What is a Slipperential?

[spiftacu1ar]

I have a question about heat. I read part of the thread, where someone mentioned that the temps stayed relatively low (116 F) after a full run. In the item description in the store, it mentions fins (or something of the sort) to help increase airflow to keep it cooled. Does this mean that if I have the diff in a sealed enclosure (i.e a sealed gearbox) it might overheat? Is there any way to prevent this. I am going to use this diff in my savage conversion, which I often also bash with, so I don't want rocks and other debris to damage the gears.

EDIT: Also, does it come stock with the 46t, 48t, or 50t spur.