An approach has been developed for the design and optimization of combined catalytic converter - mufflers for low emission diesel engines. The development of this optimizing process considered the effects of the substrate and it's properties, the flow distribution through the substrate, the total system backpressure, and it's acoustical behavior. The actual catalyst formulation is not considered in this optimization process, as this development approach focuses on other converter muffler performance issues. For a given engine and vehicle combination this design approach has fostered the determination of the appropriate substrate size, the correct level of flow uniformity, and sufficient exhaust noise attenuation, all within the appropriate back pressure limit. The resulting optimization process has proved to be a valuable tool in the development of converter mufflers for diesel applications.