Valve train geo. need opinions

cfm: The whole point of the discussion is to determine if you really should be concerned with the pattern being slightly off center. If you are set up for true mid-lift geometry, and thus have minimized the width of the contact pattern, then you need to decide if you'll fully subscribe to the theory, and accept the fact that it's a little off-center and leave it be. If you think being off-center is more important than having mid-lift geometry, then you should get a longer pushrod to force the lifter up higher on the stud, thus moving it closer to the valve and moving the pattern towards the exhaust side of the valve tip which is where you think you want to be.

Paul: sorry you're right there's a little language problem here I think. You're right, max lift happens at full rocker travel, that's a given. However, maximum valve TRAVEL is obtained when the valve side of the rocker is configured for mid-lift geometry.

The rocker tip cuts an arc, because the rocker rotates, not moves up and down. That arc has an X component (side to side, does not contribute to valve movement) and a Y component (up and down, valve movement). Given a specific pushrod configuration, the rocker will always ROTATE the same amount from the base of the cam lobe to the peak of the cam lobe, lets just say for simplicity it's 1/4 turn or 90 degrees that the rocker rotates. Draw a circle on a piece of paper. Lets say you have REALLY bad geometry and your rocker tip rotates from the 9-o'clock position to the 6-o'clock position. The Y axis (up and down, valve movement) travel is equal to the radius of the circle, or half the diameter of the circle. It also moves an equal amount on the X axis, so you can see a lot of movement was wasted moving side to side that could have gone into up and down movement. Mid-lift geometry would have the rocker swing from 10:30 position to the 7:30 position, and you can see that the Y axis travel, with the same rocker movement, is much greater than before, and also the X-axis movement is much less than before, so less valve tip wear and greater valve movement (or greater max lift). In this exaggerated scenerio, the mid-lift geometry gives 1.4x more overall valve lift than the non mid-lift geometry.

As I said before the opposite is true on the pushrod side of the rocker, because the Y-axis movement is fixed, so you can actually cause the rocker to ROTATE farther with non mid-lift geometry, but because you are traveling around the arc of the circle, and not in the most linear point of the circle (when you are tangent to it), the valve motion and acceleration gets all screwed up from the designed cam profile.

Originally Posted by Motorhead

cfm: The whole point of the discussion is to determine if you really should be concerned with the pattern being slightly off center. If you are set up for true mid-lift geometry, and thus have minimized the width of the contact pattern, then you need to decide if you'll fully subscribe to the theory, and accept the fact that it's a little off-center and leave it be.

I realise that Motorhead, hence the last post.... which was to help get it back on topic. Perhaps, I should have said in that I would leave it be....if the swipe pattern is near zero it isn't going to matter unduly.

I can see where your coming from as well.... what I think you are alluding to is that if the setup is designed for mid-lift, you must keep it set for mid-lift geometry for it valve movements to be uniform? If thats the case, your stuck with it..... you can't change/move anything anyway.