A muffler (900, 1300, 1400, 1500, 1600) for an internal combustion engine exhaust system, the muffler comprising: a housing (902, 1302, 1402, 1502, 1602) having an outer shell (904, 1304, 1404, 1504, 1604) and inlet (906, 1306, 1406, 1506, 1606) and outlet (908, 1308, 1408, 1508, 1608) headers enclosing opposite ends of the shell; a partition (910, 1308, 1408, 1520, 1608) inside the housing dividing a housing interior into a first chamber (912, 1310) extending from the inlet header to the partition and a second chamber (914, 1314) extending from the partition to the outlet header; a through pipe (920, 1330, 1430, 1530, 1630) extending through the partition and having a plurality of perforations (942, 1332, 1432, 1532, 1632) enabling fluid communication between the through pipe and the first chamber, the portion of pipe extending through the second chamber being devoid of perforations; and a valve assembly (100, 100a, 100b) having a substantially planar valve flap positioned inside the through pipe for rotation with an axle pivotally coupled to the pipe, the valve flap being rotatable between a closed position and an open position whereat a plane of the valve flap is substantially parallel to a longitudinal axis of the through pipe, wherein the valve is positioned within the second chamber downstream of the pipe perforations to separate the first and second chambers from the pipe downstream of the valve when the valve is closed, wherein all of an exhaust gas flowing through the muffler passes through the valve at a gap or passageway between the valve flap and an inner surface of the through pipe regardless of the valve flap position. a housing (902, 1302, 1402, 1502, 1602) having an outer shell (904, 1304, 1404, 1504, 1604) and inlet (906, 1306, 1406, 1506, 1606) and outlet (908, 1308, 1408, 1508, 1608) headers enclosing opposite ends of the shell; a partition (910, 1308, 1408, 1520, 1608) inside the housing dividing a housing interior into a first chamber (912, 1310) extending from the inlet header to the partition and a second chamber (914, 1314) extending from the partition to the outlet header; a through pipe (920, 1330, 1430, 1530, 1630) extending through the partition and having a plurality of perforations (942, 1332, 1432, 1532, 1632) enabling fluid communication between the through pipe and the first chamber, the portion of pipe extending through the second chamber being devoid of perforations; and a valve assembly (100, 100a, 100b) having a substantially planar valve flap positioned inside the through pipe for rotation with an axle pivotally coupled to the pipe, the valve flap being rotatable between a closed position and an open position whereat a plane of the valve flap is substantially parallel to a longitudinal axis of the through pipe, wherein the valve is positioned within the second chamber downstream of the pipe perforations to separate the first and second chambers from the pipe downstream of the valve when the valve is closed, wherein all of an exhaust gas flowing through the muffler passes through the valve at a gap or passageway between the valve flap and an inner surface of the through pipe regardless of the valve flap position.