Motorcycle Cams Are Housed Inside The Motor’s “Head”
Motorcycle cams are the same as any four-stroke motor cam. The job of the cam is to control when the intake and exhaust valves open and how much they open. Cam specifications are expressed in degrees: 0 to 360. The cam moves in relation to the crankshaft, which connects to the rods and pistons. A cam will open a valve at a certain point in the crankshaft’s rotation.
Motorcycle cam specifications indicate the point in the crankshaft’s rotation when the intake and exhaust valves begin to open and when they close. An example of how this is expressed is “274/289.” “274” specifies when the intake valve opens (at 274 degrees of crankshaft rotation in this case.) “289” species when the exhaust valve opens.
The cam profile continues to open the valve until it reaches the cam lobe. At this point, standard valves rely on valve springs to close the valves. The length of time the valve is open is called “duration” and is also expressed in degrees of crankshaft rotation.
In portions of the motor’s cycle, both intake and exhaust ports are open. Then length of time, expressed in degrees of crankshaft rotation is called “overlap.”
The cam profile also determines how far the valve is opened. The distance the valve opens is the “lift.” It is not and cannot be expressed as rotation of the camshaft, but distance: either a fraction of an inch or centimeter.
Some motors employ more than one cam and more than one intake or exhaust valve. If a motor has multiple intake or exhaust valves in the same cylinder, valves subsequent to the first valve will probably have a slightly different cam timing, allowing more design-control of the motors horsepower and torque characteristics.