The role of the valve is critical at all lift values, but a key proportion is the LD ratio — or the lift-to-diameter ratio — where the lift value equals one-quarter of the valve’s diameter. Regardless of valve diameter, at this point the valve curtain area is exactly equal to the valve head area.

“Everything below this strongly influences low-lift flow,” says Urrutia, “which is an important function of overcoming fuel-mixture inertia.”


The accompanying chart indicates standard valve lengths for popular performance engines. Important dimensions include overall valve length, head diameter, margin height and tip length.
Above this point, the engine builder needs to compare the minimum port cross-sectional area (typically the throat area above the valve) to the valve curtain area. Somewhere around the mid-lift point, the valve curtain area becomes larger than the port area and the port itself becomes the restriction. This is called the saturation lift point. At every point in this equation we are stuck with the same fixed valve shape, seat angles and valve margin dimensions that influence air movement. Above the saturation point, the seat angles are still crucial to maintaining smooth transitional flow and providing a shear edge to help maintain good fuel atomization. The particulars of this are very specific to any given combustion chamber, according to its size, shape, depth, valve position, port texture approaching the valve and, to some degree, the influence of the rising piston crown.