An exhaust system comprises at least one muffler, the back pressure generated by the muffler exponentially grows as the engine speed increases. Accordingly, fuel consumption and direct CO2 emissions are penalized due to the back pressure generated by the muffling body in order to reduce noise emissions. To obviate this, it has been suggested to construct an exhaust system with two differentiated paths according to the engine speed, so that at low speeds the exhaust gases follow a first high acoustic attenuation (high back pressure) path, while at high speeds (high exhaust gas pressure), the exhaust gases follow a second low acoustic attenuation (low back pressure) path. Simulation and experimental analysis will be presented.A control valve is provided to alternatively direct the exhaust gases along the desired path according to the engine speed. These control valves usually include an electric or electro-pneumatic actuator, but are heavy, large in size and expensive. A flow activated valve has been developed, where the valve baffle is maintained in the closing position with an elastic calibrated force. When the exhaust gas pressure exceeds a predetermined threshold value, was obtained a pneumatically originated force higher than the elastic force generated by the elastic body and the baffle moves to the opening position. An experimental sensitivity analysis will show the influence of the elastic body to the exhaust acoustic and backpressure performance.Because of low backpressure at high engine speed, was obtained a power increment (roller bench tests) and a powerful and sporty sound, accorded with the vehicle brand. Subjective evaluations correlated with psychoacoustics index analysis of the roller bench and road data will be presented.