Abstract: A high performance muffler is used to modify sound waves of exhaust gases generated by a high performance internal combustion engine. The muffler has an inlet for connection to an internal combustion engine exhaust system. A muffler body is connected to the inlet. The muffler body has a shell which has a width greater than its height. The shell includes a pair of spaced apart opposed panels. An outlet is connected to the muffler body to receive exhaust gases from the body. The muffler body has a first expansion chamber adjacent to the inlet. The first expansion chamber extends across the width of the muffler body. A first reduced opening is in the muffler body extending across the width of the muffler body. A second expansion chamber is in the muffler body adjacent to the first reduced opening. The second expansion chamber also extends across the width of the muffler body. A second reduced opening extending across the width of the muffler body communicates with the second expansion chamber.