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 wantDoh! 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...

[repenttokyo]

is it really that unusual to be running at these rpm's on the highway in 5th gear? I am above 3000 rpm at 100 km/h in my M50. My gas mileage is really good, too.

[bubba966]

A lot of 525iA's had a 4.10 diff

You also have a medium case diff. There was also a large case diff used in the M5's & 540's.

There's lots of E34 ratios in use. 4.10, 3.91, 3.73, 3.64, 3.45, 3.23, 3.15, etc.
You can get even more than that in your diff if you want.

http://www.diffsonline.com/product/customdiff1.shtml

First you should find out which ratio you have on your diff. Should be listed on a tag on the rear.

Then figure out what ratio you want to change it to. Only diffs I know that swap over to E34's are some of the E32's. But I'm used to talking about large case diffs, so I don't know which E32 diffs are medium case diffs.

[Ferret]

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>


Amusingly a 5hp18 with a 2.64 diff does 80mph(130km/h) roughly at 2000rpm, your M50 would be bogged down!

Can you remember what speed it used to do at what rpm at all on the old auto box?

3000rpm @ 60mph is extremely high for an engine this big... it should be sat somewhere between 70 and 80 at that rpm.

[Dave M]

is it really that unusual to be running at these rpm's on the highway in 5th gear? I am above 3000 rpm at 100 km/h in my M50. My gas mileage is really good, too.

Out of interest, what trannys in yours? My 525 is a 10/90 build date with the ZF 310. Oh, and i'm much closer to 2,500 rpm at 100kph.

Dave

[genphreak]

Thanks Bubba,

These are all good points. First one: Get under the car and be sure what its got... doh! Am working all day every day and its been raining for a month here (Sydney this time of year is normally dry as).. anyhow, I'll do so somehow in the wet and dark to get some clarity.

Thanks for the encouragement!.

All the six cylinder e24/28/32/34/36 cars come with a medium-case except the M-cars. Any M88/S-engined e24/28/34 will have a large case diff with the 210mm crown gears- as will any V8/V12 BMW except the M60B30 e34 (er, not sure about the M60B30 e32), I just now that the 530i had a medium case.

Mediums are interchangeable and very strong- but you will have to change the rear cover (and sometimes the input flange, which is not such a simple thing) if moving between e21/24/28/32/34 and perhaps others.

Not sure about the e39- does that use a different diff?

[genphreak]

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...Thanks Ferret, das good. If it was a Japanese vehicle I'd understand the low gearing. Will take a look and see. If it is as tall as this, the 3.45 might be a good substitute.

Anyone know hte best place to get a seal kit- BMA? Do we replace any bearings too? I've got a shop and a great friend who knows how... (I've never pried a diff apart before)

[Zeuk in Oz]

This probably doesn't actually help you Nick, but just by way of comparison, my 525ix has similar gearing - approx 105 km/h at 3,000 rpm in 5 th with its manual gearbox.

It (M50) uses perhaps 0.5 litre per 100 km more than my 528iT which is auto - 10 litres / 100km compared with 9.5. By comparison, my 525i (M20 5 speed manual) uses 8.5 - 9 litres per 100 km.

What I am suggesting is that if you can live with it, it might not be economic to spend lots on changing the diff unless the engine runs out of puff too soon or uses massive amounts of fuel.

[attack eagle]

525iA have a 4.10 rear gear, not 3.23.

I find it completely implausible that the 5 speed auto would use anything taller than a 3.7
especially since the OD ratio is the same for the 5 and 4 auto iirc.

[genphreak]

This really is a bone of contention!

Finally, its NOT raining and it isn't dark (I work long hours) so I crawled under and found the tag, cleaned it and it gives away the mystery:

3.91:1

After a long drive I can confirm; at 100kmh (62mph) the engine does easily 3400rpm.

This is too high, so I think the diff will need a swaperoo.

3400/100=34rpm per km/h with the 3.91

ie 34/3.91=8.70

Now I have a 3.45 here;

8.70*3.45=30rpm per km/h ie 3000 rpm at 100kmh

hmmm that is as per repentokyo and Zeuk's M50s. Perhaps this is the best thing... but I can't help thinking that lower would still be better.

8.70*3.2=2784rpm at 100kmh- -yeaaa!- if only I could get a 2.64 in Oz (no oil burners were imported here :(

I guess it must have that e36 gearbox... thanks for the help fellows!

[Paul in NZ]

wow...3000 at 100 k is like motorcycle gearing!My 535 does 2250 at 100 ks,some of the earlier cars(e23,e24 e28) had gearing as high as 1700-1800 rpm or so at 60 mph

[bubba966]

My 540iA M-Sport does 2,000RPM at 110kph. But it's a bit different than a 525iA.

And I'm looking to do the opposite. Not sure what's in my diff right now. Would like a 3.64 or 3.73. But I do have that 3.91 from the earlier US E34 M5. Too bad it'd be hard to find someone with a later US E34 M5 that's got the 3.73 and wants a 3.91. That'd make for a nice swap.