Originally Posted by ap72
This may or may not be correct depending on your application. If you adjust the valvetrain to give you minimum scrub this is actually incorrect. The roller tip will be furthest to the outside on the valvestem tip during MIDLIFT and will be the furtherest on the inside during zero and peak lift. That is the whole idea behind the midlift method- minimizing the scrub and side loading.
You can do it the way depicted in the picture though, and you usually end up with something like this if you adjust your rocker arms to have a centered wipe pattern rather than the smallest wipe pattern.
There are arguments on both sides, I follow the method of least scrub.
The illustration from Comp Cams is not only the WRONG thing you want, it is the worst thing you want. It is the anti-mid-lift scenario. The correct "symptom" that would be illustrated in the valve motion atop the valve tip would be A to B then back to A, there is no C. The roller sweeps out to its farthest point at precise "net" valve MID-LIFT, then rolls BACK to the inside of the valve as it reaches maximum net Valve Lift.
Secondly, any reference to the "middle of the valve" for setting, seeking, or designing is NOT correct, and it is not why MID-LIFT sets up the way it does. I have said it a million times, "where the roller sits atop the valve means nothing, providing it stays atop the valve." Because of the myriad of dimension variables involved, setting mid-lift geometry correctly, AND trying to get the roller in the middle of the valve is nearly impossible. It doesn't matter anyway, PROVIDING you have the rocker set at precise mid-lift.
The whole purpose of MID-LIFT is for CAM timing, NOT reduced wear on the guides, or valves, or minimum sweep atop the valve, or reduced drag coefficients in the rotating valve train. But these are all the "symptoms" that are beneficial, because you get ALL these when you set for mid-lift geometry. The main purpose, is to convert as much of the CAM'S information accurately, through the radial arc process with the least amount of radial converion loss. IT "standardizes" all cam testing data. Without setting the rocker geometry accurately, to mid-lift, you are wasting SEVERAL degress of cam information in the excessive motion of an overarcing pushrod and roller tip
Please NOTE that I said "pushrod" AND "roller tip." All your theories for setting rocker geometry on the valve tip, by measuring the sweep, getting it as least as possible, and making a variety of different little tools to measure this, do NOT have anything to do with what is happening on the other side of the rocker, and the MOST IMPORTANT side: the pushrod cup. This is where all the trigonometry is taking place. THIS is where ALL THREE of the cam's dynamics are either being fully "captured" for translation through the radial process, or LOST -- and the element that determines this is a 90 degree tangent point in the pushrod ball AXIS at precisely "MID-CAM-LIFT".
So when you are setting the valve side with a Comp Cam rocker, or a Jesel, or a (gag) Scorpion rocker, you're NOT setting the pushrod side TOO. You're only setting the valve, and the more important side is still over-arcing, because the position of all the other rockers, compared to the Patented mid-lift design, are TOO high in the body, so the pushrod has already passed through its mid-lift, and it is overarcing trememdously, and the pushrod is moving in out about .050" or .100" -- when it should be only moving about .013" in and out, so its harmonics are like a BASS guitar's E-string, and you've just lost about 6, 8, 10, 12 or more degrees of your camshaft THROUGHOUT the entire valve lift cycle. So that 265 degree cam (@ .050) is now only 255 degrees.
The difference can be felt and seen with an old fashioned needle torque wrench. An engine of bad rocker geometry will typically take 15 to 40 lbs more torque to turn over (without spark plugs of course), than a MID-LIFT system, properly installed. I want to emphasize this last point "properly installed." If you are off by as much as .020" of an inch (1/2 of 1 turn on an adjusting screw) you will lose 2 or 3 degrees of your camshaft. God only knows how many engines are running around, blindly happy because they don't throw their parts out the headers (yet), with .050", or .100" or .150" too long a pushrod (or worse), and stand heights all over the place.
Cam information! Getting it all through the system to the valve, with the least amount of wasted motion, so that a "standard" was set that would measure EACH cam tested without having its differences be diluted by a rocker arm stealing the information.
THAT is why I designed mid-lift. Hope this clears up some of the rhetoric. Good luck! --JM