The resistance of air flow through poppet valves has been tested experimentally by the authors. The experiments which they carried out and describe include flow in both inlet and exhaust directions, and the ranges of valve opening and of pressure head extend to or beyond the limits which normally may be utilized in practice. Individual valves were tested variously mounted, and pairs of valves were tested as grouped in an actual design. The experiments have been extended in an effort to improve the flow characteristics of the poppet valve by modification of its shape and that of the valve approaches.