Abstract: An exhaust tube having an internal butterfly valve is coupled to the exhaust end of a conventional exhaust pipe extending from an internal combustion engine. The valve plate is mounted for swinging about an axis offset from the axis of the exhaust tube. The valve plate is spring-biased to a normal position substantially obstructing the exhaust tube. At high exhaust flow conditions, the valve plate is automatically swung to an open position in which flow through the exhaust tube is essentially unrestricted. Under back pressure conditions, the valve plate swings oppositely but still constitutes a substantial obstruction to exhaust flow. Consequently, the valve automatically opens at high flow conditions and automatically closes at low flow and back pressure conditions for more efficient operation of the internal combustion engine.