Because there are no production-type sensors which are able to measure the flow directly at the intake port, it is becoming common practice to use models of varying complexity to infer the port air mass flow from other measurements. Given the tight requirements of modern air/fuel ratio (AFR) control strategies, the accuracy of these models needs to be better than ever, during steady-state of course (though λ feedback strategies are by design very robust), but mainly during transient operation. This paper describes why conventional models might be inaccurate during engine transients.By developing a simulation package (one-dimensional (1D) solution of the Navier and Stoke (N&S) equations), which can describe complex phenomena such as manifold acoustics (inertial ramming, resonance), heat-transfer, friction, backflow, in-cylinder process, moving valves and throttle, junctions, it is shown that current control models need to be modified if they are to be able to describe port air mass flow with the required accuracy during transient operation. In particular, it is discussed why one should pay more attention to pumping fluctuations, heat transfer and friction. An improved model suitable for control applications is finally proposed. Experimental data are also shown.