>From an engine designers point of view, the duration of a cam is usually
specified from the valves point of view - the cam has a short 'ramp' section
(3-4 degrees, fraction of a millimeter) before the start of the proper cam
profile, this ramp section is not included in the duration. However, this
all applies to modern hydraulic tappets (or 'lash adjusters' as they are
sometimes called), which have very little free play indeed. I have not been
an engine designer long enough to have any idea what the approach was in the
days of mechanical tappets with their acres of clearance! Having said that,
the stock clearance (10 thou) is about 1/4 of a millimeter, which is
probably about the height of the ramp section of a modern cam profile (I
haven't got any specs to check since I moved to the oil industry a few
months ago!).
Right, that's probably enough confusing and not-very-helpful information for
now!
Richard & Daffy
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