Measurements of the temporal evolution of the 3-D velocity field were performed in an IC engine during the latter part of the intake stroke using a Water Analog Engine Simulation Rig and the 3-D Particle Tracking Velocimetry technique (3-D PTV). The engine head tested was a typical 4 valve, pent-roof type combustion chamber shape with slightly asymmetric intake passages to favor a preferred swirl with one intake valve almost deactivated to reinforce the swirling flow pattern. This study was aimed at characterizing the dynamic development of the flow field resulting from this head geometry and asymmetric valve event during the latter part of the intake stroke.The most salient feature of this flow field is that this final, highly organized and energetic vortex does not emerge until relatively late in the intake stroke. Even as late at 60° BBDC, the flow field is still characterized by smaller (of the order of 1/4 or 1/3 of the bore size) structures, particularly in the tumble plane. As a matter of fact, some of these smaller structures are counter-rotating with respect to the ultimate tumble pattern. They most likely correspond to remnants of the vortices formed downstream of the intake active valve by the incoming flow. Furthermore, as the swirling flow organizes itself into a single rotating entity, its axis precesses significantly in the cylinder. At the same time, the vertical extent of the swirling motion increases significantly during the latter part of the intake stroke to finally encompass the entire cylinder.In summary, this flow field is characterized by a very organized and energetic large scale tipped swirl at BDC. However, the highly dynamic events leading to its formation provide a rich source of complex 3-D mixing, without dissipating too much momentum through destructive interactions. The combination of these two traits (highly organized BDC flow pattern and strong mixing) contribute to very desirable flow characteristics to enhance the combustion process later on and certainly contribute to the good lean burn characteristics of this engine head.