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A Yamaha YZ400, 5-valve, 4-stroke cycle, motocross racing, motorcycle engine is instrumented to provide pressure diagrams in the exhaust system which are recorded over the usable engine speed range at full throttle from 5000 rpm to 11000 rpm. The engine produces a maximum of some 34 kW (46 hp) power output. The production muffler-ended exhaust system is replaced with two alternative and more highly-tuned exhaust systems, namely a straight pipe and a straight pipe and diffuser. The complete engine geometry together with these two exhaust systems is simulated using an engine simulation software package (VIRTUAL 4-STROKE) and the experimentally-recorded exhaust pressure diagrams and performance characteristics of power, torque, etc., are compared with the predictions of the theoretical simulation. The variation of exhaust system produces differing performance characteristics whose origins the measured pressure diagrams and recorded performance characteristics struggle to explain.