E30 Diff internals compatibility

Post Sun Jun 25, 2023 8:13 am

Morning everyone.

I am currently working on an M54 swap on my E30 touring. I have started to look in to diff options and looking for some advice.

I am planning on using the 6spd box that came out of the E46 donor 330ci. Now the diff ratio in that car is either 2.93 or 3.07 from what I can see online. Therefore I would like to make sure I get a pretty low ratio diff in the E30 too so my crusiing speeds are still good. I'll have smaller diameter tyres and a less weight than the E46 too so ideally a slightly smaller ratio would be good.

Diffs around the 2.93 ratio font appear to common at all in E30s (the wiki suggests the 325e in US only?). However they are relatively common in E36s and I can see several for sale online. I know that the cases on a non-conpact E36 are different and it won't bolt straight up. However, i was wondering of anyone knew whether the internals between a medium case E36 diff and an E30 medium case doff were interchangeable?

Are there any other models which have interchangeable internal components with the E30 diff that I could look at as options?

Thanks

Post Sun Jun 25, 2023 8:48 am

I looked at what BMW did and although OEM ratios can be a little longer than ideal I think you will get away with it on an M54 swapped E30.

I looked at the Z3 3.0 with the M54 engine and am using the 3.15 Torsen LSD that was in that car. I will be using the 6-speed gearbox out of the E46 (Z3 was only ever 5-speed) but it is not a problem because most of the 5 and 6 speed have the same overdrive top gear ratio.

Post Sun Jun 25, 2023 1:34 pm

Thanks mate,

Didn't realise there was a 3.15 available on the z3. Is that one only available as a lsd? Would love to have a lsd but have been looking at open diffs to keep the costs down

Post Sun Jun 25, 2023 1:57 pm

miks15 wrote: ↑
Sun Jun 25, 2023 1:34 pm
Didn't realise there was a 3.15 available on the z3. Is that one only available as a lsd? Would love to have a lsd but have been looking at open diffs to keep the costs down

Not sure TBH whether there was a Non-LSD 3;15, but any medium case differential will fit, you just need to swap the rear case cover and sometimes the drive flanges to from your original.

Post Mon Jun 26, 2023 11:09 am

Thanks. The e36 diff has different mounts on the case itself which is why I was wondering if the internals would swap in to an e30 case.

I have seen the E34 diffs appear to use the same case though but with a different rear cover, so may look in to that. There is a 2.65 ratio available on the e34 as well which may be good. With some very rough calcs, this ratio on an e30 with smaller diameter wheels that the e46 and lighter car overall it should still out accelerate the e46 330 and gjve really good cruising rpms. And the fact it will drop straight in with only a cover change is quite appealing!