[QUOTE=TxZHP04;13891175]Popular question, seems to have been covered here 1000x before. The M3 diff isn't a direct drop in replacement for the non-M diff. For starters, it mounts differently. You can swap most of the M3 rear end over to make it work though. For more details, you'll need to search out the numerous prior discussions of this subject. Lots of good info to be found. In addition to the LSD, keep in mind the M3 diff has a 3.62 ratio compared to your stock 2.93 (assuming manual transmission) which is a little short for a 6500 rpm red line. Also keep in mind there are aftermarket LSD options which would allow for you to select the best ratio for your application.[/QUOTE]
Those seem to get pricey.
OP, search for cyberka's thread on it. He did it.