Hello, just thought i'd throw this up for info for any WR450F 2009 owners. I recently serched the IN's and OUT's of this forum trying to confirm if a FMF POWER BOMB header would fit with my Drd slip on. Well IT DOES. I replaced the original WR (smaller Dia) header, which used the step down bushing supplied with the muffer with the FMF header. Was a perfect fit. Now the bike is fractional QUIETER and SMOOTHER and a noticeable power gain through the whole range. Hope this helps someone.