I obviously don't have the book you have listing timing numbers. Is that something I can access online?
You said the 0.040" is tappet rise measured off the base circle. I got that. You went on to say that it is the valve is considered "open" at that point. Are you saying that the duration of the cam would be measured from 0.040" lift opening to 0.040" lift closing? I must really be misunderstanding something then. Take a look again at my graph. (link this time instead of embedded graphic)
Doug's Chart
The vertical axis is lift off the tappet, the horizontal is crank degrees. For 0.040" lift on opening my crank is at TDC. For closing 0.040" is at about 220 degrees. That's a lot less than the 256 duration I have seen quoted for stock cams and the opening clearly is not 18 BTDC. This is with the cam set for intake peak at 110 ATDC.
If I were to set the timing based on the zero overlap at TDC it would still not be close to having 256 degrees of duration from 0.040" to 0.040" and the opening would still not start at 18 BTDC.
As I said, I am very confused. The old chain and pulleys go back on tonight for a sanity check.
Thanks for hanging in there with me!
Doug
--Previous Message--
: The .040 is out of the book that lists
: the cam timing numbers. It is also
: referred to as "tappet rise",
: simply that much off the base circle,
: it is the starting point that valve
: action is measured from. It is measured
: directly off the cam lobe. Set cam on
: base circle, set indicator on your
: checking tappet, zero it, rotate cam
: until checking clearance is achieved,
: .040 showing on the indicator. THAT is
: the point at which effective MEASURED
: valve motion begins, and ends as it's
: closing and it's .040 from zero on the
: indicator. The opening & closing
: ramps don't do much, there is a lot of
: cam rotation in degrees for slight
: valve lift as the valve assy starts
: accelerating. Once it gets going, it
: really gets moving and this is the
: point that matters.
: Disregard the rock check for now until
: we have the base timing correct.
: If we did a graph of it, we would see
: the inlet start to open when the tappet
: has risen .040 off the base circle, and
: we would be at 18 degrees BTDC.
: I'm not a very good explainer sometimes
: but we'll get it!
: g.
:
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