A numerical model was developed for the transient heat transfer phenomena occurring in automotive exhaust systems. The model considers heat convection along the gas flow, convection between the gas and the pipe wall, conduction in the pipe wall, and radiation and convection to ambient. The model is capable of modeling pipes fabricated with layered materials or multiple walls with air gaps or insulation. The model correlates well with experimental data measured in a takedown pipe with cold start exhaust gas from a typical engine. Parametric studies provided an understanding of the effects of various pipe designs and materials on the temperature of the exhaust gas entering a downstream catalytic converter for a more efficient light-off response.