Experience in getting good results is a pretty sound basis for one's opinions; sharing those opinions freely is especially valuable and widely appreciated. Why one's methods work well can be interesting to figure out.

It would appear that variations in lash clearance, of a valve on a solid lifter cam, will determine just where along the "clearance ramps" of the cam lobe that valve lift begins and ends. Lots of duration change for very little net lift occurs when the lifter is bearing on the clearance ramps.

Isky's website has a convenient illustration and discussion of clearance ramps here: http://www.iskycams.com/degreeing.php Scroll halfway down to the discussion associated with generic Figures 9 and 10. The clearance ramps are shown as occupying about the first .016" of lift (of the lifter) off the base circle, after which lift begins to rapidly increase. (The following ignores elastic deformations in the valve train, for simplicity.)

If lash clearance at the valve stem is .018" when the lifter is above the base circle, then 1.5 rocker arm ratio makes for .012" lash clearance at the lifter. That's 75% of the way up a .016" high clearance ramp, or about 24 degrees of cam rotation from beginning of the clearance ramp shown. That's also about 8 cam degrees before when lift begins to accelerate as the cam lobe flank first comes onto the lifter, meaning 8 cam degrees of extra opening duration at each end, with scant valve opening.

More lash clearance reduces that extra duration along the clearance ramps: .020" instead of .018" at the valve would reduce low-lift duration by perhaps 2.5 cam deg. at ea end, or 10 crank degrees duration total. This would reduce low rpm exhaust backflow at overlap, presumably improving low rpm engine behavior. It also gives more margin against loss of all clearance if unintended conditions happen.

That Isky discussion is not about why lash clearance is set at a particular amount, but one inference I draw from the illustrations (and from thinking it over) is that for street use the valve should begin lifting, and be set down, with the least necessary use of the cam lobe's clearance ramps. Isky's Fig 10 shows that condition when the engine is cold. Yet lots of ramp height is provided at the cam, which caters to safe engine operation when less lash occurs.

Charles Greenlaw, Sacramento