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. The simulation, on the other hand, not only computes accurately the exhaust pressure diagrams and the performance characteristics over the entire speed range but also explains the origins of the variations in these performance characteristics. This illustrates not only the effectiveness, or otherwise, of this type of exhaust tuning on four-stroke engines but also the effectiveness of an accurate engine simulation as a design and development tool. Finally, from the experimental and simulation evidence presented the paper draws numerical conclusions on empirical design factors for the basics of exhaust system tuning.