Oval welded center/center muffler. 2.25-inch or 2.50-inch free-flow straight-through central muffler wrapped in stainless steel mesh. 6 inches x8 inches x14 inches with limited lifetime warranty. Tail modification in tail gas generally includes middle drum, tail drum and tail pipe, which is a part of many improvements, and usually has two important concepts: sound wave and back pressure. Everyone knows the noise, so what is the back pressure? As the exhaust port shrinks, the exhaust gas in front of the exhaust port gathers at the tail. When the exhaust port is exhausted, this pressure becomes negative pressure, which "sucks out" the next exhaust gas. This phenomenon is called back pressure.