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  • Diff upgrade options?

    Hi pretty new to z3m coupes, as far as I know they are an e30 style rear diff? 188mm medium case? What options have I got with upgrading to a 3.7/3.9/4.1
    Can I just use an e20 LsD or are they not up to the job? Has anyone else had a lsd built into there original z3m casing?

    Cheers

  • #2
    Originally posted by SammyZ3M View Post
    Hi pretty new to z3m coupes, as far as I know they are an e30 style rear diff? 188mm medium case? What options have I got with upgrading to a 3.7/3.9/4.1
    Can I just use an e20 LsD or are they not up to the job? Has anyone else had a lsd built into there original z3m casing?

    Cheers
    Yes it's E30 medium case.

    Stock ratio is 3:15 which I found too tall. I've tried an E30 sport 3:64 diff which I found too short so then I found the Goldilocks ratio of 3:46. Perfect!

    JC Racing at Thirsk will build a good BMW diff in a stock case or you can buy Quaife for Z3M. I'm sure there are others too.

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    • #3
      Thanks mate I was thinking of a 3.7 or 3.9 to be honest! Would I be alright with just using an e30 LSD. Will they bolt straight in?

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      • #4
        Yep, straight swap.

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        • #5
          Cheers. What sort of revs are you pulling in 5th?

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          • #6
            3.64 lsd is quite common from e30s, but they're all about 35yrs old so may need rebuild.

            Z3m lsd is stronger but ive had no issues running an e30 derived 3.46 lsd on my car for about 10yrs. Just need output shafts and backplate swapping. 70mph = 3300rpm (3000rpm stock 3.15). For mainly track 3.64 should be ok. 1st gear will be very short.

            2.2 and 3.0 sport z3s use 3.38 and 3.46 torsen lsd but clutch pack m style felt better to me.

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            • #7
              My car is LS powered with a six speed Tremec so there’s no valid comparison for you as the ratios are obviously different.

              If you do a search this has been discussed a fair bit on here before and I know that 3:46 is considered a sweet spot with an OEM five speed box too.

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              • #8
                I’ve just read c-w’s reply and I would add that I ran an E30 LSD in 3:46 with nearly 400rwhp for 10000 miles with no problems when my car was LS1 powered.

                I’m back to the OEM 3:15 now with the bigger power LS3......we’ll see how it goes.

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                • #9
                  Cool cheers guys, I’ve found an open diff from an e30 3.9 ratio. Would it best to use the gear set out of this and get it built into my z3m LSD/ casing & maybe get the ramp angles changed if I fancy it locking abit more?

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                  • #10
                    What are you using the car for? 3:9 would be very short.

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                    • #11
                      Fast road. Occasional tracj, I feel the stock diff is too long. I have driven an e46 m3 with a 4.1 and felt good so thought I’d comprised and go 3.9. Maybe 3.7 if one comes forsale

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                      • #12
                        Cant compare e46 directly as they run bigger wheels, so although the actual gearbox ratios are similar, with an same ratio diff the e46 would be relatively longer geared. 3.64 or the 3.73(?) would be good on track to use 3rd/4th/5th.

                        I put a 3rd clutch in mine for a while which is supposed to give 40% lock over stock 25%, but in all honesty it didn't need it and also i think it introduce understeer on turn in. With decent semi slicks the car doesn't need any mods to the diff from my experience. The balance of modest torque and r888s meant it could be driven flat out in dry out of corners.

                        Originally posted by fok View Post
                        I’ve just read c-w’s reply and I would add that I ran an E30 LSD in 3:46 with nearly 400rwhp for 10000 miles with no problems when my car was LS1 powered.

                        I’m back to the OEM 3:15 now with the bigger power LS3......we’ll see how it goes.
                        Yea I was thinking with a v8 it would need stock or longer, but depends on the gearbox ratios.
                        Last edited by c_w; 14-01-2020, 10:55 PM.

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                        • #13
                          Cheers for the reply, yeah might go for the 3.73 and I’m using 18s same as th e46 I’ve driven

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                          • #14
                            Is it same size tyre though? Stock z3m rear wheel is 245/40/17 but e46 m3 18” is 255/40/18, significantly bigger overall.

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                            • #15
                              235/35/18 which is what I’ll be running on the z3m anyway

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