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View Poll Results: Which diff ratio should I go for? (Please Explain!)

Voters
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  • 3.64

    3 33.33%
  • 3.73

    3 33.33%
  • 3.91

    2 22.22%
  • Keep it the same...

    1 11.11%
Multiple Choice Poll.
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Thread: Made my mind up: I'm gonna do a diff swap.. (a few Q's).

  1. #1
    Join Date
    Mar 2007
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    Default Made my mind up: I'm gonna do a diff swap.. (a few Q's).

    I looked into doing a diff swap a while ago, and time has went by and I've procrastinated... I've read all the previous threads about diff swaps in the archive but this is kind of a personal thing I find, so thought I'd ask your advice/opinon.

    Recently I asked about a diff for a friend in a scrap yard and the dude is willing to do them REALLY cheap if I take it off myself... Which in my eyes is preferrable anyway as I'll learn how to do it and make sure it's done carefully.

    So the cost factor is out the way and nothing is holding me back. I should be able to afford it and some new seals when he gets the right one in... [Currently he has a diff from an e36 323i (1998) and an e36 320i (1998 I think)... I don't know if these are suitable or interchangable with mine, I doubt it.] Now, I've done some research and found this page really helpful: http://www.vinesauto.com/diffratio.htm so I know what I'm looking for.

    First question is - What ratio would be best to move up to from a 3.46 if I want better acceleration and low end torque? The way I see it my choices are 3.64, 3.73 or 3.91. Any higher and I guess first gear is made redundant? I'm leaning towards the 3.73 but not sure, check this out:

    3.64: It's a ~5.2% increase in revs meaning @ 80mph in fifth if I'm normally at 3000revs I'd be at ~3156revs...

    3.73: It's a ~7.8% increase in revs meaning @ 80mph in fifth if I'm normally at 3000revs I'd be at ~3234revs...

    3.91: It's a ~13% increase in revs meaning @ 80mph in fifth if I'm normally at 3000revs I'd be at ~3390revs...

    Looking at that I surely wouldn't lose too much fuel consumption even using the 3.91 diff? Also, I normally shift by 5000revs in any gear meaning even in first if I wanted to get going in a hurry I would be doing the same speed if I just took it a little further to 5650revs...

    Next question, which I hope someone can help me with: Which diffs have the same flanges as mine? I really don't want to be changing the driveshaft flange as it's supposed to be a lot of messing. I don't mind changing the output flanges as they're not supposed to be as bad but I'd prefer a straight swap to be honest... I'd really appreciate any info on this.

    Ok, also, I have an LSD at present, should I replace it with another LSD? I don't want to lose out on anything by going to an open diff... Oh, and does it matter if the diff comes from a MANUAL or AUTO? I imagine not...


    Lastly, what's the deal with this 'preloading - I've read a lot of information on swapping the diff (great resource here: http://www.bmwe34.net/Wiki/tiki-inde...ferential+swap) but still in the dark with the preload. Anyone done it and can I do it?
    Last edited by e34.535i.sport; 08-29-2008 at 06:08 PM.
    1995 XJR: 4.0L S/charged straight 6 Auto

    What... It's not broken??? I can still fix it

  2. #2
    Join Date
    Sep 2007
    Location
    Seattle, WA
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    1,036

    Default

    You're going to want another LSD.

    You're going to want a diff out of an E34 (non M5/540 as those are much different) or one of the smaller ones out of an E32 (like the 735). Don't think the E36 diffs are interchangeable.

    As far as which ratio to go with? Depends on how much you want your gas mileage to decrease. But the 3.73 should be your best bet for a good balance of improved acceleration/gas mileage loss.

    Auto/manual doesn't matter in the build of the diff. Only difference between them will be the ratios.

    Reminds me I need to figure out which ratio I've got in my car now to figure out if the 3.91 LSD I picked up is too low. Couldn't pass up a diff like that for $200 though...
    Cosmos Black/Black 1995 540iA M-Sport
    BMW Individual Exclusive Edition, 1 of 65 total, 9/25/95 Build

  3. #3
    Join Date
    Jan 2004
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    Albuquerque, NM
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    Default

    Definitely go with LSD. 3.64 would be my 1st choice. We put one in my son's 535i when we did the swap and in my opinion it's perfect. I have a 3.91 in my 7 and it's a bit too shallow. A 3.73 would be my 2nd choice but they are somewhat scarce.

    Give no concern to swapping axle flanges, they pry out with a screwdriver and you'll want to put fresh seals in it anyway, so that's not an issure.

