A grind with the clearance at a lower lift value would measure more duration overall.
Most important is the acceleration of the ramp profile, the goal is to open and close as quickly as possible, w/o exceeding the limits of the valve train.
For a stock street engine, opening and closing should equal calculated LC, 18 + 180 + 58 = 256/2 = 128 - 18 = 110, and you should find the opening/closing points are correct at the .040 called out.
Glen
--Previous Message--
: OK Doug, good question, and a simple
: answer.
: 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
:
:
:
: