What's Wrong with 0.050 Duration?

Cam timing and duration are the most important camshaft parameters.  For this reason a need for a standard method of measurement was recognized long ago.  Some cam makers were using 0.004 lift, others used different numbers.  What originated as seat-to-seat duration is now known as "Advertised Duration" , i.e. you really don't know how it was measured.  Besides, it makes more sense to use numbers at a higher lift, where the valve is open enough to have significant flow.  For these reasons 0.050 lift is the chosen standard for comparison of duration.  That is not to say that seat-to-seat duration is not important, but that the duration at 0.050 lift is the best single measure of camshaft timing.

Specifying duration at 0.050 cam lift is a good standard for hydraulic lifter cams, but not for solid lifters.  The problem lies in the variation of valve clearance used.  The valve clearance typically varies from 0.010 to 0.030 inches.  For example, suppose you are comparing two cams, one has a duration of 250 degrees and the other has a duration of 260 degrees, both measured at 0.050 gross lift.  However, the first uses a lash of 0.010 (measured at the cam), while the second uses a lash of 0.025.  Which cam has the longer duration in terms of the actual or net valve lift?  There is no way of knowing without some additional information.  For solid lifter cams I propose a standard of measuring duration at 0.040 net lift.  Where 0.040 net lift is equivalent to a gross lift of 0.040 + (valve lash)/(rocker ratio).  With this definition, 040 net lift is the same as 0.050 gross lift for 0.010 lash and a 1:1 rocker ratio, and for 0.015 lash and a 1:1.5 rocker ratio.  This standard would remove the variation of valve lash from the definition.

Tip: If you have an estimate of the velocity (in/cam deg) at 0.050 lift, you can estimate the change in duration with lift from the equation:

Δ(duration) = 4 Δ(lift)/(velocity)

For the example above, the duration at 0.040 net for the first cam is 250 degrees and if the velocity is approximately 0.005 in/deg, the duration for the second cam at 0.040 net would be approximately:

260 - 4(0.015)/(0.005) = 248 degrees at 0.040 net

With some experience, you will know velocity close enough to give you a more valid comparison.  For the above example, a 25% error in velocity changes the result by only 3 degrees, which is not of much significance.  Basically, the two cams have similar duration at 0.040 net.