This paper describes a gas-dynamic model for the basic elements of exhaust mufflers in a spark ignition (SI) engine. Attention has been paid in particular to the modelling of the concentric tube resonator with a perforated pipe inside, since this element has been a problem for gas dynamic modelling (GDM) of mufflers. The model was coupled with a previously developed engine simulation code, ENGWA-1, for both acoustic and gas dynamic performance analysis of mufflers. Measured pressures in the positions of interest in a practical exhaust system and exhaust noise spectra from a tail pipe were compared with simulations for validating the model. Results show that the model can satisfactorily predict measurements. Moreover, how to use the computer code for muffler design has been discussed for the point of engineering application.