1. #1
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    Install Differential 3.91

    Hello series 8 lovers. I am researching on the differential exchange of my car. My 850ci, USA version, comes with 4-speed transmission, with Sport option, and 3.64 differential. It is equipped with the Woke V2 chip. The car performs well, but I plan on making it more fun with differential swapping by installing 3.91. My question: Is the differential 3.91 compatible with the 8 came installed only in the bmw M5 E34, or has it also fitted other bmw models of the era such as the series 3? What year / model M5 E34 came with this differential, 3.6 l or 3.8 l? Another thing: does this 3.91 of M5 need to be LSD or can it be without LSD?
    Sorry for the long text, but I'm excited about this exchange.
    Thank you

  2. #2
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    Quote Originally Posted by Faria View Post
    Hello series 8 lovers. I am researching on the differential exchange of my car. My 850ci, USA version, comes with 4-speed transmission, with Sport option, and 3.64 differential. It is equipped with the Woke V2 chip. The car performs well, but I plan on making it more fun with differential swapping by installing 3.91. My question: Is the differential 3.91 compatible with the 8 came installed only in the bmw M5 E34, or has it also fitted other bmw models of the era such as the series 3? What year / model M5 E34 came with this differential, 3.6 l or 3.8 l? Another thing: does this 3.91 of M5 need to be LSD or can it be without LSD?
    Sorry for the long text, but I'm excited about this exchange.
    Thank you
    Hi, yes in your case 3.91 LSD is optimal, it will wake up the car big time. In US 3.91 was only offered on 1991 E34 M5.
    In continental Europe 3.91 LSD also came on very few 735i E32.
    Rare any way you put it. It did not come without the LS option.
    It's a straight bolt on, just the rear cover and output flanges need to be swapped. (easy).
    If interested I have 1 left and could ship to Brazil.

  3. #3
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    Thank you Dragon850 for the clarifications. Brazilian legislation does not allow the importation of used parts. Believed, too, that due to weight, shipping is impracticable.

  4. #4
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    Has anyone ever measured the acceleration from 0-62 with 3,91 diff compared to stock 3,15 diff??

    I know many has done the swap but can anyone give some hard numbers to back up their experiences?

    Also what is the rpm when driving 62 with 3,91?

  5. #5
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    I have a auto chart somewhere, Meanwhile here's some info for the 6 speed Manual
    Attached Images Attached Images
    Last edited by 8eights; 07-01-2017 at 04:44 PM.

  6. #6
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    Use the gear ratios from here
    http://www.e31.net/technical.html

    ...and the calculator here
    http://www.endtuning.com/gearratios/

    assuming stock tire size, in 4th gear the difference is 2349-2187=162rpm
    Last edited by Budget M3; 07-01-2017 at 06:00 PM.
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  7. #7
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    Another diff question.

    My car has 3,64 diff and i wonder if i install 3,91 does it make a big difference anymore? I know folks who has euro spec cars with 3,15 diff have experience a big improvement in cars performance but since i have 3,64 allready is the change really all that dramatic ?
    Last edited by mikae31; 07-30-2017 at 05:20 PM.

  8. #8
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    The short, and real answer to you question is Yes. Going from a 3.64 to a 3.91 will wake up your car noticeably.

  9. #9
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    I haven't read much on the 3.91 LSD being installed in the 6-speed manual cars . . . just the automatics. Is there a reason for this?

  10. #10
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    Quote Originally Posted by Sniff38 View Post
    I haven't read much on the 3.91 LSD being installed in the 6-speed manual cars . . . just the automatics. Is there a reason for this?
    3.91 gearing is to short for manuals, you would need 3.15 or 3.45 for 6 speed cars.

  11. #11
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    Quote Originally Posted by dragon850 View Post
    3.91 gearing is to short for manuals, you would need 3.15 or 3.45 for 6 speed cars.
    OK, makes sense. That explains why I didn't see a 3.91 option in 8eight's spreadsheet. Just to make sure, did the U.S. spec 6-speed 850i's come with a 2.65 rear end?
    Last edited by Sniff38; 07-31-2017 at 02:02 PM.

  12. #12
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    Quote Originally Posted by dragon850 View Post
    Hi, yes in your case 3.91 LSD is optimal, it will wake up the car big time. In US 3.91 was only offered on 1991 E34 M5.
    In continental Europe 3.91 LSD also came on very few 735i E32.
    Rare any way you put it. It did not come without the LS option.
    It's a straight bolt on, just the rear cover and output flanges need to be swapped. (easy).
    If interested I have 1 left and could ship to Brazil.


    i have had my 3.91 sitting in the garage for eternity now.. so while i got the car in the air, & major surgery going on.. i might get it in. i thought i had everything for a swap, but you say the rear cover & output flanges need to be changed.. are those from the e34 m5 also? whats the part #?

  13. #13
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    Quote Originally Posted by btnh View Post

    i have had my 3.91 sitting in the garage for eternity now.. so while i got the car in the air, & major surgery going on.. i might get it in. i thought i had everything for a swap, but you say the rear cover & output flanges need to be changed.. are those from the e34 m5 also? whats the part #?
    What I mean is that you need to remove output flanges and rear cover from your original differential and swap them onto the m5 diff.

  14. #14
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    Quote Originally Posted by Sniff38 View Post

    OK, makes sense. That explains why I didn't see a 3.91 option in 8eight's spreadsheet. Just to make sure, did the U.S. spec 6-speed 850i's come with a 2.65 rear end?
    Yes

  15. #15
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    My car has 3,64 diff and i wonder if i install 3,91 does it make a big difference anymore? I know folks who has euro spec cars with 3,15 diff have experience a big improvement in cars performance but since i have 3,64 allready is the change really all that dramatic ?
    The difference between the swaps is pretty close. The 2.81 to 3.15 is an .089 percent increase.
    The 3.64 to 3.91 is an .093 percent increase.

  16. #16
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    Quote Originally Posted by rjbaren View Post

    The difference between the swaps is pretty close. The 2.81 to 3.15 is an .089 percent increase.
    The 3.64 to 3.91 is an .093 percent increase.
    Please check and edit you analysis as necessary.

  17. #17
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    I'm not sure I follow, What did I do wrong?

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