Abstract: Disclosed herein is an exhaust control system of an internal combustion engine. The exhaust control system comprises an electrically controlled valve installed in one of two tail pipes of a muffler; an engine speed sensor; an exhaust gas temperature sensor; a first control circuit for causing the valve to assume selectively a pipe closing position when the engine speed is lower than a reference level, and a pipe opening position when the engine speed is equal to or higher than the reference level; and a second control circuit for varying the reference level in accordance with the temperature of the exhaust gas.

Abstract: An exhaust control system for use in an internal combustion engine for controlling the flow of exhaust gases discharged from the engine to the atmosphere. The apparatus determines a first condition representing the engine speed being higher than a predetermined value, a second condition representing the transmission being not in neutral, and a third condition representing the throttle valve being not at or near its closed position. A control circuit generates a command signal only upon simultaneous occurrence of the first, second and third conditions. The apparatus includes a device responsive to the command signal from the control circuit for increasing the effective area of the exhaust passage.