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.
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.
Comment