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Thread: Need Differential I.D.

  1. #1
    Join Date
    Dec 2012
    Location
    topanga CA
    Posts
    483
    My Cars
    84 728i,325Ci, 77 R100RS

    Need Differential I.D.

    I found a big diff in the container today while I was looking for something else and scraped enough dirt off it to find a big white "S." Now please understand I have 40 years of stuff in these containers and the Doctors have squished my brain up a little so I have no clue what this is out of or where I got it. It's huge so I'm assuming 7 series of some description and in my experience the "S" is LSD. I pulled a few casting numbers off it but in a quick look found nothing stamped.

    If I had to guess I would say it came from a 745i or very late E23. I want to put it in an 84 728i.

    What numbers (and where) am I looking for to ID part, fitment and ratio and maybe even LSD%? Clues gratefully accepted.

  2. #2
    Join Date
    Feb 2009
    Location
    Pittsburgh, PA
    Posts
    1,718
    My Cars
    '86 635CSi, '08 128i
    That should be a rear-loaded diff...should have a tag on one of the cover bolts that will have a code. The LSDs will have an 'S' then the numeric ratio. If the tag is broken or unreadable, an LSD should spin the output shafts in the same direction when you spin the input. An open diff will spin them in opposite directions. Count the number of spins the input does when you turn the outputs one full turn to estimate ratio.

    Might be a large case (210mm) diff...those are pretty easy to tell. Just look at the output flange retainers. The medium-case (188mm) diff will have six retaining bolts. The large case will have eight bolts. The rear covers are also slightly different, but not enough to easily tell them apart.

  3. #3
    Join Date
    Mar 2006
    Location
    Sacramento, CA
    Posts
    935
    My Cars
    87 535i/84 €735i/8
    All the diffs are big and heavy
    5 and 7 series both had medium and large case diffs.
    As Dave said you can see if its medium or large casea by how many bolts are on the side flanges.
    The S painted on the case means LSD, and there should be a metal tag on 1 of the bolts that holds the rear cover on.
    --Jay3->Sic gorgiamus allos subjectatos nunc
    84 €735i manual, 88 528e/i
    Motronic 1 and 1.3 wiring
    Transmission & Diff gearing sheet

  4. #4
    Join Date
    Dec 2012
    Location
    topanga CA
    Posts
    483
    My Cars
    84 728i,325Ci, 77 R100RS
    Both outputs spin same way so yes, LSD. Didn't see a tag but...grease.

    QUESTION: Will my 84 728i accept either size case? I have halfshafts with the LSD.

    More data later.

  5. #5
    Join Date
    Mar 2007
    Location
    Quadratechtown, PA
    Posts
    741
    My Cars
    5, 7, 4x4
    medium diff has two plugs on back drain and fill. big diff has fill on back and drain on drivers side
    https://www.bimmerforums.com/forum/s...-or-Large-diff
    rotate the output shaft and count the revolutions of the input for one rotation of the output. Choices are:

    2.73
    2.93
    3.07
    3.25
    3.46
    3.64
    3.73
    3.91
    and higher.

    Should be easy to determine what the ratio is.

  6. #6
    Join Date
    Jul 2010
    Location
    Leland, NC
    Posts
    828
    My Cars
    85 €735i 08 550i/6
    Yes both sizes fit your car

    Sent from my LGLS755 using Tapatalk

  7. #7
    Join Date
    Dec 2012
    Location
    topanga CA
    Posts
    483
    My Cars
    84 728i,325Ci, 77 R100RS
    Cool. My first 106 had a really tall diff and it was fun to spool it up. The smaller 728i with 102 came with a 3.64. Gixxer has sold me a trick turbo which will now make 700RWHP at 1100 RPMS so I'm not clear what the ideal diff numbers will be but it's good to have a choice. More data soon but turbo first.

  8. #8
    Join Date
    Jun 2010
    Location
    TX
    Posts
    4,487
    My Cars
    1983/84/85 745i

    Thumbs up

    I have a 3.91:1 in my running car currently. It was good with the stock K27 and 8-12psi, but with this bigger one I could use a slightly taller gear set to let the modified K27 do the work now instead of the gears. Just seems like I get through the powerband too quick, hard to explain. Maybe not a 2.93:1/3.07:1...but like a 3.46:1 which I have sitting about. Just too lazy to swap it. The deeper gear create a hair more lag in first gear with the auto, but it accelerates much quicker due to the torque multiplication of the gears.

    My setup, with the 3.91:1, has boost threshold like stock around 2200-2300rpm and full is around 3300-3400rpm, depending how accurate that stock tachometer really is...and boost gauge.

    Hold onto that 3.64:1, those are super rare.
    I make E23 parts.
    09/1983 745i (stolen spring '13 around Houston, TX Achatgruen on nutria buffalo. 8481080)
    10/1984 745i
    11/1984 745i
    11/1984 735i (10:1-265/6)
    Ford, MB, and GM round out the pack.



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