13-16 HONDA CRF250L: Yoshimura RS-4 Full System 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 Ryan Dungey winning the AMA 450 Supercross and Motocross Championship series as a rookie in the same year, and again in 2012 with the Yoshimura-backed GEICO Honda t 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 features a polyresonant duplex chamber, a two-staged stepped baffle and an innovative carbon cone end cap. Pentagonal-shaped muffler sleeve and aluminum mount bracket. Front muffler cap functions as a polyresonant chamber which helps reduce noise.