I get that the lift will be higher along the path of the lobe. I just don't get the additional duration since the valve won't be starting or ending that lift any sooner or later. The higher rocker would only serve to change the shape of the lift curve and not the timing of it. Only way that it works to change the timing in my mind is if the point of measuring the lift and duration is off the base circle and somewhere along the travel.

 

OK, last try. That would be true IF duration was measured from zero lift. It is NOT. Duration is measured from a nominal VALVE lift (.053"). With an increased rocker ratio, upon opening the valve gets to that amount of lift SOONER, and upon closing, reaches that point LATER. Hence the change in duration/timing.

Stych out

 

Correct but again it is Lifter lift and not valve.. Technically cam timing (and lift) does not change but valve timing does.. (as does valve lift)

 

So then your basically saying that from 0-.053" its all valve train preload and actual valve lift does not occur until the valve train is preloaded which is at .053" but with a longer rocker the actual valve preload was satisfied @ or around .050 and that .003" is enough to make a 1-2* difference in the duration. I hope I'm getting this because it truly interests me. Not trying to be apita. New territory, Thanx.

 

First off you are not a PITA as you are asking a legitimate question..

You are basically correct except that your assumption (in bold) is not correct.. Typically the valve lash / preload / flex and lifter bleed is probably dome in the first 10 to 20 thou of lift on lifting the seat.. Maybe a little less.. It depends on stiffness of parts, mass and spring rate. This is in theory as I've not measured it. Take some pricey equipment and you can see some of it on you-tube.. Still the start ramp preloads train slowly up to about 0.020 lift, then accelerates the valve quickly as allowed by the design (seen and measured this). The whole reason for the 0.053 tappet lift check point is to get beyond the ramps and more on the slope of the cam as the needle on an indicator moves faster, the measurement can be more precise. On the opening side, you could easily make the measurement at a lower lift up as typically the dropping side needs more set down ramp to help take up the lash caused by the collapsing lifter.. Remember a lifter is nothing more then a lash limiting device that start to bleed down as soon as the pushrod starts to increase force on the lifter cup.