To conduct system level studies on internal combustionengines reduced order models are required in order tokeep the computational load below reasonable limits.By its nature a reduced order model is a simplification of realityand may introduce modeling errors. However what is of interestis the size of the error and if it is possible to reduce the errorby some method. A popular system level study is gas exchangeand in this paper the focus is on the exhaust valve. Generallythe valve is modeled as an ideal nozzle where the flow lossesare captured by reducing the flow area. As the valve movesslowly compared to the flow the process is assumed to be quasisteady,i.e. interpolation between steady-flow measurementscan be used to describe the dynamic process duringvalve opening. These measurements are generally done at lowpressure drops, as the influence of pressure ratio is assumed tobe negligible. As it is very difficult to measure time-resolvedmass flow it is hard to test validity of these modeling assumptions.Experimental data indicates that the model overestimatesvalve flow during the blowdown event. As the blowdown pulsecontains a significant portion of the energy in the cylinder atexhaust valve opening, it is therefore of importance to modelthis correctly. In this paper experimental results from previouslypublished research have been compared to simulationresults and the deviation from quasi-steady behavior has beenquantified. The deviation appears to be a function of pressureratio over the valve and valve opening speed. A model isproposed to compensate for the observed effects.