Made my mind up: I'm gonna do a diff swap.. (a few Q's).

[fin]

Pre-load.

Bearing in machinery are designed to run with a given amount of pressure on the sidewalls for them to run properly. It's commonest (and most critical) with cone bearings which can be thought of as three wedges (inner, outer race and the tapered bearings) working against each other. Too much preload (Torque) and the wedges bind. Not enough and the bearings are loose and can get slightly sideways changing the angle of their wedge shape. The bearing will start binding, wearing and heating the bearing at an increasing speed. In general when bearings are loose, the shaft can wobble around which transmits energy from the shaft by it's flopping motion to the bearings/races, which in turn heats things up and it's downhill from there.

Marking the nut and the input shaft when replacing a flange will put the preload on the bearing very close to what was when it 1. Left the factory, 2. was last rebuilt. You are trusting that they had it right. If the diff. is know to have a lot of miles on it, that is a very safe assumption.

If you really want to dial it in. Determine if the replacement flange is thicker or thinner and adjust the angle between the marks on the nut and the shaft to compensate for the difference. So if the replacement flange is 0.25mm smaller and thread pitch of the shaft is 1.0mm, then you would turn the nut a quarter turn past the matching mark point. (I don't know what the true pitch of the shaft is, don't bet your rear end on 1.0mm.)

Don't sweat it, the real difference in the flange thicknesses isn't such that the torque values would be outside the tolerance of the bearing's recommended amount. I don't think fine German engineers would neglect to think of that when writing the tolerances for the flange thicknesses.

At the factory, the nuts are put on with torque drivers that are regularly tested to ensure the proper torque being used. When you move into that world you will start hearing words like 'running torque', 'static torque' etc. That is why using the torque value to loosen the nut is not the value used to torque it on. Cleaning and greasing will greatly affect those numbers.

More than you probably wanted to know, but I just had to say it.

Cheers,

Ned

[Jeff N.]

What JoeS said. I have a 3.73 LSD. First gear becomes optional. Fuel usage at highway cruise is accelerated. A 3.64 is probably the best gearing if you want to shorten it up a bit.

Pros of the 3.73: No need to swap the input shaft. Since they are sourced from a 89 525 stick, (looow power), they tend not to be trashed. You will need to swap the output flanges put they just pop out and move over from your current diffy.

The real issue I think with the E34 is not the diff gearing, it's the long spacing in the gearbox between 2nd and 3rd that's the issue.

[e34.535i.sport]

Thanks for all the input so far guys... I've been away for a few days so haven't been able to respond.

I think the 3,64/3.73 may be the way to go by the looks of things, but I wanted the difference to be noticible- otherwise the effort of changing it won't be worth it if you know what I mean. However, I recently found that I can get some damn good mpg on the motorway which I wouldn't want to totally ruin!

I need to ask - can anyone outline to me plainly which diff's I can use that will NOT require preloading? Also, I read that there are a couple of different housings, which ones won't fit mine? I know the m5 diff is a no go...

I'm looking at a 525i/m (89/90) 3.73 (LSD hopefully) OR a 735i/m (89/90) 3.64 (LSD hopefully)...

It's a shame as a guy I've been to before has said he will do me a 535i auto 3.91 diff for cheap if I come and take it off myself. Tempting but possibly too short for me gear wise.

[bubba966]

I need to ask - can anyone outline to me plainly which diff's I can use that will NOT require preloading? Also, I read that there are a couple of different housings, which ones won't fit mine? I know the m5 diff is a no go...

I'm looking at a 525i/m (89/90) 3.73 (LSD hopefully) OR a 735i/m (89/90) 3.64 (LSD hopefully)...

It's a shame as a guy I've been to before has said he will do me a 535i auto 3.91 diff for cheap if I come and take it off myself. Tempting but possibly too short for me gear wise.

The M5 & 540 diffs aren't direct swaps. The 518/520 might not be as well, but I don't know for sure. M5 & 540 use 'large case' diffs, 525, 530, 535 use 'medium case' diffs. Not sure about the diffs in 5's, but in the 3's the 318 had a smaller diff than the 325. Which is why I think you should ask someone that knows Euro E34's better about the 518/520 diffs if you should come across those.

I'm not sure but I think the 735 diff is a medium case diff just like the ones in the E34's. I know the 740 & 750 used the same large case diffs the 540 & M5 had as those swap right in.

