The unsteady flow effects of a Close-Coupled Converter (CCC) are investigated. The aim is to improve the catalytic efficiency by uniformly utilizing the entire converter volume. Experimental and computational methods were used to evaluate the differences between unsteady and steady-state flow. Catalytic converters placed close to the engine have individual inflow pipes from each cylinder and suffer extreme filling and emptying processes. There is severe interaction of the pulsating exhaust flows. A wide range of engine operating points were studied at different engine speeds and load conditions. Also described is the influence of the gas pulsation on the conversion characteristics.