Previous papers from The Queen's University of Belfast have shown the application of digital computers in simulating the unsteady gas flow and thermodynamic processes in single cylinder engines having various types of exhaust systems. This paper outlines the results of an investigation of twin cylinder engines of the outboard motor type where compact and complex exhaust systems are used to optimise the performance characteristics within a specified package size. Measured and predicted pressure-time histories for the exhaust and open cycle cylinder are presented for a 350cm3, twin cylinder test engine, which has been extensively modified to emulate the porting configuration and performance characteristics of two production outboard motor engines. Also compared are the measured and predicted output performance data for the engines, where all of the predicted data is produced by a new twin cylinder simulation program incorporating a simple constant pressure junction model. The propagation and reflection of the exhaust pressure waves, in the complex exhaust system considered, are also extensively described with the aid of pressure-distance diagrams shown at discrete crankangle positions for a particular engine speed (5000 rev/min).