A theoretical and experimental study of three-dimensional steady and unsteady compressible non-reacting flow inside double flow of monolith catalytic converter system attatched to 6-cylinder engine with two Y type junctions at inlet exhaust pipes was performed for the achievement of performance improvements, reduction of light-off time, and longer service life by improving the flow distribution of pulsating exhaust gases. To obtain the boundary conditions to CFD analysis, one dimensional non-steady gas dynamic calculation was also performed by using the method of characteristics in intake and exhaust system. Studies indicate that unsteady representation is necessary because pulsation of gas velocity may affect gas flow uniformity in the monolith. Also simulation results show that the level of flow maldistribution in the monolith heavily depends on curvature and angle of separation streamline of mixing pipe which homogenise the exhaust gas from individual cylinders. It's also found that on dual flow converter systems, length of mixing pipe and junction geometry influence greatly on the degree of flow distribution.