The 3.91 is going to be deep enough to effect your mileage noticeably. Not only that but if you don't have enough HP for deep gearing like that it'd actually accelerate from a stop slower than a gear ratio that's not as deep. And the gear ratio of the diff is only part of the equation as you need to look at the overall gear reduction which includes the gear reductions in the tranny.

For instance, my truck used to have a TH-350 tranny and a 4.10:1 diff. The TH-350 has a 2.52:1 first gear. That combined with the 4.10:1 rear end produced great acceleration. In fact it almost lifts the front tires off the ground.

But then I changed the tranny out to a 700R4 that has a 3.06:1 first gear. That combined with the 4:10 rear end gave me slower acceleration than I had previously with the TH-350 and didn't lift the front end nearly as much at launches.

Mentioned that to the guy that built my custom 700R4 during the follow up call and he didn't understand how that could be. I was putting more torque to the ground with the 700R4, but was getting weaker/slower launches. Which makes no sense.

But then I told him that the engine in my truck was built for torque, not HP and doesn't build HP very quickly. (torque is like the brute force, and HP is more about the speed of the power buildup. Which is why ricers with no torque don't get going well off a start, but go fine once moving along)

What was happening was that the 700R4 and 4.10 rear end was actually too deep of a gear reduction combination for the HP curve of my engine. As the engine didn't rev up quick enough, it wasn't putting out enough RPM's for the lower gearing to keep the old acceleration level.

Yes, it puts more power to the ground than it did (which is why I've snapped a pair of good quality 20,000 pound tow straps, and can lay 10" wide strips of rubber for blocks). But it'd doesn't accelerate from a dead stop as fast as it used to.

Anyways, I don't think a 3.91 is too deep to negatively effect acceleration. And I'm not even sure a 4.10 would be. But the ratios that are higher than 4.10 would probably be too deep for acceleration improvement.

I still think your best bet for optimum improvement and minimal loss of mileage is the 3.73 if you do actually have a 3.46 in the car now. Best make sure you've got a 3.46 and not a 3.23 or 3.15 or something even taller than that.

[e34.535i.sport]

The M5 & 540 diffs aren't direct swaps. The 518/520 might not be as well, but I don't know for sure. M5 & 540 use 'large case' diffs, 525, 530, 535 use 'medium case' diffs. Not sure about the diffs in 5's, but in the 3's the 318 had a smaller diff than the 325. Which is why I think you should ask someone that knows Euro E34's better about the 518/520 diffs if you should come across those.

I'm not sure but I think the 735 diff is a medium case diff just like the ones in the E34's. I know the 740 & 750 used the same large case diffs the 540 & M5 had as those swap right in.

The 3.91 is going to be deep enough to effect your mileage noticeably. Not only that but if you don't have enough HP for deep gearing like that it'd actually accelerate from a stop slower than a gear ratio that's not as deep. And the gear ratio of the diff is only part of the equation as you need to look at the overall gear reduction which includes the gear reductions in the tranny.

For instance, my truck used to have a TH-350 tranny and a 4.10:1 diff. The TH-350 has a 2.52:1 first gear. That combined with the 4.10:1 rear end produced great acceleration. In fact it almost lifts the front tires off the ground.

But then I changed the tranny out to a 700R4 that has a 3.06:1 first gear. That combined with the 4:10 rear end gave me slower acceleration than I had previously with the TH-350 and didn't lift the front end nearly as much at launches.

Mentioned that to the guy that built my custom 700R4 during the follow up call and he didn't understand how that could be. I was putting more torque to the ground with the 700R4, but was getting weaker/slower launches. Which makes no sense.

But then I told him that the engine in my truck was built for torque, not HP and doesn't build HP very quickly. (torque is like the brute force, and HP is more about the speed of the power buildup. Which is why ricers with no torque don't get going well off a start, but go fine once moving along)

What was happening was that the 700R4 and 4.10 rear end was actually too deep of a gear reduction combination for the HP curve of my engine. As the engine didn't rev up quick enough, it wasn't putting out enough RPM's for the lower gearing to keep the old acceleration level.

Yes, it puts more power to the ground than it did (which is why I've snapped a pair of good quality 20,000 pound tow straps, and can lay 10" wide strips of rubber for blocks). But it'd doesn't accelerate from a dead stop as fast as it used to.

