Weight Differences - Large Case Rear End versus Medium Case Rear End

[TGreene]

It has been proven that the medium case rear ends can handle much more power than the M5 and 540i put down. There are also many more ratio options available for less money with the medium case rear ends. This leaves me wondering how much of a difference there is in weight between the two. Even if the sub-frames are the same, the half axles are at least a little different and the large case differential must have at least ten pounds on the medium case.

Pros:

1) More diff ratios to chose from.
2) Medium case diffs are cheaper and more common.
3) A few pounds weight saved.
4) Less mass for the drive train to spin. Between the half axles and larger gears in the diff it might only be a pound or two, but if physics apply here the same way they do with light weight flywheels, it should be considered.

Cons:

1) Labor

Does anyone have any data they could share? I have a large case half axle I can weight.

If dropping a subframe on a M5 or 540i for whatever reason, might it make sense to swap to a medium case setup while you are at it?

[imae30driver]

The medium case diff will not fit the subframe i believe. Also, DS and potential half shaft issues .

Its just not worth while, 3.15 lsd is the proven method

[TGreene]

The medium case diff will not fit the subframe i believe. Also, DS and potential half shaft issues .

Its just not worth while, 3.15 lsd is the proven method

RealOEM shows that all subframes are the same except for the M5 subframes.

[TheStigg]

E34 medium case = approx. 82 lb. E32 750 large case = approx. 106 lb. (bathroom scale accuracy). As mentioned, there are dimensional issues to swap them. At a minimum, the rear driveshaft would have to be lengthened to mate a medium case into a large case chassis. The input flange and axle flanges cannot swap large to medium or visa-versa, but if the driveshaft cv joint has the same hole pattern, it will mate large or medium. Also, if the cv axles are the same hole pattern, they will mate large or medium.

Here are some measurements I made a few years ago comparing the 2:

http://www.bimmerforums.com/forum/showthread.php?1470479-Nitty-gritty-on-Large-vs-Medium-case-diff-swap

[atl530i]

I'm happy with the large case s3.15 in the 540. Only complaint I have is that it would have been nice to add another clutch/dog ear to it without machining the carrier cover to accept the additional clutch/dog ear. It's as simple as removing the spacer in my medium case s3.23 to add another clutch and dog ear.

[TGreene]

E34 medium case = approx. 82 lb. E32 750 large case = approx. 106 lb. (bathroom scale accuracy). As mentioned, there are dimensional issues to swap them. At a minimum, the rear driveshaft would have to be lengthened to mate a medium case into a large case chassis. The input flange and axle flanges cannot swap large to medium or visa-versa, but if the driveshaft cv joint has the same hole pattern, it will mate large or medium. Also, if the cv axles are the same hole pattern, they will mate large or medium.

Here are some measurements I made a few years ago comparing the 2:

http://www.bimmerforums.com/forum/showthread.php?1470479-Nitty-gritty-on-Large-vs-Medium-case-diff-swap

If your scale is correct then that is 24lbs difference not including the small difference from the half axles. That is more than I would have thought.

Some complications if you want to piece something together using your existing large half axles/rear wheel hub, but if swapping everything over from the CSB back, it might be very simple.

[janders211]

If your scale is correct then that is 24lbs difference not including the small difference from the half axles. That is more than I would have thought.

Some complications if you want to piece something together using your existing large half axles/rear wheel hub, but if swapping everything over from the CSB back, it might be very simple.

I thought this sounded heavy for a large case...so I just went out and weighed my large case 2.93 and my large case S3.15..both raw weight, and me holding them, minus my weight to compensate for scale zero errors.
large case 2.93 was 92-95lbs, large case lsd from a 750il S3.15 was 100-105 lbs. wow- I guess they are that fat.

[ross1]

Cars with large case also use different hubs, NO?

[Layne]

Cars with large case also use different hubs, NO?

That's correct.

I had the same thought a while back. http://www.bimmerforums.com/forum/showthread.php?1797153-Differential-downsizing

24lbs under the rear makes NO difference at all. The differential is located in the least relevant place for weight savings. Whether the difference in parasitic loss matters is debatable, but the fact that it's not enough difference to justify the swap is not debatable. You would have to custom modify a driveshaft and replace the trailing arms and CV axles. Not worth it.

[Racerhoze]

I'm happy with the large case s3.15 in the 540. Only complaint I have is that it would have been nice to add another clutch/dog ear to it without machining the carrier cover to accept the additional clutch/dog ear. It's as simple as removing the spacer in my medium case s3.23 to add another clutch and dog ear.

bimmerdiffs will ship you a pre-machined one and you ship them back a core.

[TGreene]

I disagree. It makes a small difference.

Now I am considering also the weight difference between the G280 and the G260/6 from the 535i. The G280 is expensive, loud and clunky. If I could throw a G260/6 and everything that goes behind it, it might make for a good way to spend a weekend. Lighter car, less rotating mass, quieter gearbox, and more gear ratios to play around with.

[BruinBimmer]

I would keep the extra weight if I were you, especially since you have an extra bit of iron block hanging over the front axle. I've played around with my suspension damping/height at autocross, and the handling can be pretty sensitive to weight distribution. I had some mad rake for kicks one time, and it was helpless on turn-in - just plowed straight on. Too much weight bias over the front. Raised the front about an inch or so, and it had just a hint of rotation when braking on turn-in. Nicely swung the rear end around the tight apex, so I was pointed in the direction I wanted to go sooner, and could get on the power sooner. The way the car behaved with those two front suspension heights was night and day.

I won't speak for M5 owners since I've never driven one, but I have no complaints about the easily attainable 2.81, 2.93, 3.15, and 3.64 ratios available for the large case.

[TGreene]

That is a very good point.

[wanganstyle]

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