    One issue with auto vs manual diff's will be the input flange if it's different than yours, but you can swap that too. Replace the seal if you have to swap input hubs, but don't mess with it if the donor hub is the same.
    gale
    92 735i 5-spd, turbo project finally underway!


  4. #4
    Join Date
    Mar 2007
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    Default

    Quote Originally Posted by gale View Post
    Definitely go with LSD. 3.64 would be my 1st choice. We put one in my son's 535i when we did the swap and in my opinion it's perfect. I have a 3.91 in my 7 and it's a bit too shallow. A 3.73 would be my 2nd choice but they are somewhat scarce.

    Give no concern to swapping axle flanges, they pry out with a screwdriver and you'll want to put fresh seals in it anyway, so that's not an issure.

    One issue with auto vs manual diff's will be the input flange if it's different than yours, but you can swap that too. Replace the seal if you have to swap input hubs, but don't mess with it if the donor hub is the same.
    Just the man - I was hoping you'd see my thread! I was reading yours from a while back recently and saw you had done the swap!!! Did your son find it much better off the line - a lot more torquey so to speak? I know what you mean about going too far and having the gears too short, but I just want the best compromise really.

    I'm interested in this preload - If I get a manual diff should the driveshaft flange be the same then? Will I need to worry about preloading anything when putting the diff in and attaching it to the driveshaft? Thanks in advance!
    1995 XJR: 4.0L S/charged straight 6 Auto

    What... It's not broken??? I can still fix it

  5. #5
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    Mar 2007
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    Default

    Quote Originally Posted by bubba966 View Post
    You're going to want another LSD.

    You're going to want a diff out of an E34 (non M5/540 as those are much different) or one of the smaller ones out of an E32 (like the 735). Don't think the E36 diffs are interchangeable.

    As far as which ratio to go with? Depends on how much you want your gas mileage to decrease. But the 3.73 should be your best bet for a good balance of improved acceleration/gas mileage loss.

    Auto/manual doesn't matter in the build of the diff. Only difference between them will be the ratios.

    Reminds me I need to figure out which ratio I've got in my car now to figure out if the 3.91 LSD I picked up is too low. Couldn't pass up a diff like that for $200 though...
    Thanks, I was thinking I should stick with the LSD, I just hope I can find a good one.
    1995 XJR: 4.0L S/charged straight 6 Auto

    What... It's not broken??? I can still fix it

  6. #6
    Join Date
    Sep 2007
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    Seattle, WA
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    Default

    Quote Originally Posted by gale View Post
    Definitely go with LSD. 3.64 would be my 1st choice. We put one in my son's 535i when we did the swap and in my opinion it's perfect. I have a 3.91 in my 7 and it's a bit too shallow. A 3.73 would be my 2nd choice but they are somewhat scarce.
    What diffs were in those cars before? And what tranny is in the 535?
    Cosmos Black/Black 1995 540iA M-Sport
    BMW Individual Exclusive Edition, 1 of 65 total, 9/25/95 Build

  7. #7
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    Default

    Quote Originally Posted by bubba966 View Post
    What diffs were in those cars before? And what tranny is in the 535?
    Someone correct me if I'm wrong, but I believe ALL e34 535i manuals have a 3.46, at least in the US. Getrag 260/6 was used in all of them,
    gale
    92 735i 5-spd, turbo project finally underway!


  8. #8
    Join Date
    Jan 2004
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    indiana
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    Default It depends....

    if you are going to see the highway or interstate at all the 3.64 would be the best, but want to exploit the torque go with the 3.73. Running 80 -85 down the interstate chews thru the fuel with a 3.73.

    I AM VERY INTERESTED buying your 3.46 lsd when you pull the trigger.

    Joe S
    JoeS
    _____________________________________________

  9. #9
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    Default

    and all the auto 535 had the 3.64 in the US, correct?

    i'm not familiar wit hthe 535 driveshafts but on the ones with carrier bearings, you can mix and match the rear shafts to make it fit whatever you want.

    manual front + auto back would match up with the tranny and diff and not require the risky business of preloading without knowing exactly what you are doing. Probably cost the same or less to have it balanced vs. having an expert swap flanges 'the right way'.
    Last edited by attack eagle; 08-29-2008 at 11:00 PM.

  10. #10
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    Default

    if you have 535iS you prolly already have the 3.64....what revs are you running at 60
    mph???I have a 3.64 and run about 2250 at 100 kph indicacted ..if i were you i would be going the other way...
    Gone but not forgotten

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