Anyways, I don't think a 3.91 is too deep to negatively effect acceleration. And I'm not even sure a 4.10 would be. But the ratios that are higher than 4.10 would probably be too deep for acceleration improvement.

I still think your best bet for optimum improvement and minimal loss of mileage is the 3.73 if you do actually have a 3.46 in the car now. Best make sure you've got a 3.46 and not a 3.23 or 3.15 or something even taller than that.

Thanks that was really helpful... I've discounted the 3.91 as I think you're right. I would prefer to get my hands on a 3.64 if possible as it should improve acceleration without having negative effects being relatively similar to my own. However If I come across a 3.73 in good nick it may be worth a punt if I can get it cheap. Either way if I don't find it better I can always throw mine back in! If I do mine will be for sale...

I'm pretty certain I have the 3.46 but may jump under tonight to check for sure. I assume I should be able to easily see the number without dropping the diff or anything? At least I hope!

[bubba966]

Thanks that was really helpful... I've discounted the 3.91 as I think you're right. I would prefer to get my hands on a 3.64 if possible as it should improve acceleration without having negative effects being relatively similar to my own. However If I come across a 3.73 in good nick it may be worth a punt if I can get it cheap. Either way if I don't find it better I can always throw mine back in! If I do mine will be for sale...

I'm pretty certain I have the 3.46 but may jump under tonight to check for sure. I assume I should be able to easily see the number without dropping the diff or anything? At least I hope!

The M5 diff I picked up has a steel tag attached to the rear diff cover that's got the ratio stamped into it. It's also marked somewhere else that I'm not recalling at the moment. You should be able to find the ratio on the diff without pulling it out of the car.

A 3.64 will be an improvement over a 3.46. But I honestly think you'd enjoy driving a 3.73 over a 3.64 if you've got a 3.46. Shouldn't hurt to put either into the car. and should you get one of those and decide that you didn't care for it and want to try another one I'm sure you could sell the LSD you don't like for about the same price as you pay for it.

BMW LSD's are always in demand. Makes it easy to try different ratios if you're able to sell one for what you paid for it. Which is why I didn't mind picking up that 3.91 LSD without knowing if that was too deep or not for my car. It was $200, I should be able to sell it for more than that if I want to. And as I've bought new parts for it (bolts/seals/etc.), I will want more than $200 should I sell it.

[Paul in NZ]

The M5 diff I picked up has a steel tag attached to the rear diff cover that's got the ratio stamped into it. It's also marked somewhere else that I'm not recalling at the moment. You should be able to find the ratio on the diff without pulling it out of the car.

A 3.64 will be an improvement over a 3.46. But I honestly think you'd enjoy driving a 3.73 over a 3.64 if you've got a 3.46. Shouldn't hurt to put either into the car. and should you get one of those and decide that you didn't care for it and want to try another one I'm sure you could sell the LSD you don't like for about the same price as you pay for it.

BMW LSD's are always in demand. Makes it easy to try different ratios if you're able to sell one for what you paid for it. Which is why I didn't mind picking up that 3.91 LSD without knowing if that was too deep or not for my car. It was $200, I should be able to sell it for more than that if I want to. And as I've bought new parts for it (bolts/seals/etc.), I will want more than $200 should I sell it.

AFAIK if you have an lsd you can swap the clutch pack from one diff to another....or swap different ratios frm one case to another,as long as the diff is the same..ie two med case BMW diffs...one 3.46 lsd and one 3.73 open...you can make a 3.73 lsd with those two diffs...

[Paul in NZ]

Thanks that was really helpful... I've discounted the 3.91 as I think you're right. I would prefer to get my hands on a 3.64 if possible as it should improve acceleration without having negative effects being relatively similar to my own. However If I come across a 3.73 in good nick it may be worth a punt if I can get it cheap. Either way if I don't find it better I can always throw mine back in! If I do mine will be for sale...

I'm pretty certain I have the 3.46 but may jump under tonight to check for sure. I assume I should be able to easily see the number without dropping the diff or anything? At least I hope!

I asked you before....if you have a 535 sport,you should already have the 3.64...what revs are you running at 60 mph?

[GoldenOne]

eh if you somehow want a S3.91, i have one for sale!

[e34.535i.sport]

eh if you somehow want a S3.91, i have one for sale!

It seems I just might witht the recent revelations... how much are you looking for and what car is it off (535i?)?

It's probably going to work out a lot dearer with shipping etc - what do you think?