This paper details results from a research project to study the influence of exhaust port pressure on the gas exchange process in a reciprocating four-stroke engine. To achieve optimum cylinder charging, it was observed that during the exhaust event, the phasing of compression and rarefaction waves in the exhaust port was of critical importance. Discrete periods during the exhaust event were identified within which compression and expansion pressure waves were best phased to enhance cylinder breathing. The pressure wave resonance characteristics of exhaust manifold components were investigated and a method of identifying the most significant pressure wave components arriving in the exhaust port was developed. The novel application of an acoustic model is shown to give rapid identification of the resonance characteristics of manifold pipes. The findings from these studies are used to derive exhaust system design guidelines that define the optimum exhaust system geometry to give effective scavenging and good cylinder charging.