After doing some research online, it appears newer rear differentials in the 335i are one piece. The ring gear can't be unbolted for installation of an aftermarket limited slip differential. If it really is one piece.. installing a mechanical rear lsd will be VERY expensive. I get the feeling bmw will not offer rear lsd's as an option because it leaves the 135i with a major performance handicap that will prevent it from embarrassing the M3.
I guess I'll wait and see if the 135i really comes with a one-piece differential. If true, I won't order one. Coming from an awd car there's no way I'll sell it for a rear drive car with electronic differential. I wonder if the entire rear axle could be retrofitted from an older M3?
http://www.turnermotorsport.com/html...=100-100-00008
http://www.activeautowerke.com/drive...92335/main.php