The problem of defining mathematical descriptions for instantaneous discharge coefficients of valves and ports for turbocharged engine has been solved. The developed procedure for identification is based on the simultaneous consideration of the mathematical model of the gas-exchange processes and the experimental ‘pressure-crank angle’ histories for the same process. As a result the instantaneous discharge coefficients can be defined by the valve / port geometry, valve lift and pressure ratio. An actual problem for a two-stroke uniflow-scavenged turbocharged engine has been solved. The obtained mathematical descriptions for the discharge coefficient can be used for other engines.