Abstract: An exhaust system includes an exhaust pipe attached to the engine. Attached to the exhaust pipe is a muffler which has an exhaust gas inlet, a plurality of expansion chambers through which exhaust gas flows, and at least one wall between adjacent expansion chambers. There is an opening or orifice in the wall between adjacent expansion chambers. A flap which covers the orifice or the opening in the wall between adjacent expansion chambers. The flap opens in response to a pressurized surge of exhaust gas entering into the expansion chamber. The muffler also has an exhaust gas outlet for allowing exhaust gas to pass from the second chamber of the muffler. The exhaust system typically has a plurality of chambers attached to one another such that the exhaust gas travels along a tortuous path between the exhaust gas inlet and the exhaust gas outlet. The flap is pivotally attached to the wall between adjacent expansion chambers. The flap is also positioned so that it closes due to gravitational forces.