FMF Racing Yamaha YZ250F 06-09/YZ450F 06-09/WR250F 07-13/450F 07-11 Q4 S/A MufflerA 4 Stroke header system that incorporates FMF's patented Power Bomb module at a precise location in the initial stage of the header system; by positioning the "Bomb" at this key point, the expansion and contraction of exhaust sonics through the system resulting in a flow increase of nearly 10%. The surface of the perforated core lets the sound exit into the absorbing material (steel wool packing) and then allows the air to expand, cool and ramp back