Quote:
Jayson said:
Why not use stainless steel. This never happened to the BMW.



As others have said, they are stainless steel. Stainless steel does not always live up to its English name.
I think it is particularly prone to discoloring if subjected to high temperatures in wet and dirty conditions.

Your BMW's exhaust tailpipes would not have got as hot as those of your Porsche because the former are much further from the engine than the latter.