What Is a Valve Float? Definition, Meaning and Concept

When an internal combustion engine experiences valve float, the lifters do not follow the camshaft profile correctly. This can be caused by many factors, the most common being poor or weak valve springs. During valve float, the valves in the cylinder heads do not close completely and cause the engine to misfire and lose power. Valve float can cause piston, valve and camshaft damage. Extended periods of valve float can lead to catastrophic engine failure, including an engine that explodes.

The camshaft has been called the brain of an internal combustion engine, and the specific grind or profile of the camshaft determines the power output of the engine as well as the power curve. When the camshaft rotates inside the engine, the lifters ride on the cam lobes and use pushrods to operate the rocker arms. The rocker arms push on the valve stem tips and open and close the valves to allow an air/fuel mixture to enter the combustion chamber and let exhaust out. The powerful springs attached to the valve stems are the only method of keeping the valves closed.

There are two styles of valve lifters commonly used on an internal combustion engine: hydraulic and mechanical lifters, or solid lifters. There are three basic types of lifters used in the combustion engine: flat tappet, mushroom tappet, and flat tappet roller lifters. The roller is the most used. At high engine speeds, the lifters can begin to bounce off the camshaft lobes due to failure of the valve spring to close the valve. This is valve float.

Virtually non-existent in everyday street use vehicles, valve float is typically experienced in high speed racing engines where sustained engine speeds can be close to the engine's rev limits. Many engine manufacturers use a rev kit to remove valve float in the engine. This is nothing more than extra springs and a plate mounted under the cylinder heads that apply extra pressure to the lifters to keep them pressed towards the camshaft lobes. The downside of the rev kit is that it requires several horsepower to compress the extra springs and robs the engine of critical horsepower.

The use of stronger valve springs is a typical method of dealing with valve float. Multiple springs are often used to provide the spring resistance required to maintain contact with the lifters and camshaft. Advanced computer designed camshaft profiles are also used to create an easier to follow camshaft lobe profile that helps eliminate valve float.