08-18 YAMAHA WR250R: Yoshimura RS-4 Slip-On Exhaust (Race/Stainless Steel With Carbon Fiber End Cap) The work that went into developing the RS-4 muffler design paid big dividends in 2010 with winning the AMA 450 Supercross and Motocross Championship series, and again in 2012 winning both 250 Supercross Lites titles. That same R&D has been directed to developing the current generation RS-4 - a unique design that provides an unrivaled combination of power and a quiet exhaust note. The RS-4 Slip-On uses the same technology that goes into the full systems, but features an easier-to-install application. The RS-4's premium muffler packing helps meet the U.S.F.S. noise specifications while maintaining performance.