Computer simulations of internal combustion engine cycles utilize empirical information, such as the flow characteristics of valves. This paper presents comprehensive data on the flow area of piston controlled ports. Detailed results of systematic steady flow tests on both inlet and exhaust ports are presented. New aspects of the work are that it covers a wide range of pressure ratios for normal and reverse flow directions and that comparative results, with and without the restrictions of an exhaust belt, are included. The concept of effective flow area is briefly discussed and relevant equations given. Previous experimental work is reviewed.