1. ABSTRACTAutomobile exhaust system engineers have conflicting goals which must be achieved to meet customer demands for improved vehicle performance and quietness. They must reduce exhaust system pressure loss which improves volumetric efficiency of the engine and increases power output. At the same time they must reduce the discharge noise from the exhaust system to achieve desired noise levels for the vehicle.This study presents an analysis of the basic characteristics of discharge gas noise of internal combustion engines and presents solutions which reduces exhaust system pressure loss and reduces discharge noise level. The solution involves a technique utilizing a flow control valve to control discharge gas flow and control the propagation of sound waves toward the exhaust piping system. The effect is to muffle the subharmonic element in the low speed range of the engine. This element is resonant and becomes amplified in the piping system increasing noise level. The valve also reduces jet noise and pressure loss at the end of the tail pipe during high speed operation of the engine.