From Helix Racing Products - High-Density Fiberglass Muffler Re-Pack (1" x 11" x 16"). This tight weave high density muffler packing is preferred to control sound and back pressure. When being installed, it can be applied to muffler core in layers that allow for fine tuning of the back pressure that effects the torque curve to achieve desired feel and performance. Every muffler needs repacking after several hours of riding. Quiet performance is the way to go for todays power sports. For street or off-road machines.