Mid lift provides the least amount of scrub. Low point (2/3 lift) gives you the least amount of scrub under load. That is, the scrub occurs while the load is low and then when the load is high there is very little scrubbing action.
I think most of the serious valve train designers migrated to the low pivot point design a number of years back. The math is fairly complicated but if you put the rocker arm perpendicular to the valve stem at 2/3 lift you're in the ballpark. The scrub pattern is larger than with the mid-lift design, but the sideways force applied to the valve goes down.
I can understand the theory behind the 2/3 concept, but only at very high lifts and very high spring pressures. Even then, I find it hard to believe that the side loads would be substantially reduced. You cannot change the fact that the highest acceleration of the valve is when the rocker is 90 degrees (perpendicular) to the valve. That means at 2/3 lift, the valve only has 1/3 left to decelerate before the closing sequence starts instead of half of the lift. That will require more spring to control the valve and subsequently increase the load, both at lower lifts and higher lifts. With the added sweep at lower lifts and increased spring loads, at best I would think the scrub loads would be a wash between the two, but at the expense of valve control and spring wear with the 2/3 lift concept. Also, I believe the 2/3 idea was started long before the first roller rocker was designed, and was intended for non-roller type rockers with heavy spring loads because the scrubbing action was far more dramatic with that type of rocker. But then, I'm not really serious about valvetrains.
Mike Beachel
I didn't write the rules of math nor create the laws of physics, I am just bound by them.
