The Exhaust Noise is one of the major noise pollutants. It is well-known that for higher noise reduction, the engine has to bear high back pressure. For a race car, back-pressure plays a major role in engine's performance characteristics.For a given condition of engine rpm & load, conventional muffler has a fixed value of back-pressure and noise attenuation. Better acceleration requires low back-pressure, but the exhaust noise should also be less than the required (Norm) value (110 dBA). This contradicting condition is achieved here by using a ‘Butterfly Valve’ in this novel exhaust muffler. The butterfly valve assumes 2 positions i.e. fully open & fully closed. When the valve is fully closed, the noise reduction will be higher, but the back-pressure will also shoot up. When open, noise reduction will be less and so the back-pressure. So, when better performance is required, the valve is opened and back-pressure is reduced.The muffler is designed for a 4 cylinder 600 cc engine. The sound transmission loss (STL), which is the measure of effectiveness of the muffler, is also verified experimentally. The reduction in the back pressure at 11000 engine rpm is around 40% with open valve, as compared closed valve condition. The increase in STL, with closed valve, is around 15∼20 dB higher when compared to open valve condition.