The K&N RU-4600 Air Filter is a Round Tapered Universal Air Filter. The base OD is 7.5" and tapers down to a 4.5" OD. The overall length of the filter element is 4" (not including the flange). This shorter overall length give the RU-4600 much more flexibility in tight-space applications. The RU-4600 is commonly used in Honda cold air intake systems where the filter is positioned behind the front facia. The filter has a black rubber top. K&N Air Filters utilize a oiled cotton media, which provides excellent filtration while allowing it to be less dense to improve flow rates. This filter is compatible with all BPI Flow Stacks.