High cell density ceramic and metallic substrates provide significant increases in substrate geometric surface area versus standard designs used in U.S. Tier 1 and earlier model light-duty vehicles. Larger substrate geometric surface area translates into more efficient contact between the exhaust gas constituents and active catalyst components displayed on the substrate channel walls. The result is more emission conversion efficiency per unit volume of substrate as cell densities are increased. Increasing the substrate channel density also results in smaller channel flow dimensions, which in turn improves mass transfer between the flowing exhaust gas and active catalyst sites on the walls of the substrate. High cell density substrate designs that utilize thinner ceramic or metallic walls separating flow channels further reduce the thermal mass of the substrate and facilitate heat-up. In this way, the overall mass of a given sized substrate is reduced relative to older designs with lower cell density and thicker wall dimensions. The resulting lower thermal mass is able to heat-up more quickly during critical start-up operations and contribute to improved performance during cold and warm-start driving modes, making these advanced high cell density substrates ideal for close-coupled converter applications.