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  • Swapped my Diff yesterday

    Yesterday I changed my LSD diff to a slightly lower ratio one and thought I'd do a write-up (don't think there is one on here already?). It's not too bad a job to do, the only annoying aspect of it is getting the bolts back in through the subframe to bolt the diff up; they're quite a fine thread and have to be aligned perfectly before they will screw in.

    Ever since buying my M Coupe I’ve always thought that the gearing feels a little long legged and widely spaced, particularly for tighter roads where I found myself using one gear (3rd) since it pulls ok from about 50 into three figures but only feels properly strong from about 70. The ideal would be to fit a 6-speed M3 EVO gearbox and fit a lower ratio diff too but I’ve read the shift isn’t as nice as the 5-speed and they’re also not as strong. Just fitting the lower diff is also a lot cheaper and easier! The only downside is increased revs at motorway cruising speeds.

    Having read the thoughts of owners who’ve changed their diffs (all positive!) I decided to change my own diff. The standard diff ratio is 3.15 and I always knew that fitting something like a 3.64 (off an E30 325i Sport) would be too low for me; even though I’d know it would make it a lot of fun to drive I think the 15% drop in gearing would be just too much when cruising. The other problem I had with the 3.64 is that it’s like to be 20 years old and of a largely unknown condition. The ratio I had in mind was the 3.46 - a drop of 10% and is similar to the ratio %age change I made in my 205GTi which really transformed the car. It turns out that the 3.46 LSD doesn’t seem to exist and would require a custom build using a pinion and wheel set from a 5-series. It would also mean donating my old diff, sending it off for a rebuild with my car off the road for a while and it would also mean it would be difficult to change back if I didn’t like it.

    It was searching the American forums (where they seem to be well ahead of us in terms of modifying their Z3s) that I read of a few who’ve changed their LSD diffs for a later Torsen (Torque Sensing) type LSD diff from a Z3 Sport. These were fitted to the 2.2 and 3.0 Sport Z3s - a few of the "higher profile" US forum members have fitted these and ran them without problems. I couldn’t find a 3.46 from a 3.0 but found a 3.38 LSD (around 8% difference) off a 16,000 mile 2.2 from a breakers.

    Torsen diff with which has an assortment of worm gears inside


    The Torsen diff works differently to the Z3M clutch pack diff and actually acts like an open diff when there is no load/torque running through it. It requires a certain resistance (traction) at both wheels to create the limited slip though. For a racer the clutch pack would be preferable since it delivers torque to both wheels all the time and is ok if you lift a wheel; but since I’m unlikely to lift a wheel the Torsen diff should be fine!

    Changing the Diff

    - Jack the car up and put on axle stands (you could use ramps if you preferred).
    - Remove the exhaust backboxes; I've read it's possible to do this job without removing them but I took mine off to give more space (having done this job I wouldn't like to try it with them fitted). Unbolt the pairs of 12/13mm nuts and bolts at the joints and undo the 4 brackets held on by 13mm nuts under the boot flor.
    - Undo the 12 “star” bolts on the half-shafts (E12 socket required; don’t risk using a 8mm or 3/8” socket as you WILL round them (I tried!). I just zip-tied the shafts round the anti roll bar to support them.
    - Undo the 4 16mm nuts on the prop-shaft flange.
    - Place a jack under the diff to take it’s weight and remove the 19mm nut and bolt through the rear mount. Here you’ll find out just how much of the diff weight this mount takes!
    - To enable the subframe to come down a bit more and give more room I slackened off the 2 large subframe bolts at either side and also remove the sideplate bolts (two allen keys).
    - Lower the jack and the diff should lower down twisting the subframe down with it to give better access to the 4 diff bolts (16mm). You can easily get to the front 2 with a ratchet and socket but the higher rear 2 require a spanner.
    - With those 4 bolts undone you can lower the diff down (it requires a bit of jiggling to come down and away from the prop-shaft flange studs)




    - Drain the oil on the bench; just hang the drain end over the edge of the bench and drain into a bucket tipping it up from the front to ensure it’s fully drained.

    Z3M diff (left), Torsen diff (right)


    Z3M Clutch pack LSD


    - Swap the half shafts over; they just prise out with a little tap - the Z3M has slightly larger cups than the 2.2 but they are otherwise identical (internal side). I imagine the 2.2 are the same as the E30 325i.
    - Then swap the back plates over. I bought a new gasket from BMW (about £1). I kept my old speedo sensor in but for some reason it didn’t work so swapped to the similar looking 2.2 sensor which worked fine.

    Z3M back plate has a much larger finned cooling area


    3.38 Torsen LSD diff with Z3M back plate


    - On re-fitting the diff I balanced it on a block of wood on a jack and raised it up carefully. I coupled it to the prop shaft then rasied it up until I could connect the rear mount; this meant it was more or less aligned to the subframe bolt holes. Getting the first bolt in took me almost an hour! once the first bolt goes in the rest go in easier.

    First driving impressions; it does feel like it now has “crisper” acceleration, the gears feel slightly closer together. 70mph is now about 3300rpm and I wouldn’t really want to go any lower than this. I’ve not done much of a test drive so might add any further thoughts later.
    Last edited by c_w; 12-08-2007, 03:24 PM.

  • #2
    Nice write up c_w. Always fancied lowering my diff ratio.
    So what was the overall cost to do this and the approx installation time?
    Waiting for the report after the test drive.

    Greenbat

    Ex MC Owner

    Comment


    • #3
      Likewise, nice writeup.

