The effect of exhaust system configuration upon performance of two-stroke engines is explored. Computer predictions of gas dynamic behavior in the exhaust pipes are compared to experimental results of real pipe tests. Predicted pressure, velocity and temperature histories at key points in the exhaust system explain the relative power differences for the test pipes. Predicted volumetric flow rates show the effectiveness of exhausting gases at larger pipe cross sectional areas in reducing noise output.The application of the computer predictions to a Yamaha RS 100 offers an explanation for the experimentally measured loss of power at mid-range engine speeds encountered using one of the two test pipes. A relative comparison of the two systems shows that the differing pipe geometries cause predicted pressure and velocity histories to change. These changes are correlated to actual power curves.An experimental exhaust system is presented for the Yamaha 360 MX which increases power and is quieter than the stock system. Computer predictions are used to explain the increased performance.