Recent regulatory requirements have introduced, for the first time, catalyst exhaust systems with closed loop air/fuel control into the severe environment of stern-drive and inboard-powered pleasure marine vessels. These engines often maintain consistently high power levels due to vessel drag. Sea water used to cool the engine and exhaust is corrosive, and the engine experiences high g-loads when the planing vessel is used in wavy sea conditions. Engineers must face these challenges in order to develop a durable, efficient, clean-operating, and affordable marine engine.Computational fluid dynamics (CFD) has become a key tool to drive the design optimization of catalyst exhaust systems for marine applicatons. CFD models are used to simulate the unsteady exhaust gas flow of a fired engine. In particular, CFD is used to develop an exhaust system which will promote efficiency, low emissions, and robust closed-loop air fuel control. Increased gas residence time via catalyst flow uniformity and balanced cylinder flow streams at the oxygen sensors are required to achieve an optimal design.A recent study was conducted in order to establish a correlation between unsteady exhaust flow CFD and physical testing for catalyst flow uniformity and oxygen sensor placement. This was done with prototype marine catalyst exhaust systems running on an eight cylinder gasoline engine. The desire was to prove that the CFD method provides accurate design direction to the team responsible for optimizing the exhaust system. The strong agreement established in this paper provided the confidence necessary to employ CFD in the development of future marine catalyst exhaust systems.