FMF Racing Factory 4.1 RCT Aluminum Slip-On Muffler w/Carbon End Cap for Yamaha YZ450F 14-17/YZ450FX/WR450F 16-18FMF Factory 4. 1 RCT Slip On Exhaust Yamaha YZ450F YZ450FX WR450F If you are looking to drop weight, improve looks and most importantly improve your bikes power band then look no further. The FMF Factory 4. 1 RCT Slip On muffler was designed to bring out the hidden power potential in your machine by improving top end pull and low end torque. All 4. 1 RCT mufflers are manufactured in FMF's Southern California factory using aerospace grade materials