The Factory 4.1 RCT incorporates the latest in FMF Resonance Chamber Technology both internally and externally. In addition, we achieve a shorter overall length that is less susceptible to crash damage and complementary to our Factory Forward Engineering - moving the muffler closer to the motor to centralize mass. The Factory 4.1 RCT is constructed from only aerospace grade materials that meet our exacting specifications and is manufactured 100% at our Southern California factory. Available in Natural Aluminum Muffler with Carbon End Cap Massive Power gains throughout the RPM range. Huge weight savings New RTS (Rapid Tuning System) FMF uses the highest quality proprietary blend of alloy grade titanium made especially to our standards. Can be used with FMF or Stock OEM Header Fits: Yamaha WR450F 2016-2018 YZ450F 2014-2017 YZ450FX 2016-2018