HMF 6946006 Quiet Core for Exhaust The HMF Quiet Core is an insert made of 201 stainless steel and installs on most HMF exhausts in seconds. It's designed for sound sensitive riding areas and decreases the sound level approximately 3 - 6dB.Quiet Core fits the the following HMF exhausts:Peformance Series.Swamp and Swamp XL Series.Penland Series (Torque Range Modifier removed).Signature X Series HMF recommends re-jetting the exhaust whenever a Quiet Core is installed. Failure to do so can lead to several problems and can damage the exhaust and void the warranty. You cannot use the Torque Range Modifier with a Quiet Core at the same time, only one can occupy the exhaust space.