Small Case and Large Case

[kylebes1]

Ok, I know for a fact they are interchangible. I had a 3.91 small case in my 1984 318i. When I did the m50 swap I installed a 3.73 LSD, large case.

The point of this discussion is why do people assume the strength of the small case is that much less than a large case? (In general)

I have talked to a few people...

One guy at the local dealership did an s50 with a small case 3.91. Ran for two years with no trouble. I think a guy at AKG and another at Euro depot told me the same thing....

Other than mileage, why would a small case not hold up?

O and the guy with the s50 tracked th ecar, so you know the diff was abused...

[e9nine]

Sizing is Small - Medium - Large.

Driving style, condition of the diff with regards to fluids and prior use/abuse determine how long a diff. will last among other variables.

Tracking a car is a broad term. There is a diff. between the occasional track session, seasonal track driving and regular track driving.
Same principle applies for why some cars need diff coolers or cooling fins and why a pure race diff costs more than most people's cars here besides the obvious gearset and lockup variations.

If you're going to drive it on the street in a sane manner without doing insane burn outs etc, a medium case will last you just fine.

[AdamF 88iS]

The large case carries almost double the amout of fluid as well, which makes a huge difference in the life of it, especially if it's LSD.