      Any idea of the difference in weight between the two?
      Strongstrut CL front brace-Yellowstuff pads/Castrol SRF race brake fluid-Rogue top mounts/Rear support bushes-H&R ARBs-Whiteline droplinks-Bilstein Sport shockers-H&R Springs-Black Halo Angel Eye headlight units-Sachs lightweight flywheel/matching clutch-Supersprint exhaust/race catalysers-BBS RSGTs with Falkens-K&N High Flow Air Filter-Recaro Pole Position seats - Exdos mod - ACS flippers

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      • #4
        Thanks, I was lucky with the diff itself, being at a breakers just nearby and with just 16k on it - I paid not much more than the cost of that 3.46 gearset! I was all set to buy one the same for £275+carriage but luckily came across this one and was local too.

        I would say give time for a day to swap the diff, it isn't a partciularly hard job when you look back at whats required and could be done in half a day but factoring in cleaning up bits (I cleaned and painted the backplate) and swapping stuff over best to say just under a day's work. I also did the job on my own so raising the diff on a single jack whilst trying to couple it to the propshaft and get it on to the rear mount was fun! hehe

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        • #5
          I should have weighed them both but I can't see there being a great deal in it, but at a guess the Torsen may be slightly heavier due to more gears inside it. There wasn't anything obviously noticeable when moving them around on the bench though.

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          • #6
            Superb post - look forward to hearing how it differs when driving.

            I'm unsure what the M3 engined 325 sport I ran against had diff wise but at around 100 he was loads faster as hit the limiter in 3rd.

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            • #7
              Doing this job again recently I didn't remove the backboxes which saves some time. Access to the bolts on the subframe is still tight but I wouldn't say the exhausts are in the way.

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              • #8
                has anyone ever priced up changing crown wheel and pinion to achieve desired ratio and at what cost / who could do it.

                I take mine off road in wet months and have plenty of time to do it then.

                looked at diffs before and put other jobs ahead of it. with those done now I would like to change the diff to improve the feel as cw explains.

                I much prefer the cluch pack diff though and my opinion on the standard set up is that it's a little on the soft side having ran more agressive diffs on other vehicles I've owned. I threfore don't want a less agressive torsen type as described though I respect what has been done.

                Comment


                • #9
                  Ace write-up, C_W! I really enjoyed the read.

                  How many miles has your car done?
                  Out: 99 S50 Titan Silver V876 KKO
                  In: 98 S50 Estoril.......

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                  • #10
                    Originally posted by ever88 View Post
                    has anyone ever priced up changing crown wheel and pinion to achieve desired ratio and at what cost / who could do it.

                    I take mine off road in wet months and have plenty of time to do it then.

                    looked at diffs before and put other jobs ahead of it. with those done now I would like to change the diff to improve the feel as cw explains.

                    I much prefer the cluch pack diff though and my opinion on the standard set up is that it's a little on the soft side having ran more agressive diffs on other vehicles I've owned. I threfore don't want a less agressive torsen type as described though I respect what has been done.
                    All BMW LSDs have 25% lock, I'm not sure what that actually amounts to but both the plate and torsen diffs have the same lock of "25%". The plated LSD does feel different though, probably best to describe it as more adjustable on the throttle before traction is lost. I don't know why but they dropped the older style plated LSD for Torsens on the Z3s (non-Ms) to 2002 and the E46 M3 onwards has some kind of fancy hydraulic LSD. I think there might be advantages and disadvantages to both. I certainly feel/felt no problem with the Torsen LSD and it would slide out just like the plated OE diff.

                    To change the diff and crownwheel it requires a fairly comprehensive strip down and resetting of the preload/backlash so I think it's a professional only job to get it right. It also depends what ratio you want to change to; the only ratio I would consider to swap internals over is the 3.46 because going slightly lower (ie 3.64 and 3.77) are common ratio LSD diffs available from the E30 325i Sport. The only problem with the latter two is that they're going a bit on the extreme side for this car IMO (3.64 is a fairly hefty 15% drop vs 10% drop of the 3.46). With a 3.46 you see 3300rpm@70mph (it doesn't sound like a massive difference but it is in reality, but again, not too ott)
                    Last edited by c_w; 24-03-2009, 02:19 PM.

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                    • #11
                      Originally posted by h4pry View Post
                      Ace write-up, C_W! I really enjoyed the read.

                      How many miles has your car done?
                      Thanks, it's on about 105k I think.

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                      • #12
                        did not know that the lock up was about the same.

                        this has changed my mind somewhat.

                        strange how you can feel the difference though. I wonder how much of that is because you know something has been changed.

                        Comment


                        • #13
                          great write up, you were brave doing that on your own, the diff must weigh around 30kg - lots of backs have been put out handling them...

                          I finally got a 3,64 on my car last month, and am very happy with it. Completely transforms the car.

                          You can get the diff rebuilt with crown wheel and pinion for a reasonable price. i know www.moseleymotorsport.com can do it, and they are a pleasure to deal with.

                          I would do that before buying a diff i didnt know off t'internet again, at least that way you know what you are getting.

                          steve

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                          • #14
                            It's not too bad removing and refitting on your own - luckily as the base of the diff is largely flat it is quite easy to balance it on jack and lower and raise it up. The worst part of it is locating the subframe bolts.

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                            • #15
                              Dropping the subframe at the same time apparently makes it much easier as you then have easier access to line up and tighten the diff to subframe bolts. I haven't done the diff on an MC, but when replacing diff pinion seal on my E30 (which has basically the same backend as an MC), I found the diff/subframe bolts to be bloody awkward and a struggle. Diff itself was surprisingly lighter than expected (thank god). It was only afterwards that someone suggested dropping the whole subframe.

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