DNA Air Filters - Air Filter for BMW C 400 GT 18-23 / C 400 X 18-23 / C 600 GT 11-20 / C 500 Sport 11-20 / C 650 Sport 11-20This filter features DNAs advanced FCd1(Full Contour design). Installation of this DNA Air filter is very easy; simply follow the installation instructions included in the BMW workshop manual. The filtering efficiency2 is extremely high at 98 99% filtering efficiency (ISO 5011), with 4 layers of DNA Cotton. The flow of this new DNA filters set is +35. 29% more than the stock filters! DNA air filters flow: 85. 58 CFM (Cubic feet per minute) @1,5H2O