The U.S. Environmental Protection Agency has completed a program to demonstrate the feasibility of using integrated catalyst-muffler exhaust systems for nonroad spark ignition gasoline Class I engines (sub-19 kW, less than 225 cc). Integrated catalyst-muffler systems were developed for 4 different Class I engine families. Passive secondary air-injection systems were used with most of the systems to provide an exhaust feed-gas composition that was slightly rich of stoichiometry when used in conjunction with unmodified “Phase 2” carburetor A/F ratio calibrations. Catalyst sizing, PGM loading, and secondary-air venturi design were selected to limit CO oxidation and the typically resultant high heat rejection at high load operating points while still providing good NOx and HC emission control. Infrared thermal imaging was used to assess heat rejection at the EPA A-cycle operational points and during simulated hot soaks for selected configurations. Emissions of NOx+HC were reduced by approximately 35 to 65% at low hours and approximately 35 to 50% for configurations tested at near the end of the 125-hour regulatory useful life.