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What is a Valve Float?

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  • Written By: Lori Kilchermann
  • Edited By: Lauren Fritsky
  • Last Modified Date: 30 August 2016
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    Conjecture Corporation
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When an internal combustion engine experiences valve float, the lifters are not following the camshaft profile properly. This can be caused by many factors, with the most common being poor or weak valve springs. During valve float, the valves in the cylinder heads are not closing completely and cause the engine to misfire and lose power. Valve float can result in piston, valve and camshaft damage. Extended periods of valve float can lead to catastrophic engine failure, including a blown engine.

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. As the camshaft turns inside of the engine, the lifters ride on the cam lobes and use push rods 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 into the combustion chamber and let the exhaust out. Powerful springs attached to the valve stems are the only method of keeping the valves closed.

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

Virtually non-occurring in everyday street-driven vehicles, valve float is typically experienced in high-revving racing engines where sustained engine speeds can be near the rev limits of the engine. Many engine builders use a rev kit to eliminate valve float in the engine. This is nothing more than additional springs and a plate mounted beneath the cylinder heads that applies added pressure to the lifters to keep them pushed down onto 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 power.

Using stronger springs on the valves is a typical method of confronting valve float. Often multiple springs are used to give the required spring strength to maintain contact with the lifters and the camshaft. Advanced computer-designed camshaft profiles are also used to create an-easier-to-follow camshaft lobe profile that aids in the elimination of valve float.

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