Differensial for lower gearing

[Brein]

I have a 518i with 4 speed manual box, when i am driving highway speeds i am driving with High rpm(don’t have rpm gauge so don’t spesifik rpm) looked into changing the 4 speed to a 5 speed, but that was not a Easy job when you have the M10, so i thought of changing the dif to a lower gear ratio diff for better cruising speed on the higway.
Has anyone done this before? Is there a diff that Will fit my car or do i need to change axels? Looking for a cruising speed of 110 km/h

[Brein]

did some research and currently i have a 4.10 diff, at 110 km/h i should have a rpm of 3800, if i change to a 3.64, ot should drop to a 3300 rpm, i can go to 3.25 but i think it would be to low with the 4 speed, found a e30 325i 4x4 3.64 diff that is for sale in my area, does anybody if this will fit my car or have any experience with this?

[Mike W.]

4.10 is really short gearing, no wonder it's buzzy. It doesn't appear that the 325ix diff will fit. Looking at Realoem.com, which I encourage you to check out, all they show under the diff is a gasket set. Nothing else. All the other E30 models they show lots of parts. Actually they do show it as a 188 which means the size of the ring gear which is the medium sized diff they typically used with 6 cylinder engines. But no parts, no list of the ratio or anything like that, so I suspect it's different. Now knowing BMW it may just be a detail difference, or it may be completely different, I can't tell. And you can't go off the picture because they just use a generic diff drawing, not one necessarily for that exact model. But I doubt it would fit.

Be careful of too big a change, I would look for either a 3.91 or a 3.73 at most, otherwise you run the risk of the car feeling like a dog that doesn't have any power, can't get out of it's own way and always being in the wrong gear. I had an E21 long ago I did an engine change on, replacing the 2.0 with a newer 1.8, almost the same power, but less torque. Originally it had a 3.64 and it just felt all wrong. I put in a 3.91 and it felt just right. Granted it was a 5 speed, but even in the lower gears it didn't feel right and didn't drive well to me.

[Ju@n]

I wouldn't go too low with the ratio, as Mike says, you'll end up with too long gears.

Reading your comment Mike, I have a 316 (a dog slow version of the 320i that got in the US). I changed the 4.10 diff for a 3.64 and I actually liked the difference, not sure about the gearbox on 320i s, but now 1st gear is usable, and got some extra kph for the same rpm at highway speeds

[1st 5er]

I have a 518i with 4 speed manual box...

Diff swap should be pretty straight forward.
Installing an E30 diff should only require cover and output shafts to be swapped.

[Brein]

Okay, so i use the cover and outputshaft from a e30 on my car?

[turbodan]

Be aware that the 325iX diff uses a viscous coupling type limited slip which requires the use of the matching stub axles. The e28 cv axles may not be compatible.

I don't know if the 518 has the same axles as the US market e28s but I would look into this issue specifically before you get too far along.

[Mike W.]

Okay, so i use the cover and outputshaft from a e30 on my car?

Now I don't technically know since I've never seen an E30 325ix diff, but at least on regular E30s the answer is no, at least on the rear cover plate. The output flanges are smaller on E30s than on US E28s, but we only got 6 cylinder E28s.

Again, spend some time on Realoem.com, it can help. It's not perfect, but it should help.

[1st 5er]

Okay, so i use the cover and outputshaft from a e30 on my car?

No.
The opposite. E28 cover and output shafts on the E30 diff.

[nikos_kalogridis]

I have a 5 speed gearbox taken out of my '84 E28 518 when converted to an M535i. Unfortunately its in Greece but if you are interested let me know.