14-18 YAMAHA YZ250F: FMF Powercore 4 HEX Slip-On Exhaust When designing the PowerCore 4, FMF looked at their top of the line products to deliver race-inspired performance at an economical price. The end result is a quality muffler with improved torque, increased horsepower and rugged durability all at a price that won't break the bank. Overall increase in power and torque. Durable stainless steel and aluminum construction. Space Age internal packing material for maximum longevity.