Difference between the diffs. 3,6 and 4,2?

Hello.
can anyone tell me what the difference, or the differences, between the differential of the 3,6 to that of the 4,2 is? I'm mainly concerned with the Torsen.
I'm gonna have to say that the 4.2's "locks" more than the 3.6's. I can't imagine, but maybe someone knows more.
I would like to install a torse in the gearbox of an Audi 80 quattro type 85 (autocross vehicle). I was told that the diff of the 4.2 fits there (small change necessary).