Originally Posted by swamp2
This is not only a way to save cost but also to throttle the performance back compared to other vehicles in their line up. Although they do not have to be so concerned about E46 M3 vs. 335i on quite a few tighter tracks a LSD would likely put the 335i ahead of the E46 M3. Porsche clearly left the LSD off of the Cayman S to prevent it from spanking much more expensive 911's.
Agreed about managing product lines. Not so sure if cost has much to do with it though. It really doesn't add that much. My '93 Miata had it. Current Miata's offer it as an option for less than $500. I know the 335 has more torque, therefore a 335 LSD would cost more, but still. The aftermarket ones seem to sell for ~$3k, which means BMW could put one in for a difference of about $1k or so. At least as an option. Anyway, I'm going off topic here obviously...