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The two-stroke engine, by its nature is very dependent on the unsteady gas dynamics within an exhaust system. This is demonstrated by the tuning effects on two-stroke engines, which have been well documented. In consideration of current emissions legislation, a two-stroke engine can be fitted with a catalytic converter for the outboard, utility or automotive markets. The catalytic substrate represents a major obstruction to the flow of exhaust gas, which hinders the progression of the main exhausted pulse, and in turn effects the scavenging of the cylinder and ultimately the performance of the engine. Within this investigation, a 400 cc direct injection two-stroke engine was used with various catalysts positioned at different distances from the exhaust manifold. Comparison tests were performed between a fully lit off catalyst and a non-operational bare substrate.