There is no exhaust that exactly emulates the finish on the OEM 135i tips. The OEM BMW tips are polished aluminum and then are TypeII anodized. There is currently no commercially viable method to electrochemically treat the surface of 304 stainless steel using a anodizing or magnetic dyeing process. Your only real option is to use a black ceramic coat that is specifically designed for automotive uses (i.e. high heat) such as the one we offer for any of our exhaust systems.
WARNING: GINORMOUS PICTURE.
http://www.berktechnology.com/produc...eramic-coating