This paper presents a single-cycle gas simulation of the scavenging process in a two-stroke cycle engine. The apparatus used is described in the most detailed fashion and the experimental procedure is covered completely. On the apparatus is placed some eleven differing cylinders of a Yamaha 250 motorcycle engine and the scavenging efficiency - scavenge ratio characteristics of each determined experimentally. The results of these experiments are compared with the known performance characteristics of the same eleven cylinders which were obtained under firing conditions for variations of power, torque, air-flow, fuel consumption and scavenging efficiency at several speeds and throttle positions. The correlation, between the ranking of the several cylinders determined on the scavenging simulation apparatus with the performance characteristics obtained under firing conditions, is very good. Further tests on the gas simulation apparatus show that the test method is accurate and repeatable, and that there is a strong correlation between the magnitude and spread of the values of scavenging efficiency measured under both simulation and firing situations. The use of carbon dioxide and air as the simulation gases gives the correct density ratio for the simulation of the real scavenge process and the Reynolds numbers for the flow are always in the turbulent region. The design of the apparatus is particularly suitable for the improvement of the scavenging process during prototype engine development and also for the future need to correlate the predictions emanating from computational fluid dynamics methods with real experimental data.