Emission engineers are currently evaluating several catalyst substrate designs, such as monoliths and beads, for use in automotive converters. One of their problems is to design converters that provide adequate control of the engine exhaust emissions but that do not significantly increase back pressure. In this paper, the design tradeoffs between pressure drop and conversion efficiency are examined for monolith, bead, Raschig ring, and wire mesh catalyst substrates. It is shown that monolith substrates provide the best flexibility when designing converters to satisfy vehicle installation space and engine back pressure requirements. Bead substrates have higher potential conversion efficiencies than monoliths but are less adaptable to vehicle space requirements.