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What Diff Ratio and From What?

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  • What Diff Ratio and From What?

    Hi guys,

    I'm new to the forum having recently purchased a Z3m Coupe with e39 M5 drive train transplant. I've spent the past 7 weeks working my way through the car, snagging and detailing to bring the car up to a standard that I wanted the car to be.

    Next on my list is to change the rear diff or the crown wheel and pinion. It currently has a 3.15 final drive ratio which was standard fitment in the e39 but as I have no interest in doing 200mph, I would like to lower this to make the car pull even harder. I'm thinking about the 3.62 final drive ratio from the e46 M3 and wondered if anyone on here has ever rebuilt the e39 diff with this crown wheel and pinion?

  • #2
    My MC is LS1 powered with a 6 speed Tremec T56 gearbox and I found the stock 3:15 diff way too tall.
    I swapped to a 3:64 but found that too short and revvy and it didn't suit a V8 power plant.
    I'd say that the Goldilocks diff is 3:46. Mine suits the car perfectly. It's a medium case LSD, originally from E30 325 Sport.

    I don't know what E39 gearbox ratios are but my Monaro Tremec details are here if you want to compare.........https://en.m.wikipedia.org/wiki/Borg...6_transmission

    Good to hear that you're improving your car, I saw the first incarnation of the transplant and it wasn't pretty!
    It appears we are now an official V8 MC owners club! UK membership so far....2 :)
    Last edited by fok; 18-05-2016, 05:51 PM.

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    • #3
      Which gearbox from that table is yours? I would be very interested in comparing gear ratios before making a decision. I've had a quick look at the gear ratio calculations and thought the 3.64 might suit the high revving nature of the BMW V8.

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      • #4
        I found the original 3.15 felt too tall, but that is on he stock engine - I bet with 400lb/ft it pulls those gears well though? But sometimes shorter gearing is nicer/sportier to drive.

        A 3.64 would suit a 3.2 engine well, but would be best with a 6speed conversion to retain cruise gear. Since you have the 6sp, the 3.64 might work ok with the m5 v8 as it still revs to similar rpm. The 3.64 is also a much more common ratio from E30 (to take the CWP from) than the 3.45 and 3.46 which are pretty rare ratios.


        There is also a 3.38 ratio fitted 2.2 Z3 Sports, it's a Torsen based diff but the CWP should still work if you just wanted that. I bought one of these really cheap from a breaker years ago because few people realise its an LSD.

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        • #5
          Mine is the Monaro / GTO chart.

          My LS1 redline is set at 7000 and whilst it's not got a wild cam compared to some, it still likes to rev.....it's not a lazy V8 by any stretch so it's not going to be too disimilar to an S62 in terms of delivery.

          The 3:15 made everything feel a bit too lazy but I felt that the 3:64 made 1st and 2nd too short and necessitated gear changes too frequently to enjoy the surge of torque associated with big displacement.

          3:46 was the perfect compromise for me.

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