Flow fields generated during the intake stroke of a 4-stroke I.C. engine are studied experimentally using water analog simulation. The fluid is seeded by small flow tracer particles and imaged by two digital cameras at BDC. Using a 3-D Particle Tracking Velocimetry technique recently developed, the 3-D motion of these flow tracers is determined in a completely automated way using sophisticated image processing and PTV algorithms. The resulting 3-D velocity fields are ensemble averaged over a large number of successive cycles to determine the mean characteristics of the flow field as well as to estimate the turbulent fluctuations. This novel technique was applied to three different cylinder head configurations. Each configuration was run for conditions simulating idle operation two different ways: first with both inlet ports open and second with only the primary port open. As expected, the resulting flow fields were found to he very different, and the level of detail provided by these experiments went far beyond the conventional swirl and tumble ratio estimates usually obtained with steady flow rigs. This new approach yields valuable insight into the complex fluid mechanics of the intake stroke and the effect of geometry changes on the process. The results of this investigation were successfully correlated with combustion data available for the simulated conditions.