E30 diffs

Post Sat Oct 26, 2013 10:11 am

Hi guys, i've got an m50 engine'd e30 with the e34 gearbox (1:1 5th gear). Originally it had the standard 316i diff which was pretty much un-driveable. It's now got a standard 325i open diff but this is still too low geared. Stupidly i sold the 3.23 LSD that came with the donor car for silly money (cheap) when i was thinking of getting rid of the car and now i regret it totally.

Anyway, now i have a 325i 'medium case diff' that i believe will house the internals of other diffs, what other diffs are actually compatible? i.e. which internals are actually an option?

cheers

Post Sat Oct 26, 2013 1:54 pm

You need a diff with a ratio around 3:1/3:25...
Some E28's can provide a suitable diff,but you will need an E30 type back plate and E30 medium case output flanges.
E30 M3 also used suitable diffs,but beware of 'M3' tax regarding price.
It would be worth your while to research Z3 diffs,some used the M50 type engine and direct drive 5th 'box with an E30 based rear suspension.

Or you could just use an ex-M40 gearbox(overdrive 5th) with a 3:64 medium case diff!

Post Sun Oct 27, 2013 9:41 am

Best diff will be a 3.07 from an E28 525e. Swap the output flanges and rear cover from the 325i diff, job done.

Post Sun Oct 27, 2013 10:02 am

3.07 will give you a ratio that is the equivalent of a 3.79 used with a standard E30 box.

Post Sun Oct 27, 2013 8:45 pm

The E36 328i manual had a 1:1 5th gear too didn't it?

Post Sun Oct 27, 2013 9:43 pm

Pud237 wrote:The E36 328i manual had a 1:1 5th gear too didn't it?

It did indeed. These boxes were designed as a five speed from scratch, whereas the E30 box is evolved from a four speed design, and essentially has a bit grafted on the back with the fifth gear. Since the original fourth gear is 1:1, to provide a fifth gear it had to be an overdrive gear. The same thing has happened with the E34/36 five speed box with a bit grafted on the back to make a six speed.

Post Tue Oct 29, 2013 12:33 am

Brianmoooore wrote:
Pud237 wrote:The E36 328i manual had a 1:1 5th gear too didn't it?
It did indeed. These boxes were designed as a five speed from scratch, whereas the E30 box is evolved from a four speed design, and essentially has a bit grafted on the back with the fifth gear. Since the original fourth gear is 1:1, to provide a fifth gear it had to be an overdrive gear. The same thing has happened with the E34/36 five speed box with a bit grafted on the back to make a six speed.

Thanks for that - always wondered why they used an overdrive 5th in the E30s but a direct 5th in the E36s.

My E30 has an E36 gearbox in it at the moment and the diff is small case, either 316 or 318 (and welded!). As you can imagine it is thirsty and a pain to drive! 2nd gear is for pulling away, 1st gear is for hills.. I'm about to start de-bruvving it, rubber band 16" tyres will be going, get rid of the silly camber, lift the rideheight 10-15mm to fit the tyres in the arches. Picked up a medium case 3.6 diff tonight which should help a bit, and I'm going from 195/40/16 front and 215/40/16 rear to some 205/50/16 front and 225/45/16 rear which I've got sitting about. I just hope the slightly taller diff and slightly taller tyres will do enough..

Post Tue Oct 29, 2013 1:23 am

daimlerman wrote:You need a diff with a ratio around 3:1/3:25...
Some E28's can provide a suitable diff,but you will need an E30 type back plate and E30 medium case output flanges.
E30 M3 also used suitable diffs,but beware of 'M3' tax regarding price.
It would be worth your while to research Z3 diffs,some used the M50 type engine and direct drive 5th 'box with an E30 based rear suspension.

Or you could just use an ex-M40 gearbox(overdrive 5th) with a 3:64 medium case diff!

1.8 8v M40 gearbox bolts up to the M52 2.8 24v engine? What clutch & FW? This might be my solution.. Will the box take the power & torque of the bigger 6cyl?

Post Tue Oct 29, 2013 7:55 am

Pud237 wrote:
daimlerman wrote:You need a diff with a ratio around 3:1/3:25...
Some E28's can provide a suitable diff,but you will need an E30 type back plate and E30 medium case output flanges.
E30 M3 also used suitable diffs,but beware of 'M3' tax regarding price.
It would be worth your while to research Z3 diffs,some used the M50 type engine and direct drive 5th 'box with an E30 based rear suspension.

Or you could just use an ex-M40 gearbox(overdrive 5th) with a 3:64 medium case diff!
1.8 8v M40 gearbox bolts up to the M52 2.8 24v engine? What clutch & FW? This might be my solution.. Will the box take the power & torque of the bigger 6cyl?

There's plenty using this combination!
You need an M20 flywheel/clutch/starter(ring gear has a different pitch to M50/52)

An ex 320 gearlever support bracket will locate the stick centrally in the tunnel hole,with a linkage from the same source.

Mine,using an M50 that's standard,has been built several years.covering some 45k trouble-free miles.
A tuned M52,say in excess of 250bhp,may well shorten gearbox life,though.

Post Tue Oct 29, 2013 3:53 pm

Andyboy wrote:Best diff will be a 3.07 from an E28 525e. Swap the output flanges and rear cover from the 325i diff, job done.

Like the one I have . It's strong and it's for sale

Post Tue Oct 29, 2013 10:01 pm

pacerpete wrote:
Andyboy wrote:Best diff will be a 3.07 from an E28 525e. Swap the output flanges and rear cover from the 325i diff, job done.

Like the one I have . It's strong and it's for sale

Strong you say? And for sale as well?

The two main reasons the direct drive box was used were closer ratios suiting the 24v engine, and lower mechanical losses.

Post Thu Oct 31, 2013 7:55 pm

daimlerman wrote:There's plenty using this combination!
You need an M20 flywheel/clutch/starter(ring gear has a different pitch to M50/52)

An ex 320 gearlever support bracket will locate the stick centrally in the tunnel hole,with a linkage from the same source.

Mine,using an M50 that's standard,has been built several years.covering some 45k trouble-free miles.
A tuned M52,say in excess of 250bhp,may well shorten gearbox life,though.

Thanks!

OK, M20 flywheel (bolts too?), clutch kit (recommendations?) and starter motor added to my ever growing shopping list!

Gearlever support bracket & linkage specifically needs to come from a 320? I've got a nice short-shift kit of some sort in there at the moment, would that not be compatible with the M40 box?

Reckon my M52 must be about 200bhp tops as my 250bhp Alfa (200kg heavier) is a good bit quicker.

Post Sat Nov 02, 2013 5:42 pm

If i could just steal this thread back, does anybody know conclusively if different model bmw diffs used different gear axis offset on the hypoid gears? or are they all the same?