This all-new Factory 4.1™ Muffler incorporates the latest FMF Resonance Chamber Technology®. The patented chamber design enhances and refines exhaust pressure. Its patented core shape lowers sound level while maintaining maximum power output, and the patented canister shape increases internal volume for more sound-absorbing material. Factory Forward Engineering centralizes mass and is less prone to impact damage. Maintenance is made easy with the removal of four screws on the front end cap, and the innovative rear end cap simplifies installation of the spark arrestor (sold separately) or tuning tip inserts. Includes QuietCore™ 94 dBA insert. Meets AMA® and FIM sound requirements. Used by the JGR and Star Racing factory-support teams. Compatible with stock or FMF® header. Built 100% in the U.S.A.
This accessory is restricted to closed-course use.

(Shown here on YZ250F)