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Design of automotive catalytic converters is a complex process involving the optimization of many physical and chemical variables. Often, simple characteristics such as geometric surface area and space velocity are used to compare alternative designs. Unfortunately, these parameters do not account for all of the relative variables affecting emissions performance. The effectiveness-Ntu methodology, developed for heat exchanger design, can be extended to catalytic converter design through the heat and mass transfer analogy. This technique allows comparisons to include all physical variables affecting emissions performance. This paper presents Ntu-based, catalyst performance criteria for steady state and transient emissions and exhaust flow restriction. The theoretical analysis includes the effects of washcoat on cell geometry and, subsequently, on heat/mass-transfer performance.