The emission and flow restriction characteristics of three different ceramic substrates with varying wall thickness and cell density (400 cpsi/6.5 mil, 600/4.3, and 600/3.5) are compared. These 106mm diameter substrates were catalyzed with similar amounts of washcoat and fabricated into catalytic converters having a total volume of 2.0 liters. A Pd/Rh catalyst technology was applied at a concentration of 6.65 g/l and a ratio of 20/1. Three sets of converters (two of each type) were aged for 100 hours on an engine dynamometer stand. After aging, the FTP performance of these converters were evaluated on an auto-driver FTP stand using a 2.4L, four-cylinder prototype engine and on a 2.4L, four-cylinder prototype vehicle. A third set of unaged converters was used for cold flow restriction measurements and vehicle acceleration tests.Independent FTP results on the auto-driver 2.4L engine and the 2.4L vehicle demonstrated that the 600 cpsi catalysts had significantly better emissions performance than the catalyst using the 400 cpsi substrate. In general, the high geometric surface area of the 600 cpsi substrates exhibited superior HC and NOx emission performance throughout all phases of the FTP. The bulk density of the substrates appeared to significantly affect NOx emissions during the cold start portion of the FTP. The results suggest that 600 cpsi substrates can be used to reduce catalyst volume while maintaining equivalent emissions performance to a 400 cpsi substrate.With respect to the cold flow restriction tests and the vehicle accelerations, the 400/6.5 product and the 600/3.5 product exhibited similar performance. The 600/4.3 product was more restrictive and negatively impacted vehicle acceleration.