Performance improvements, of catalytic converters, and longer service life can be achieved by improving the flow distribution of exhaust gases. Computational Fluid Dynamics (CFD) is an excellent and relatively inexpensive technique for rapid and efficient optimization of the flow. Studies indicate that a 3D representation is necessary because 2D is insufficient. The computations are confirmed by measurements of steady flow. A number of design parameters are systematically investigated and their effects on an index of flow uniformity established. The parameters include the geometry of the inlet tube and inlet cone, the geometry and placement of the monoliths, and the shape of the exit cone. The difficult flow conditions in close coupled converters are examined. The flow path is improved and the best location of the HEGO-sensor found.