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Thread: 3.46 diff swap?

  1. #1
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    3.46 diff swap?

    Hi guys. While my M sadly sits in storage for another month or so, I've searched and searched and searched all over this board about those who've swapped out their stock 3.15 diff for a shorter ratio. I've learned a lot. I must agree that as much as I love my S54 MR, the gearing is way too tall. I love shifting and I'm hitting almost 110 at the end of 3rd! I understand why people think the S54 has less grunt at lower RPMs than an S52. It's just geared so freaking long. For example, I was shifting into 6th gear by the time I hit 110mph in my former DD Mazdaspeed3. Granted, the M gets there just a tad faster Just bought a never used/installed Shark tune off eBay for half price, so I'm very excited for the "hopefully" better throttle response so I can finally nail my perfect rev matched downshifts everytime like before with every other car I've driven. Being that my car is just a fun, weekend toy, and will never see an auto-x event, I'm just looking for making it more fun to drive. That's the most important thing to me. And it's 97.3% there. From what I've gathered, a 3.46 differential would be ideal for my needs, about a 10% drop in gearing from what I've read. Anything higher like a 3.64 or 3.73 appears to make first gear basically a waste and highway cruising (which I do here and there) at too high of an RPM. And I'm not swapping in a 6 speed, too much work and cost. Plus I think the stock 5 speed (with ZHP shift knob) feels perfect! This diff swap would definitely require me to use my local indie mechanic (the most honest, inexpensive, knowledgeable about all BMW anything guy I've been ever so lucky to meet). I know swapping in a diff from an E30 is supposed to be pretty easy and basically bolt on. But my main question, besides asking if you guys think a 3.46 is worth the upgrade and would make the driving experience more fun, is can I just purchase a matched ring and pinion gear set and have it swapped into my current diff case? I see these kinds of sets online for a decent price. Is this a more or less expensive approach than just buying a used 3.46 diff considering I'd be paying for installation? I want to retain the LSD I have and have no need to keep the old 3.15 diff intact. I appreciate your thoughts!

    Steve
    Last edited by spikeisgr8; 02-15-2017 at 04:20 PM.

  2. #2
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    Cant imagine a new gear set being cheaper than a used diff (~$400-500) and you can sell your old diff if you keep it intact. A 3.46 will pair nicely with the S54, though Id recommend the sport mode mod over the shark injector personally.

  3. #3
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    Thanks for the reply. Well, I suppose it's important to mention the previous owner fitted my car with a Rogue Engineering diff cover which looks great. So in order for me to keep that cover, wouldn't I want to swap out the gear set in that one for a 3.46?

    - - - Updated - - -

    And on the sport mode comment, the idea behind getting the Shark is if/when I plan on switching out the diff, I can increase the redline to 8,100. The rod bearings have already been replaced so might as well take advantage of that!

  4. #4
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    You can buy a complete diff and just swap the covers

  5. #5
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    I swapped from 3.15 to 3.46 in my 2.8 and I feel like it's perfect. One of the best things I've done to the car. Made it go from lazy to peppy feeling. I scored a diff off of eBay for $300, but mine's just a Torsen. I'd imagine you could do the same and just swap over the ring/pinion, and keep your diff cover.
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  6. #6
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    There are calculators all over the internet that will give you the numbers. I have a spreadsheet that allows you to pick the tranny, tire size, and gear sets. It will give you RPM before and after for each shift, speed at each shift and torque multiplier figures. Just email me. screenname@screenname.com Don't PM. I never remember to look to see if I have a message.
    As for doing a gear swap vs a used pumpkin. I would just go used. Believe it or not, though fairly simple, getting the correct lash on these things is an art. Best if you just find a decent used case. They are very easy to source.
    If you have an area to work on the car, it really is a very simple swap.

