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Thread: Diff question

  1. #1
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    Diff question

    I'm new to BMWs and I just picked up an e36 325is. Looking to start a drift build. From what I understand the 325is (1994) comes with a LSD. My main question was what size diff is it? I have heard its a 3.91 but the previous owner told me it was a 3.23. Not sure if he knows what he was talking about. It is a cold weather package car I believe. (heated power seats)

    I ask because I have a chance to pick up an already welded diff for cheap and I just want to be sure that it will fit my car. The already welded diff is a 3.91, he also has a 2.91 that's welded for sale. Is there a difference once they are welded? Is one more durable than the other once welded? Any input is appreciated, Thanks

  2. #2
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    So the size is all the same (for majority of e36 models, 318s have different size diffs), the ratio is what differs. I think your 325 comes with a 3.15 diff, which means that the driveshaft/diff will rotate 3.15 times while the wheels make one complete rotation. 3.91 diff would rotate 3.91 times by the time the wheels rotate one complete time. I would pick up the 3.91 welded diff, most guys on here are running welded 3.91s including myself. Durability depends on the welds so as long as you trust the person who did it then you should be ok, and the difference is once you put in the welded diff the wheels will always rotate the same amount (which is why you hear cars with welded diffs tires "skipping" when theyre making tight turns) so this puts some more stress on the axles but welded diffs will always be "locked" while drifting when an lsd sometimes won't stay locked while drifting

    Someone may be able to explain better but I think i hit most of the points lol

  3. #3
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    I think that explains it pretty well. Thanks

  4. #4
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    http://www.bokchoys.com/differential/GearRatios.htm

    there are of course exceptions, but when trying to drift with stock, low power cars you want to run the lowest ratio (highest number) you can get you hands on, which would be the 3.91. Some of the Nissan guys go as far as swapping in pathfinder 4.60s. The lower the ratio the faster acceleration you get, but you lose top end so you will find yourself shifting more / possibly having to shift at weird places on track. This is why the pros run quick change diffs, so they can dial in the rear end ratio to the speeds and shift points they want to achieve depending on track layouts.
    Last edited by Novablue454; 07-30-2018 at 03:33 PM.

  5. #5
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    dont believe you have a LSD until you visually verify its an LSD. there were so many options and ways to get LSDs and so much diff swapping that it will be impossible to tell if its LSD until you verify what you have. If you really want, jack up the rear and spin 1 tire by hand, both should spin if you have an LSD.

    anyway, if your car has its factory install optioned LSD, it will be a 3.15, which is decent for drift. the 3.91s came from automatics and are super short, good for 3rd gear track. 1st gear will be non existent. if its a 3.23 diff you have then it was swapped from an OBDII M3.

    Heres where things get interesting as far as drifting goes. the LSDs are a good option, however they need to be rebuilt. they never locked hard from the factory and the clutches are more than likely worn over the last 20 years. Also, my opinions is that LSDs transition kind of funky. I use a welded dif in my 328. As for ratios, the 2.93 you said he also has should be a pass. its a long gearing and while ive never drifted one, i cant see it being too fun to be clutch kicking it all day on certain courses. the 3.91 is something you should definitely get. everyone seems to be about them for drifting. i have one, but to be honest, im just not about it. its too short and our events here are generally high 2nd gear tracks. 3.15 diff welded (not LSD) is my ideal diff. it just seems to work for me. for the local tracks it keeps me in 2nd the whole course and im not clutch kicking my life away in 3rd and downshifting to 2nd like the 3.91 has. However, course layouts change and sometimes one diff would be better than the other, so its nice to have 2 because they arent hard to changed once you get it down.

  6. #6
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    That is extremely helpful thank you. The track I will be starting on is pretty small and low speed. So I'll start with the 3.91 and if I find myself having to shift too much or I go to a larger track I will look into a different setup.

  7. #7
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    Quote Originally Posted by RandomHero27 View Post
    dont believe you have a LSD until you visually verify its an LSD. there were so many options and ways to get LSDs and so much diff swapping that it will be impossible to tell if its LSD until you verify what you have. If you really want, jack up the rear and spin 1 tire by hand, both should spin if you have an LSD.

    anyway, if your car has its factory install optioned LSD, it will be a 3.15, which is decent for drift. the 3.91s came from automatics and are super short, good for 3rd gear track. 1st gear will be non existent. if its a 3.23 diff you have then it was swapped from an OBDII M3.

    Heres where things get interesting as far as drifting goes. the LSDs are a good option, however they need to be rebuilt. they never locked hard from the factory and the clutches are more than likely worn over the last 20 years. Also, my opinions is that LSDs transition kind of funky. I use a welded dif in my 328. As for ratios, the 2.93 you said he also has should be a pass. its a long gearing and while ive never drifted one, i cant see it being too fun to be clutch kicking it all day on certain courses. the 3.91 is something you should definitely get. everyone seems to be about them for drifting. i have one, but to be honest, im just not about it. its too short and our events here are generally high 2nd gear tracks. 3.15 diff welded (not LSD) is my ideal diff. it just seems to work for me. for the local tracks it keeps me in 2nd the whole course and im not clutch kicking my life away in 3rd and downshifting to 2nd like the 3.91 has. However, course layouts change and sometimes one diff would be better than the other, so its nice to have 2 because they arent hard to changed once you get it down.
    I drifted with a 2.9 for 3 years, even with almost 400 ft/lbs it was terrible. Going to a 3.6 was much nicer, but I still prefer the 4.10 in my FC

  8. #8
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    I don’t know if anyone covered this but the 5-speed in a BMW has no overdrive gear. This plays a huge factor when deciding what gear ratio to use for your rear end because the ratios in a BMW transmission are COMPLETELY different. Just because some guy in a 240 or an FC praises his 4.10 or 4.30, that doesn’t mean a damn thing to us. You’ll be shifting way too much with anything more than a 3.91 and even then you may be shifting to much.

