What is camshaft duration, and why is it so important?

Duration is the period of time, measured in degrees of crankshaft rotation, that a valve is open.  Duration (at .050" lifter rise) is the deciding factor to what the engine's basic RPM range will be.  Lower duration cams produce the power in the lower RPM range.  Larger duration cams operate at higher RPM, but you will lose bottom end power to gain top end power as the duration is increased.  (For each ten degree change in the duration at the .050", the power band moves up or down in RPM range by approximately 500 RPM.)

In order for duration to have any merit as a measurement for comparing camshaft size, the method for determining the duration must be the same.  There are two key components for measuring duration--the degrees of crankshaft rotation and at what point of the lifter rise the measurements were taken.  Advertised durations are not taken at any consistent point of lifter rise, so these numbers can vary greatly.  for this reason, advertised duration figures are NOT good for comparing cams.  Duration values expressed at .050" lifter rise state the exact point the measurement was taken.  These are the only duration figures that are consistent and can accurately be used to compare camshafts.

(Crane Cams Catalog part # 99193-98)

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