DG Performance 04-2103 - RCM Steel Exhaust (Black) for Honda TRX 250X /300EX (1987-1993) RCM - Great performance and phenominal value! The midpipe diameter and gets larger as it gets to the muffler body to help scavenge exhaust gases from the engine and increase power Better throttle response and a smoother power delivery RCM stands for Reverse Cone Megaphone. Steel contruction