Cam Duration

Cam durations are expressed as the number of crank rotational degrees that the valve face is a particular distance from its seat. Motorcycle cams are usually reckoned at .040" (1mm) lift; automotive at .050." In special cases, the cam producer may specify at some other lift; usually .020." When a valve is lifted the requisite distance, the crank rotation specifies the duration. Remamber that the crank turns twice as fast as the cam. Thus the cam _could_ have 360° duration and still feed the cylinders - albeit very poorly - without having "full circle" lobes. Factory manuals sometimes list close to 300° duration for cams that have quite a bit less. It appears that the mfg considers any lift at all "duration" even if it is due to the gradual accelleration ramps used to more gently open and drop closed the valves.

In general, thhe more duration a cam has the more fully the cylinders may fill and also purge exhaust gases. Long duration means that the intake and exhaust are both open at the same time - this makes for hard starting and low throttle lack of power, but it isn't a linear relationship. A motor may have a lot of duration, but wide LBA (lobe center separation) will mitigate this to a degree. Engines designed for forced aspiration may have a lot of duration, which seems like a scheme to push unburnt mixture straight out the exhaust, but with wide lobe separation there is not so much valve overlap to allow this to happen.

More to come . . .

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