XSeries-forum

After much head scratching I have decided to replace both front and rear diffs to a taller ratio - mainly because after speaking to various BMW breakers - the 3.91 diff are very weak and will at some point break.
So do I go for 3.73 or 3.64 ratio. I will be towing a twin axle caravan in the future so dont want to go too far away from 3.91 but I think the power the 3.0d has - it will still pull comfortably.
any suggestions???

FWIW, I'd go with the 3.64 - if those are your options. If you drove 2 otherwise identical cars, one with the 3.73 and one with 3.64, back-to-back I doubt you'd notice the difference between them.

In my younger days I did ratio swaps on several different cars, and the only time there was any real difference in the way a car drove was when I went from 4.27 to 3.64 in my 320i E30 (that was from the 2nd lowest option to the highest in 1 jump).

As you say, it shouldn't be a problem for the M57 engine - especially as the ratio options on the E53 went right up to 2.81.

I took the plunge and sought a pair of 3.64 diff froma 4.4 petrol X5. They arrived today and hope to have them fitted in a day or so. wish me luck!!
I'll soon have a working albeit redundant 3.91 front diff and rear diff in bits if anybody needs them?
The rear diff casing is intact along with pinion and crown wheel, other gears showing signs of wear but no cracked teeth.

ex4x4 wrote: Wed Aug 28, 2019 6:46 pmwish me luck!!

You got it!

Keep us updated, please.

Well they're in and what a difference
Took a few days on and off but got there in the end.
A few points to note when changing diffs.
>Drain oil out first!!!
>Rear diff is easy by youself if you have a high lift trolley jack. - the two half shafts simply unbolt from diff and bend out of the way. No need to remove wheels or try to drive half shafts out of bearings.
>A block of wood to jam wheels saves alot of time removing drive shaft/ prop shaft bolts. Leave car in neutral and release parking brake, shaft turns by hand to locate bolts them jam wood under wheel to prevent shaft turning when undoing or tightening bolts.
> front diff is a royal PITA to get out of engine bay. it does not just slide out!!
unbolt steering rack from subframe - 2 bolts.
I had to unbolt engine mounts and jack up engine about 2 " via trolley jack, then with a 2nd trolley jack to support subframe - unbolt subframe bolts - leaving drivers side bolts in by about 3-4 threads then completely remove passenger side bolts and control lowering via 2nd trolley jack. the diff will then come out sideways towards wheel hub.
>when trying to replace diff, insert top two mounting bolts through casing before offering up to engine, as once in place, the top two bolts can not be inserted as their path is blocked by steering pipes / bodywork.
> steel strengthening plate - remove only 5 bolts and leave rearmost drivers bolt loose but in place - plate can then be swung out of the way. This way it is very easy to locate back in place and bolt up.
> to remove front half shafts - remove road wheel and unbolt control arm ( the bottom one parallel with drive shaft) from the car chassis - this will give more than enough room to remove halfshaft.
Whilst I had car jacked up on stands - I took the oportunity to drain and fill auto transmission. I did a pan off refill last year with new filter - but this time just dropped anout 3.5litres and replaced.
So all in all a doable job on your own - but plan for a good couple of days. A universal hub puller definately advantageous , a 36mm 12 point socket a must and a good high power impact gun work wonders on hub nuts and every other tight bolt and nut.

On test drive - the car is marginally slower on acceleration but by no means sluggish, more than compensated by an increase in MPG - I hope!!! - I calculate about a 7% increase but will feed back when confirmed.

Glad you got it all sorted out and thanks for the info on the job.

Alan Gunn wrote: Sun Sep 01, 2019 8:58 amGlad you got it all sorted out and thanks for the info on the job.

Seconded!

Well done on getting it sorted

thanks!
Now I know what is involved - if anybody needs some help doing a diff change - count me out!!!! lol
I get back with MPG difference - if any in the coming weeks.