Camshaft Duration, Lift, and Overlap

Before taking camshaft lobe measurements, the camshaft must be carefully inspected for straightness, scoring, and galling. Camshaft lobes wear, and effect valve timing and operation. Camshaft duration, lift, and overlap are important factors that contribute to valve timing and ultimately engine performance. Take valve clearance measurements with the intake and exhaust valves completely closed. This is done with the cam contacting the lifter at the base or bottom of the camshaft lobe.

Camshaft lift and overlap

Duration: Duration refers to time; how long the valve stays open in relation to crankshaft rotation. It's measured in degrees of crankshaft rotation. If this measurement is out of specifications, the valve will be opened or closed at the wrong time in relation to piston stroke. This will definitely result in a loss of engine performance.

Lift: This measurement refers to how far the camshaft's lobe opens the valve into the cylinder. If the valve doesn't open far enough into the cylinder, there will be a restriction in the air/fuel charge. This too will result in poor engine performance. This measurement can be measured with a micrometer. Measure from the nose to the heel of the cam lobe, and then deduct the base circle. This results in an accurate measurement of cam lobe lift.

Overlap: As the engine cycles, there’s a time when the intake and exhaust valves remain open at the same time. This is a good thing and known as valve overlap. It's an important function in valve timing, because the exhaust gases flowing through the exhaust valve and manifold help draw a fresh charge through the intake valve and into the combustion chamber.