The design and development of high performance racing engines is greatly facilitated by the use of accurate simulation of their performance characteristics. The reverse is true if the simulation is inaccurate. This paper reports on certain aspects concerning the accuracy of engine modelling and simulation. The aspect considered is the coefficient of discharge of poppet valves in the engine cylinder head. It is required to know such discharge coefficients, as a function of lift and pressure ratio, so that, in the mathematical modelling process at each juncture during the open cycle, accuracy can be obtained for the mass flow rates of gases, the magnitude of pressure waves created, and the in-cylinder thermodynamics. The discharge coefficients, Cd, are required for both valves and for inflow and outflow at each valve and are measured for each set of valves in a four-valve pent-roof cylinder head of a 600 cm3 high performance motorcycle engine. They are measured in the traditional manner, by steady flow with air, and analysed to acquire both the ‘ideal’ and ‘actual’ discharge coefficients. The ‘ideal’ Cd is determined in the manner traditionally found in the literature, and is shown to be greatly in error in certain circumstances. This reinforces previous findings regarding ports in a 2-stroke engine. The effects of valve masking at the bore edge on the discharge coefficients are also investigated. The several measured Cd maps are inserted into the GPB engine simulation code for a 4 litre, V8, high performance automobile engine, where it is found that significant changes ensue in the prediction of performance characteristics.