This paper deals with some recent advances in the field of 1D fluid dynamic modeling of unsteady reacting flows in complex s.i. engine pipe-systems, involving a catalytic converter. In particular, a numerical simulation code has been developed to allow the simulation of chemical reactions occurring in the catalyst, in order to predict the chemical specie concentration in the exhaust gas from the cylinder to the tailpipe outlet, passing through the catalytic converter. The composition of the exhaust gas, discharged by the cylinder and then flowing towards the converter, is calculated by means of a thermodynamic two-zone combustion model, including emission sub-models. The catalytic converter can be simulated by means of a 1D fluid dynamic and chemical approach, considering the laminar flow in each tiny channel of the substrate. The main chemical reactions (oxidation of CO, C3H6, C3H8, H2 and reduction of NO) occurring in the solid phase, within the wash-coat, are taken into account in the model. The predicted reaction rates are used to determine the specie source terms to be included in the one-dimensional fluid dynamic conservation equations, to allow for the modeling of chemical specie transport with reactions in the unsteady flow. The heat released by the exothermic reactions is treated as a source term in the heat transfer equations, to calculate the thermal transient of the catalyst and wall and gas temperatures.