The paper discusses the application of maps of measured discharge coefficients for poppet valves, cylinder ports, and in-pipe throttles within a theoretical simulation of the unsteady gas flow through an internal combustion engine. The maps provided cover both inflow and outflow at the discontinuity being discussed and are displayed as contour maps of the discharge coefficient as some function of the geometrical flow area of that discontinuity and of the pressure ratio across it up to a maximum value of 2.0.An engine simulation package is used for both a four-stroke and a two-stroke engine to determine the typical pressure ratio and area ratio characteristics which pertain at all such discontinuities at representative engine speed and load conditions. From this information it becomes possible to ascertain the regions of the discharge coefficient maps which are indexed more frequently during the course of the engine simulation and which have a greater, or lesser, influence on the computational outcome, the engine gas flow rates, and, ultimately, on the actual performance characteristics of the simulated engine. Hence, it also informs the experimenter which regions of a contour map of discharge coefficients for a particular pipe end discontinuity must be measured with the greatest rigour.From the information presented it is clear that the traditional method of measuring discharge coefficients of valves, ports, etc., over a range of discontinuity area but at a single, low, pressure ratio is quite inadequate for the acquisition of data to accurately guide a theoretical engine simulation package. The evidence also shows that testing at a single pressure ratio can be potentially misleading even for the experimental development of the gas flow characteristics of valves, ports, etc., as a means of enhancing engine performance.