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In order to study alternate methods of Air Fuel ratio (A/F) perturbation for maximizing three-way catalyst conversion efficiency, two methods for measuring the Oxygen Storage Capacity (OSC) of Catalyst were developed on an engine test bench. The first is to measure just the break-through Perturbing Oxygen Quantity (POQ, which is defined as the product of A/F amplitude, perturbation period and gas flow), and the second is to measure the response delay of the rear A/F sensor, which has been improved to be very similar to the former. Then, the OSC values of many catalysts were investigated with different perturbation parameters. The results show that OSC would not be affected by amplitude, period of perturbation and gas flow, and that the best conversion efficiency is obtained when the value of POQ is about 1/2 of the value for OSC. These results suggest that the best way to control perturbation is to keep POQ at 1/2 of OSC by setting perturbation parameters.