Diff shopping

[locknload]

What's the rule for swapping diffs in an e30?  I think I read somewhere, any small or medium case but it must come from E30 or E36.  Can anyone verify that?  Are any diffs from other cars (Z3, E34, E46) easily swapped?

[DesktopDave]

No 'normal' E36 diff will fit unless you swap the gearset into an E30-style case. The only exceptions are the small case diff from a 318Ti (touring) or any Z3. The Ti diff may require E30 output shafts. The Z3 are all medium case AFAIK. If you get really lucky, you'll find a six-cyl Z3 with a torsen diff...or an MZ3 with the cooling fin diff cooler.

You can also fit an E28 diff if you swap the rear cover. That includes the late E24 from '82 onward. They're all medium case except the M5 & M6.

If you find an E23, those will also work except for the Euro 745i model. That has a large case diff.

There are a few early '88 E34 and E38 that can potentially work too. They use an entirely different mounting system, but the early ones were drilled for both models. Again, you'll need to swap the rear covers.

[locknload]

Thanks Dave.  My 318i is running the original 4.10 LSD.  Would like to go a bit taller, I've heard 3.73 is a good ratio.  Most of the Z3 diffs I've seen are 3.45.  Would that be too much for a li'l ol' M42 to pull off the line?  I suspect the Torsen unit you speak of is taller still?

[DesktopDave]

I'm running a 3.73 open diff with stock wheels right now. It's a very common E30 ratio, most of the M20B25 cars used it. It is a great mod for highway use, but it does take a little off the car's acceleration. If you're also running larger circumference tires it'd even be worse. I don't mind, and I typically start in 2nd gear to save synchro & thrust bearing wear.

I'd assume the taller diff probably saves a bit of gas too. I don't have a 4.10 baseline (that diff was broken when I bought the car). My fuel logs reported an average of 29MPG at 60% highway before I did any other fuel saving mods.

I just got lucky, the Z3 donor I found (locally ) had a 3.46. That's slightly low for a stock E30 M42...but ideal for an E24 with a "dirty" Euro M30 turning a Getrag 265.  IIRC the donor was a relatively rare M54B22 5-speed ('02 Z3 2.3i?).

[E36-italia]

Would like to go a bit taller, I've heard 3.73 is a good ratio.  Most of the Z3 diffs I've seen are 3.45.  Would that be too much for a li'l ol' M42 to pull off the line?

I had a 3.15 in my E36 318iS for a while.. not bad under acceleration (enough to keep up with traffic without flooring her all the time) .. but when it was on steam (read: 140+ km/h) it was quick! i ran out of balls at 220km/h (speedo speed, not gps)

Today the car has a 4.45 .. but i haven't driven it yet...

[locknload]

So, I did locate a 3.15 diff from a late 2.8 5sp.  My E30 is bone stock 5 sp.  Would this be a practical choice or should I keep looking?

[Darky]

Hi

3.15 is way too tall for an e30. 3.6 is about as tall as e30 went.
3.15 is a e36 ratio they changed the gearing considerably going from e30 to e36. Unless of course you also put the e36 gearbox in too!

Cheers Rohan

[DesktopDave]

I'd keep looking too. The 3.15 can take a huge amount of torque, but ratios that tall are best for FI/boosted cars. A 3.15 would be wunderbar for hours of autobahn triple digit speeds. That's not a problem I have, LOL.

IMHO the stock M42/G240 combo is well suited to a 3.73 (or even a 3.64) for excellent highway manners and fuel economy.

If you're looking to perk the car up a bit, I'd recommend a reputable chip and smaller circumference tires with the stock 4.10. The rare 4.43 is best for acceleration and will also help to compensate for larger tire circumferences. I'd bet a diff that short makes first gear redundant.

[Slowered318]

Given the age and NLA status the hardest part will be finding a quiet and leak-free diff. Make sure it's from drivable car or someone who will refund/exchange within a few months.

I went with a 3.91:1 and I absolutely love this ratio! First gear is more useful, didn't notice any performance loss and the RPM sits a little lower when highway cruising.

From stock (4.10) the 3.91 is only a 3.5% difference. Going to a 3.73 would be 10% "taller" and I think it would slow you down, even with a 195/60 series tire.

This might help. http://www.e30zone.net/e30zonewiki/index.php/Differential