How to Understand Camshaft Specs

by John Stevens J.D.

The purpose of a camshaft is to open the intake valves to allow the air/fuel mixture to enter the cylinders and to open the exhaust valves to allow the exhaust gases to exit the cylinders after the air/fuel mixture has ignited. The majority of aftermarket camshafts feature specifications different from the stock camshafts they are designed to replace. These camshafts are either designed to increase fuel mileage or increase power. Camshaft manufacturers provide several different specifications for each camshaft and assume that all consumers will know what these specifications mean. Fortunately, understanding these specifications is fairly straightforward.

Locate the "duration" figure on the camshaft manufacturer's specifications sheet. Duration refers to the length of time the camshaft will open the specified valve. Camshaft manufacturers sometimes expressly state the given duration for the intake valve and for the exhaust valve. However, the majority of the time the manufacturer will instead provide a shorthand notation in the form of XXX/YYY, such as 262/272, for example. The first digit, 262, represents the amount of time measured in degrees that the intake valve will remain open. The second number, 272, represents the amount of time measured in degrees that the exhaust valve will remain open. Generally speaking, the greater the duration, the more horsepower the engine will produce. However, as duration increases, low-end torque will decrease.

Determine the "lift" figure on the camshaft manufacturer's specifications sheet. As its name suggests, lift refers to the distance that the valve will be pushed out of its seat by the camshaft. As with duration, sometime manufacturers will expressly note the lift specifications for both the intake and exhaust valves, but more often will instead provide a shorthand notation in the form of AAA/BBB, such as .420/.442. The first number, .420, indicates that the intake valve will be lifted a total of .420 of an inch off of its seat. The second number, .442, notes the distance for the exhaust valve.

Locate the "lobe separation" or "overlap" figure on the camshaft manufacturer's specifications sheet. The terms "lobe separation" and "overlap" represent the same concept. The choice of terms depends on the manufacturer although lobe separation is more commonly used. Lobe separation refers to the amount of time, measured in crankshaft degrees, that both the intake and exhaust manifold valve are open simultaneously. For example, a lobe separation of 112 means that both valves are open for 112 degrees simultaneously. Generally, the wider the lobe separation angle, the more top-end power the engine will produce. The tighter the lobe separation, the more low-end power the engine will produce.

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About the Author

John Stevens has been a writer for various websites since 2008. He holds an Associate of Science in administration of justice from Riverside Community College, a Bachelor of Arts in criminal justice from California State University, San Bernardino, and a Juris Doctor from Whittier Law School. Stevens is a lawyer and licensed real-estate broker.