AS above, just removed the ones under the car, they fitted a resonator box in place of both cats, not sure if I'm that happy, sound has changed, a bit more muted, not that growl the 330i is known for.....performance, feels a bit nippier, seems to rev a bit cleaner, and I think my fuel consumption has improved a bit....not sure if I want to remove the header cats, would be nice to get a complete replacement header that has been de-catted....