Milltek Sport | Secondary Catalyst Bypass (F80 M3, F82 M4)F80 M3 Cat Delete Milltek Secondary Catalyst Bypass (F80 M3, F82 M4) Some vehicles sport two catalysts due to their dual pipe assembly. The F80 M3 Cat Delete bypass pipe removes the secondary catalyst to further improve gas flow and reduce the restrictions caused by catalysts within the exhaust system. All our performance exhaust systems are designed with you in mind, and we cater to an ever expanding range of vehicles. We believe in continuous