Rear diff swap

[William316i]

Hello, having a bit of difficulty deciding what to do having some problems with my rear diff, I currently have a 316 coupe I am swapping to a m20, I’ve been looking on multiple different discussions online and not really got a clear answer, I believe I currently have an open 4.10 ratio in my diff and was wondering if that would be fine to leave in there or would I be best off changing, I’m not bothered about the smiles per gallon as it’s just gonna be something to go have fun in on a weekend, as this is a small case diff do I have to change anything to get a medium case to fit ?

Any advice would be helpful.

Thanks

[Brianmoooore]

4.1 with a M20B25 would soon get tiring on the road, but if you're happy to live with the revs., then it'll be fine. BMW considered the small case diff. to be fine for a M20B20, but not strong enough for a M20B25, but it'll last OK if not abused too much.
Changing from a small case diff. to a medium case only requires a slight change in the length of the adjustable rear section of the prop. shaft, along with setting the centre bearing pre load.
When fitting the replacement diff, you will swear that two of the fixing holes on the subframe are in the wrong place, but they aren't.

[William316i]

4.1 with a M20B25 would soon get tiring on the road, but if you're happy to live with the revs., then it'll be fine. BMW considered the small case diff. to be fine for a M20B20, but not strong enough for a M20B25, but it'll last OK if not abused too much.
Changing from a small case diff. to a medium case only requires a slight change in the length of the adjustable rear section of the prop. shaft, along with setting the centre bearing pre load.
When fitting the replacement diff, you will swear that two of the fixing holes on the subframe are in the wrong place, but they aren't.


I thought as much, I can deal with the noise as I won’t be doing a massive amount of motorway miles, I did actually purchase a medium case 3.73 diff but what I didn’t realise is that it was off an 525 e34, i looked into trying to get that to fit but it seems like it’s gonna be too much work, I will keep looking for another diff for now though.

P.s does anyone know anybody wanting to buy an e34 diff

[Kevinboydunning]

Hi, so you have a LSD that will fit into an E30 318??

I have an M50 motor 318is set up and wanted to increase the ratio (lower revs at higher speed) and add the LSD.. I have read so many forums but unsure of the current ratio and therefore what ratio to aim for... as you can tell im new to this E30 stuff! someone said a Z3M diff fits well and will give a good ratio.

[Brianmoooore]

someone said a Z3M diff fits well and will give a good ratio.

It would be in the ballpark if you are using a M50/52 gearbox, but if you are using an E30 gearbox the ratio is numerically too low.
5th gear on an E30 box is 0.81 : 1, whereas on the later box, it is 1 : 1, the same as 4th gear on the E30.