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What is an Exhaust Valve?

Lori Kilchermann
Lori Kilchermann

An exhaust valve is found in the cylinder head of an internal combustion engine. When the fuel and air mixture has been ignited in the cylinder, the spent gasses are sent out of the engine through this valve. In the typical internal combustion engine, the exhaust valve is larger than the intake valve. This is due to the fact that it is more difficult to clear the cylinder of exhaust gasses than it is to introduce fuel and air into the combustion chamber.

A major feat in building horsepower and fuel mileage with the modern engine is being able to coordinate the exhaust valve opening and closing at the optimal time. The camshaft is at the heart of the situation and has its lobes ground to take full advantage of the valve's ability to clear the cylinder. By opening the exhaust valve at the critical time in the combustion process, the piston is able to push all of the cylinder's exhaust gasses out of the combustion chamber without sacrificing the intake charge's flow into the cylinder.

Man with hands on his hips
Man with hands on his hips

Several factors are involved in assisting the exhaust valve in its task. A well-prepared valve pocket in the cylinder head is crucial. Along with the exhaust valve, the valve pocket or bowl must be made free from restriction to allow the spent gasses to exit the combustion chamber speedily and without restriction. The exhaust port must also be free from obstruction, and gasket matching is a popular method of accomplishing this.

By grinding the exhaust port of the cylinder head and the exhaust manifold to the exact size as the opening in the exhaust gasket, there is no interruption in flow caused from the gasses making contact with the blunt edge of the gasket or the exhaust manifold. This smooth transition eliminates back pressure, which can hinder the flow of exhaust from the engine. By smoothing the radius of the exhaust bowl in the cylinder head, the flow is also aided by providing a smooth and unobstructed path out of the cylinder.

When grinding the valve seat on an exhaust valve, there can be as many as six different angles ground onto a single valve face in a high-performance application. Each angle corresponds with an existing angle ground into the valve seat in the cylinder head. The meshing of the angles not only provides an adequate seal when the valve is closed, but it supplies a smooth path around the valve's edge for the gasses to flow. The typical street-driven vehicle utilizes a three-angle valve grind, as the speed of the exhaust flow is not as important as that of a racing engine.

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Discussion Comments


@Soulfox -- And timing is even more critical when you start talking about multi valve engines. Having four valves per cylinder means the engine's performance and efficiency will go up considerably, but it also means that more complexity is involved.

With complexity comes problems, and it is not uncommon to see the timing go out of whack as an engine gets older.


That is precisely why timing is so important in an engine. If the timing chain is off just a hair, you can have all sorts of problems. When it comes to valves, the timing chain being off could mean that exhaust could be sucked back into the cylinders, for example.

When an engine is running rough, the timing chain may be the culprit. A good mechanic can adjust one quickly, but getting to the thing takes a lot of work.

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