This paper describes the experimental study of a tumble-flap mounted in the intake port of a single-cylinder spark-ignited gasoline engine. The research question addressed was whether an optimal tumble level could be found for the combustion system under investigation. Indicated fuel consumption was measured for a number of part-load operating points with the tumble-flap either open or closed. The experimental results were subjected to an energy balance analysis to understand which portion of the fuel energy was converted to work and how much was lost by incomplete combustion, heat losses to walls and to the exhaust gases, as well as to pumping losses.Closing the tumble-flap resulted in reduced fuel consumption only in a small area of the operating map: only at low-speed, low-load operation, a benefit could be obtained. One way of achieving this was by means of de-throttling due to increased valve overlap, since the combustion system could tolerate a higher level of internal exhaust gas recirculation (EGR) when the tumble-flap was closed. Improved combustion efficiency was another way of achieving lower fuel consumption.For higher engine speeds and loads, fuel consumption actually deteriorated for closed tumble-flap operation, indicating that an optimal tumble level exists and that it was reached in this study. The reason for the worse fuel consumption was that the intake ports in the baseline combustion system already set up such a strong charge motion that a further increase of turbulence (by closing the tumble-flap) no longer contributed to improved conversion of fuel into products or a higher tolerance to internal EGR. On the contrary, it was found that restricting the intake port resulted in increased pumping losses. A tendency of increased wall heat losses for the highest tumble levels could also be observed. Combustion duration generally became shorter for higher levels of in-cylinder turbulence, but for some operating points with extreme tumble level combustion speed actually decreased.