3.91 Final Drive Swap on 1995 318iS

[gj124]

Hi, new member and longtime lurker here! I'm from the UK but you guys seem way more knowledgeable than the guys on the UK forum so I decided to ask my question here instead. I'm desperate for a higher final drive in my 318iS Sprint car which I believe has a 3.38 ratio 168 diff as standard. I also have a spare medium case 3.15 from a 325. Both are open diffs and will have to stay that way as the LSD's cost 1K+ and I won't be spending that much money. My plan is to purchase an E30 325 open differential with the 3.91 ratio, split the casing and swap the gear unit over as one piece to one of my E36 diffs (as they are supposedly the same?), but I am dubious if this will actually work so any help, advice or other suggestions are greatly appreciated!
Thanks.

[dominator293]

It should work ok. I believe the 188mm diff gears from the E28, E30, E32, E34, E36 are all interchangeable whereas E46 and later aren't interchangeable. I'd get a proper diff shop to do all the work for you though, you'll need all new seals and bearings as well as the backlash and whatnot all set up correctly with the new gearset.

[XnWarden]

You can't swap a medium case diff into a small case car without swapping a few other things as well.

And you can't swap medium case internals to a small case.

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[TspencerJ]

If I remember right the 318is has a 3.45 ratio standard. As other's have said the E30 325 diff will be medium case 188mm pumpkin size so won't fit into your 3.45's small case (168mm) diff housing. If you want to use the 188mm 3.91 ratio from the E30 325 diff, you've got 2 main options:

1. Swap entire open pumpkin + pinion from E30 diff to a 188mm E36 medium case diff.
To do this properly will want new pinion + carrier bearings, potentially require different thickness shims to get correct carrier preload, pinion preload set correctly via a crush washer and correct backlash set.

2. Remove 3.91 ratio from E30 diff pumpkin and install to the E36 medium case diff pumpkin.
Provided the E36 diff carrier bearings are in good shape (no pitting/marks on races or rollers etc.) this option means you only really need to replace the pinion bearing and set pinion preload (as you are retaining the correct carrier preload by using original bearings). This may also require a spacer for the ring gear as higher gear ratio's sit in a different position to lower ratios on the pumpkin (I swapped a 3.46 ratio into my 3.15 M3 LSD and needed the spacer for correct mounting).

Both options will likely require you to pay a gearbox/diff shop to do if you don't want to do yourself/invest in the tools. Having done a full LSD rebuild and ratio change, it's not an impossible job but is quite involved and really needs a press (for bearings + races), dial indicator for backlash and in/lb torque wrench for measuring pinion preload (which are rare as rocking-horse shit in the UK!!). You'll also need to obtain a medium case car's rear half of prop and medium case shafts for correct fitting + driveshaft length.

If you are purely looking to increase your acceleration on a budget, I'd instead recommend looking for a 4.44 small case open diff off of a 316i auto, 318i auto or 318is auto. That will give you your acceleration boost and only run you £60-£100 tops instead of several hundred to rebuild a diff to your desired ratio, but will reduce top speed a fair bit (think it's ~110mph @ 6500rpm).

Hope that's of help - feel free to PM if you want any advice on rebuilding a diff yourself, I've done a write-up in the past I can link too.

[gj124]

Thanks for the informative reply mate, I've managed to get my hands on a 4.1 ratio 168 E30 Diff pretty much for free, and to be honest I thought I could bodge the internals (pumpkin) and the pinion into my 3.45 small case with some new seals, a pinion bearing and a lot of luck but it seems to be a bit more involved than I was hoping, though I'll give it a go if I get a spare weekend. I did think about the 4.44 but I'm sure it'd be a right pain having to change gear all the time and wouldn't actually be much faster. In the meantime I think I'll keep an eye out for a small case LSD.

[TspencerJ]

You can definitely bodge it together, but if you end up with too much/little preload the bearings won't last particularly long haha. Saying that, people have reported success just impacting the pinion nuts on and off when swapping input flanges so it's not impossible!