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I believe the secondary cats on the N54 are outside the O2 sensors loop so no emissions issues. I have an N55 with PE and resonator and it sounds about perfect with no drone. Like many have said it should have come that way from the factory.
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12' 135i DCT, Convertible, Triple black, Nav, Apps, BMW Shadow Grilles, LCD interior lighting, BMW PE, Michelin PSS.
06' M Roadster, SMB/Red, AA Tune, K&N, Remus Exhaust, ZHP shift knob, Hyperblack M Rims.
09' BMW 328i Touring, Monaco Blue, Panoramic roof, Nav.
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