  7. #7
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    Thanks for you input BimmerBreaker, Tim95M3, and PbFut. I've searched eBay and Craigslist for a couple weeks and haven't found anything yet. But I have found this (http://www.ebay.com/itm/BMW-E30-E36-...1XvltH&vxp=mtr). You guys don't think this would be a good option? Is there somewhere better to look? Or do I just have to wait until a used diff from an E30 shows up. I'm in no rush. And I'm not THAT mechanically inclined, so I'd be letting my mechanic do the work on this one!
    Last edited by spikeisgr8; 02-15-2017 at 04:19 PM.
    2001 BMW M Roadster - Steel Gray over Imola Red/Black (1 of 8), Eisenmann sport exhaust, KW V3 coilovers, Strong Strut engine brace, Randy Forbes subframe reinforcement, GSR Technik tune, 3.64 Torsen LSD...Weekend toy!
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  8. #8
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    Quote Originally Posted by spikeisgr8 View Post
    ...... Anything higher like a 3.64 or 3.73 appears to make first gear basically a waste and highway cruising (which I do here and there) at too high of an RPM.
    well, i'm currently running 3.64 gears and sometimes I wish I had went to the 3.73 or lower. I do have a 6 speed but as far as I know the 1st 5 gears are basically the same as the 5 speed with 6th being overdrive. I would like to say that 1st gear is in no way useless and currently I have 460rwhp so I see no issues in a car with 300rwhp. cruzing @70 is around a little under 3k in 5th ... and 6th gear is like 3.1 - 3.2k @80 or something like that as I never really paid that close of attention. all I know is the needle is around that 3k mark .... lol

    I would hope that with a red line of 8k, cruzing at 3k wouldn't be an issue and i know lugging an engine isn't good either. i rarely cruze @2k my engine stays at or above 3k all day long.

    hope this helps some.

    oh .... and go used carrier lots cheaper then swapping gears.
    Maddog


  9. #9
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    I have 4000 rpm 80mph in 5th with 3.64.
    -Abel

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  10. #10
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    Maddog You smoke'n too much of that California Gold. A 15 plus torque multiplier behind 460 RWHP is not particularly manageable even at the best bleach box.
    That or you are launching at a relatively low RPM to build torque slowly.
    Hell I am only at 8.35 TM and 425 RWT and will scorch a set of hot Hoosiers out of a slow corner if not careful.

  11. #11
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    Quote Originally Posted by PbFut View Post
    As for doing a gear swap vs a used pumpkin. I would just go used. Believe it or not, though fairly simple, getting the correct lash on these things is an art. Best if you just find a decent used case. They are very easy to source.
    If you have an area to work on the car, it really is a very simple swap.
    This. A 3:38 would also be a nice upgrade.
    ICS built Turbo M Roadster

  12. #12
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    Quote Originally Posted by spikeisgr8 View Post
    Thanks for you input BimmerBreaker, Tim95M3, and PbFut. I've searched eBay and Craigslist for a couple weeks and haven't found anything yet. But I have found this (http://www.ebay.com/itm/BMW-E30-E36-...1XvltH&vxp=mtr). You guys don't think this would be a good option? Is there somewhere better to look? Or do I just have to wait until a used diff from an E30 shows up. I'm in no rush. And I'm not THAT mechanically inclined, so I'd be letting my mechanic do the work on this one!
    You could buy that, but itd be a lot more expensive. Either way you are paying a mechanic to swap pumpkins but if you buy just the gear set you are also paying to swap gears - and hoping the mechanic knows how to do so properly - plus you arent able to sell the old unit to recoup costs. Seems a bit wasteful imo.

  13. #13
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    That's next on my mod list. I intend to use the case, and pinion gear from a 2001 Auto 3.0, and swap my LSD onto the new ring gear. I'm just going to find the diff on car-part.com and have it shipped here - my budget is $400. I may go ahead and put in new seals while it's out of the car.

    I thought about it for a while and I think the 3.73 is too tall - plus the option of quickly and easily doing the swap myself is really appealing.
    Last edited by CoMZ302; 02-16-2017 at 05:19 PM.
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  14. #14
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    Quote Originally Posted by PbFut View Post
    Maddog You smoke'n too much of that California Gold. A 15 plus torque multiplier behind 460 RWHP is not particularly manageable even at the best bleach box.
    That or you are launching at a relatively low RPM to build torque slowly.
    Hell I am only at 8.35 TM and 425 RWT and will scorch a set of hot Hoosiers out of a slow corner if not careful.
    LOL ... Dan, I do have a good supply now that it's legal in Cali. I got your back if you need some.