  9. #9
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    Quote Originally Posted by RandomHero27 View Post

    Heres where things get interesting as far as drifting goes. the LSDs are a good option, however they need to be rebuilt. they never locked hard from the factory and the clutches are more than likely worn over the last 20 years. Also, my opinions is that LSDs transition kind of funky.

    I have to disagree with you there. Ive had a few BMW 2 clutch LSDs that have lots of miles on them that lock great stock. My black car had 190K on it when I bought it and right out of the gate it locked great. I rebuilt it for a 3 clutch setup because I had the whole rear end out to weld in subframe reinforcement plates and figured "Why not" Honestly the didn't really change anything since it locked up great before. As far as being funky in transitions I much prefer the clutch LSD to any car I have driven with a welded. Its a personal preference thing and I love the bmw factory diff. The thought of putting a welded in my car is a pretty off-putting thought imho.

  10. #10
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    I've opened a TON of factory LSDs. Only 1 was badly worth and to the point where it should have been chucked. Otherwise, you can very easily use a factory LSD with no mods. Refreshing with new clutches/dog ears and even a 3 clutch is super easy. I hate welded diffs when not drifting. Even getting around the paddock is a pita.

    Status: Someone put glitter in my oil. Wait. Why's all my oil outside the engine? What's that knocking?

  11. #11
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    I’ll never go back to a welded unless it’s a strictly track only missile car. Welded is for super budget drifting, nothing else. If you can’t transition properly with a LSD, you need to change your driving style, because that’s the first time I’ve heard anyone use that argument.

  12. #12
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    E36 - 4.23, 2.52. 1.66, 1.22, 1.00

    Z32 - 3.21, 1.92, 1.30, 1.00, .752

    T56 - 2.66, 1.78, 1.30, 1.00, .074, .050

    damn, no wonder my 1-2 gears seem so long. But the T56 is pretty close to the E36 ratios, just all shifted a gear. The 3.6 in my car puts me in basically the exact wrong place for most track layouts, either limiter in 2nd or bogged in 3rd.

  13. #13
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    Question 325i with a 318 diff... does it work?

    Hello,

    I had a quick question about something. I recently installed a 318 locked diff onto my 325i, i had to swap out the axle stubs from the 318 to the 325 ones. which had me noticing something. They were sticking out a little bit more than the 318 ones. but they seemed to work right. After installed the diff and driving on it for awhile I've started to noticed that one of the sides of the diff is leaking out where the axle stub goes into. Could it be the seals or could it be that the diff just doesn't work? Please help me out!

    - Andrew A.

    Last edited by S.C.B.I.M.M.E.R; 08-22-2018 at 12:46 PM.

  14. #14
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    Quote Originally Posted by S.C.B.I.M.M.E.R View Post
    I had a quick question about something. I recently installed a 318 locked diff onto my 325i, i had to swap out the axle stubs from the 318 to the 325 ones. which had me noticing something. They were sticking out a little bit more than the 318 ones. but they seemed to work right. After installed the diff and driving on it for awhile I've started to noticed that one of the sides of the diff is leaking out where the axle stub goes into. Could it be the seals or could it be that the diff just doesn't work? Please help me out!

    - Andrew A.
    318 had a completely different dif in it. I really surprised all you noticed was the axles. The 318 dif is smaller and weaker. But you really need to start your own thread for this. Do not reply in here any more.

  15. #15
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    Quote Originally Posted by Liquidity View Post
    318 had a completely different dif in it. I really surprised all you noticed was the axles. The 318 dif is smaller and weaker. But you really need to start your own thread for this. Do not reply in here any more.
    I have but i'm still having trouble finding answers. Search up "325i Diff Problem"

  16. #16
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    The diff just doesnt work is the answer lol

  17. #17
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    Yo. Because I like to spend money...I'm looking for e46 Mini Spool.
    Struck out with Summit, Motive Gear, Condor Speed Shop, and ECS Tuning.
    Or... talk me out of it? I'm dumb, but I'm not stupid.
    https://www.motivegear.com/auto-part...erential-spool

    s-l1000.jpg
    spool-installed.jpg
    Last edited by satakal; 03-11-2019 at 12:52 PM.
    none

  18. #18
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    Or like... weld your diff

    Status: Someone put glitter in my oil. Wait. Why's all my oil outside the engine? What's that knocking?

  19. #19
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    Quote Originally Posted by satakal View Post
    Yo. Because I like to spend money...I'm looking for e46 Mini Spool.
    Struck out with Summit, Motive Gear, Condor Speed Shop, and ECS Tuning.
    Or... talk me out of it? I'm dumb, but I'm not stupid.
    https://www.motivegear.com/auto-part...erential-spool

    s-l1000.jpg
    spool-installed.jpg
    Just weld it. This is a complete waste of money. Your axles are the failure point, not the dif.

  20. #20
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    Welded. Got it.
    What use cases are most likely to fail the axle? Race track? Autocross? 2nd gear drifting? 3rd gear? Power shift? Clutch kick?
    Uncharted territory for me. Want to know if I should include an axle in my set of spares. (stock power e46 330)
    none

  21. #21
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    launches and clutch kicks. I've broke ~5 axles now and every one of them has been launching

  22. #22
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    Grip and max torque applied to the axle are the main factors in breaking them. The other possibility is high angles on axles (like dumped cars) tend to snap.

    Status: Someone put glitter in my oil. Wait. Why's all my oil outside the engine? What's that knocking?

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