Camshafts - Lift & Duration

Camshafts

Camshafts, or cams as it is commonly called, is the part that determines how much, when and how long the valves should be open. In other words, these determine the character of the engine. By changing cams, you can make an engine behave completely differently. You can move the power band to an area that suits the purpose of the car. The main difference between the cams is Duration (How long the cam keeps the valve open) and Lift (How much it opens the valve). Both of these play a big role in the character of the engine.

Understanding Lift and Duration and how it affects the engine stroke

Lift: Lift is how much the camshaft opens the valve at the highest point.

How do you measure lift?
Lift is most easily measured by measuring the total height of the cam and the base circle of the cam. The lift is then the height of the Cam minus the Base circle.

How does different lift affect the character?
Increasing the lift without affecting the Duration would maintain the previous power band but increase the effect throughout. However, this is basically impossible as the shape of the cam needs to be changed to be able to get a higher lift, which means that the duration would automatically increase as well.

Duration: Duration is the angle of the cam that causes the valve not to touch the valve seat.

How to measure Duration? One way to measure Duration is to turn the cam in a peak and have an indicator dial on the valve and a protractor on the camshaft.

How does Duration affect the character? An increased Duration gives the engine more power at high revs, but does so at the expense of the low range. In other words, it's always a compromise, you simply move the power band. Car manufacturers are always looking for the best compromise for drivability, which means they often settle on a cam that provides plenty of torque in the low and mid range. However, this means that there is a lot to be gained at top revs with a different camshaft, in other words there is a lot of power to be gained here.