    Don't know how much it matters but my rwt is 322. more then 100 less then you lol

    but I wasn't talking about doing standing 1/4's at the stop light. for that you would defiantly need some serious tires.... lol
    I was talking about just driving on the street. rolling in 1st and just stand on it with good tires and you'll just scoot with just a little squeal and defiantly be careful in a turn with traction control off ... lol.

    also we (me) have to be careful when talking about gearing cuz really a lot of things come into play not only gear ratios but you have to consider tire sizes which makes a huge difference in over all gear ratios. I believe i'm running a 27" tire (295/35R19) vs stock somewhere in the 24" range (245/40R17).
    Maddog


  15. #15
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    Guess I'll hold off until I see an E30 diff pop up somewhere for sale. In the meantime, looking forward to trying out the Shark tune once I take my car out of storage after winter. I'll let you guys know my impression then!
    2001 BMW M Roadster - Steel Gray over Imola Red/Black (1 of 8), Eisenmann sport exhaust, KW V3 coilovers, Strong Strut engine brace, Randy Forbes subframe reinforcement, GSR Technik tune, 3.64 Torsen LSD...Weekend toy!
    ">

  16. #16
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    It's a standard medium case diff. There were more models than just the e30 it was used in.
    Check out real oem. I think you can still put a part number in and retrieve all the models it was used in.

  17. #17
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    The list of potential donors is far larger than ETK (realoem et al) will show. Minor easily changeable parts such as the output flanges and rear cover make for many variations and part numbers on the same basic pumpkin.


    /.randy

  18. #18
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    I believe I got my 3.46s from E28s, but can't find verification on that at the moment.


    /.randy

  19. #19
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    OK, I'm reviving my thread lol. I've done a ton more research. If given the choice between a used 3.64 (Torsen) diff from a Z3 2.5 auto or a 3.73 (LSD) from an E30, which would you guys recommend? Would the difference be negligible? Is going to Torsen a step down from my current clutch pack diff (and will it mess with DSC)? Again, I'm not tracking this car. Just looking to make it more fun to drive for my weekend enjoyment. And I like to shift!

  20. #20
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    I have run all four. Torsen or clutch type, .64 or .73 won't make much of a difference. Getting those 3.15 out will. If the 3.73 feels too low try some 275/40/17 tires. Need more torque? Remove the cats.
    Great looking car-by the way!

    -Ed Hands
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  21. #21
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    Thanks for the advice, E.Hands! I figured there wouldn't be much difference. And yeah, I'd love to remove the cats to get more torque. But $2k for headers isn't in my budget right now lol. And thanks! Love it. Estoril Blue was my second choice. Still would love to see one in person. The three M's I've seen in the year I've owned mine were black, silver, and red.
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  22. #22
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    Well, my final decision comes down to which you'd think is easier to install. For example, I'm looking at these two diffs on eBay right now. First one is from a Z3 with a 2.5 auto (3.64, Torsen, 62k miles on it, $400). The second one is from an E30 (3.73, LSD, 98k miles on it, $600). I'd assume the Z3 diff would be easier? Again, my Indie mechanic is going to do the job. Would rather give him the easier job to do to ensure it goes right.

    http://www.ebay.com/itm/02-BMW-Z3-M-...pX0gZ5&vxp=mtr

    http://www.ebay.com/itm/BMW-E30-S3-7...FYrGrJ&vxp=mtr
    Last edited by spikeisgr8; 02-27-2017 at 06:40 PM.
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    ">

  23. #23
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    I can't see where the install would be any different with either one. But cheaper, newer, less mileage and clutch less(nothing that wears out) seems like the way to go to me.

    -Ed Hands
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  24. #24
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    Its the same job either way. Removing old diff and installing the new one. Maybe swapping the diff cover if needed or bushing...

  25. #25
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    The only downsides to a Torsen is that when the wheel comes OFF the ground completely, it behaves like an open diff. This is usually only encountered in high performance race cars, usually on slicks.
    Otherwise, the Torsen is much better for lack of maintenance (No clutches to wear out. A clutch pack doesn't really last more than 50k miles. It slowly becomes a full time open diff.) Heat is also not a problem.
    -Abel

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