Stringent emission regulations around the world necessitate the use of high-efficiency catalytic converters in vehicle exhaust systems. Therefore, determining the optimum geometry of the honeycomb monolith structure is necessary. This structure requires a high surface area for treating gases while maintaining a low pressure drop in the engine. In this book, an adapted sub-grid scale modeling is used to predict the pressure loss of square- and hexagonal-cell-shaped honeycomb monoliths. This sub-grid scale modeling represents the actual variations in the pressure drop between the inlet and outlet for various combinations of wall thickness and cell density. A comparison is made between the experimental and numerical results established in literature. The present approach is found to provide better and more comprehensive results than the single channel technique.