Ceramic monolithic catalyst supports have been an integral part of automotive emissions control systems since the early 70's. This investigation examines the impact of physical (cell density, frontal area, volume) and material (porosity, thermal mass) design parameters on vehicle emissions and pressure drop. This study indicates that CO and HC emissions can be reduced by larger volume and/or higher cell density substrates. Materials changes have little or no impact on catalyst performance. Pressure drop is increased by using a longer substrate, and dramatically reduced with a larger frontal area part.