This paper describes methodologies developed to reduce the noise radiated from silencers while keeping or improving engine performance; i.e. whilst minimising the pressure loss through the silencer. This work was successfully developed to help motorcycle designers to meet the current legal noise limitations with minimum detriment to engine performance. Innovative designs for reactive silencers have resulted, with improved acoustic and engine performance levels, compared with traditional systems.The methodology was developed using the boundary element method (BEM). This method is able to calculate the acoustic variables in the interior and exterior fluid of the silencer. Furthermore, it is possible to simulate the temperature gradient along the silencer. A further important characteristic of the method is the ability to simulate the proper boundary conditions at the outlet without the need of hand calculations or “guesses”.