Who knows about differentials?

[genphreak]

I have a RHD 8/90 build 525i M50 with the 5 speed 5HP18 tranny (Japanese delivery)

I bought it cheap, fixed it up and then the tranny went. I wanted to rebuild it, but my mechanics had a manual and it saved me some money and a lot of time...

However, now the car seems to have a very short final drive ratio, which I don't get as they both have 5 gears...

The motor now does about 3000 rpm at 100kmh (60mph)

I found this resource which shows that they should both be 3.23
http://igge.se/bmw530/final%20drive/final.htm

1. Is the ZF box a serious overdrive or have they installed an e36 box with different ratios? (If this is possible...)

2. I have a 3.45 open diff here or should I get a 3.91?

3. Can one swap the innards of the LSD and use the crown wheels from an open diff? (I know it'd be hard- I am wondering if they will fit/mate at all)

Thank you for reading my queries... Nick

[Dave M]

I don't know bmw diff ratios, but i can add that the resource you posted doesn't differentiate between the drivetrains delivered on M20 and M50 525s.

If you went from the Getrag to the ZF.....310Z (which it sound like), and are now experiencing higher than normal rpms at 100kph, there must be a difference in the diffs used between the getrag and ZF boxes as my ZF....310Z 525 manual spins at a ~2500 rpm at 100kph.

Sorry I can't be more help,

Dave

[repenttokyo]

my car does around those RPM at that speed....

[Dave M]

Don't recall where I collected this, but its obviously from an e30 related site. I would like to assume that Getrag 260 and ZF 310 ratios remain the same between applications (e30 vs. e34).

E30 Getrag 260

3.83 1st
2.20 2nd
1.40 3rd
1.00 4th
0.81 5th


E36 ZF 310/320

4.21 1st
2.49 2nd
1.66 3rd
1.24 4th
1.00 5th

So, if you're diff ratio hasn't changed between trannys, would this explain your spinnies ?

Dave

[Ross]

3000 revs gets me around 80 mph. A LOWER numeric ratio is what you need, if you have 3.54 now 3.91 will increase your revs.
Crown and ring gears are a matched set, mixing won't work.

[healtoeit]

3000 revs gets me around 80 mph. A LOWER numeric ratio is what you need, if you have 3.54 now 3.91 will increase your revs.

I 2nd that!
A 3.91 will boost your acceleration but do the opposite of what you want

[genphreak]

I 2nd that!
A 3.91 will boost your acceleration but do the opposite of what you want

Doh! Of course... I have a 3.45 here and a 3.91, but no 2.91 or whatever... where can I find that... hmm

(Thanx everyone)

Looks like this is an e36 box, huh!

[genphreak]

Don't recall where I collected this, but its obviously from an e30 related site. I would like to assume that Getrag 260 and ZF 310 ratios remain the same between applications (e30 vs. e34).

E30 Getrag 260

3.83 1st
2.20 2nd
1.40 3rd
1.00 4th
0.81 5th


E36 ZF 310/320

4.21 1st
2.49 2nd
1.66 3rd
1.24 4th
1.00 5th

So, if you're diff ratio hasn't changed between trannys, would this explain your spinnies ?

Dave

Yep- looks like this is the case Dave, thanks! Easily 20% is what I am noticing. At 35mph its already doing 2000 which is way too high for cruise... the auto was barely ever selecting 5th gear at this speed (ie only when on the flat or downhill and under a low throttle condition)

Blast! What to do now... get a 2.64? No diesel e34s ever came to Australia... and would it be available as an LSD? I doubt it... <cries>

Would a 2.81 from an e39 fit?

[Ferret]

From what I can make out of E34.de, some of the 525i's were also fitted with 3.73 and 4.10 diffs depending on the gearbox, have you actually checked what yours is fitted with?

Will any E30 boxes actually mate up properly with the M50 engine?

I'd have a crawl under and look at its ratio tag, if it had an older auto box in it I'll bet it's got one of the horrifically high ratio diffs in it... I bet if you drop the 3.45 in it you'll notice an improvement!

[Ferret]

Doing some further research on bimmerfans.info and plugging in the fact it's a japanese vehicle, it gives me the choice of diffs:

3.91
4.1 (Not sure if this ones allowed)
4.44

Bet I know which one you've got...