When a cam is ground, the opening/closing ramps are fairly gentle, in terms of valve acceleration, not much opening/closing action per degree of rotation until it takes up the running clearance and gets the initial motion going and it gets on the actual part of the lobe where there is real acceleration. The cam designer will call out a spec on this amount of valve lift (checking clearance) to do the initial setup. There is no universal standard for the checking clearance, I've seen as low as .015ish to over .050.
The checking clearance is vital to know when setting up a cam, short of lining up the dots!
HTH, let me know if not.
Glen
--Previous Message--
: I don't mean to sound dense so forgive
: the following question as it surely
: sounds like I don't know anything about
: what I am doing. In the A- and
: B-series world I am used to the cam
: timing is handled in a very different
: way.
:
: What exactly is "checking
: clearance"? I may be
: misunderstanding the term. As I
: understand it, it is the amount of lift
: on the cam at which angular crank
: measurements are recorded. In this
: case (based on Paul G's data) I should
: see the crank at 18 BTDC when the
: intake cam lobe is providing
: 0.010" of lift... and 58 ABDC when
: the closing intake lobe is providing
: 0.010" of lift.
:
: I do not have my Excel data with me at
: work. However, I do remember looking
: at this and found for 0.010" of
: lift the crank was at about 19 and 57
: degrees respectively which is close to
: Paul G's data and probably within
: measurement error for such low values
: of lift.
:
: With max lift occurring where it should
: be (110 ATDC and BTDC for intake and
: exhaust respectively) I'm not sure
: where the 4 degree advance on the
: overlap point is coming from. It
: suggests that the duration is not what
: it should be or that one or both lobes
: are worn or not symmetrical. But how
: would I know or measure it more
: accurately? I may try moving the
: indicators to valves 1 & 2 and
: repeating the measurements tonight.
:
: I also may put the old paint marked
: original pulleys and chain on this
: evening and look at where the crossover
: point is with those components. I
: doubt it will be much different but
: maybe I will gain a better
: understanding of what is going on.
:
: You said: "For the setup I would
: use opening/closing points, if they =
: 110 lobe center."
: Can you clarify what you mean?
:
: Doug L.
:
:
:
:
:
: --Previous Message--
: OK if you go by opening & closing
: points, at specified checking
: clearance, what is the LC?
: The number difference sounds like 4
: degrees advance in the grind. The
: difference between the lobe separation
: angle & lobe center is the
: advance/retard ground into the cam.
: For the setup I would use
: opening/closing points, if they = 110
: lobe center.
: Glen
:
: