jcb999

http://www.wighat.com/fcr3/confusion.htm
Pushrod Length- Incorrect pushrod length can be detrimental to valve guide wear. Most sources say that centering the rocker contact patch on the valve stem centerline at mid valve lift is the correct method for determining the optimum pushrod length. This method is wrong and can actually cause more harm than good. The method only applies when the valvetrain geometry is correct. This means that the rocker arm lengths and stud placement and valve tip heights are all perfect. This is rarely the case. To illustrate this, think of the valve angle and the rocker stud angle. They are usually not the same. If a longer or shorter valve is installed, then the relationship of the valve tip to the rocker stud centerline has changed. Heads that have had multiple valve jobs can also see this relationship change. Note, the rocker length (pivot to tip) remains unchanged, so the rocker contact patch will have to move off the valve centerline some particular distance for optimum geometry to be maintained.

The optimum length, for component longevity, is the length that will give the least rocker arm contact area on the valve stem. In other words the narrowest wear pattern. This assures that the relationship is optimized and the rocker is positioned at the correct angle. This means that the optimum rocker tip contact point does not necessarily coincide with the valve stem centerline, and probably will not. What is the acceptable limit for being offset from the valve stem centerline? That will depend on the set-up. A safe margin to strive for is about +/-.080" of the centerline of an 11/32 diameter valve stem. This means that no part of the wear pattern should be outside of this .160" wide envelope. As the pushrod length is changed, the pattern will change noticeably. As the geometry becomes closer to optimum, the pattern will get narrowest. If the narrowest pattern is too far from the valvestem centerline, then the valve to rocker relationship has to be changed. In this case, valve stem length will need to change.

sleeper

Sure it makes sense. Eccentric loading of any bearing surface will cause wear or clearance issues.

I think that it's pointless to even try making a street engine to the tolerances that the article is talking about. Mass produced parts will always have some sort of variance in them. Are you planning to have all the valvetrain components CNC machined? Even if the pushrods are perfect the lifters, valvesprings, rocker arms etc... will not be.

This kind of precision is more warranted on a 12000 rpm F1 engine.

jcb999

i tend to agree with u.. I am about to order PRs and thought I had a pretty good grasp on the geometery issue. But this article tossed that out the window.

The previous pushrods were too long (using the standard rule of thumb about centering the rocker on the valve at midlift), but i really saw no issues associated with it. The PRs I was going to get do bring the rocker tip closer to the intake side a low lift and less towards the exhaust at max, but they don't accomplish the thing the guy was talking about in the tread. I would like to accomplish what he said, but I don't think it is cost effective (or maybe required) with what we are talking. Since you can't change the rocker stud angle very easy, the only way i see to accomplish it is through valve length changes and I don't think that will address the entire problem.

I think this would be much more important on a motor that has really big valve springs.
The guides would take much more of a beating when the roller tip was at the extreme side of the intake or exhaust valve.

Thanks for input. Something I just recently found out,, u know the F1 motors are turning 18K to 18.5K rpm now?



[This message has been edited by jcb999 (edited July 31, 2001).]

Guest

Thats the way I set up mine, smallest centered contact patch on the valve I can get.

RB83L69

That's the way I do mine. Smallest contact patch = least side-to-side motion of rocker across valve stem. The valve does not care if the point that the force is applied to it comes from somewhere other than the center; as long as it's pushing straight in line with the valve. Centering the wear pattern is great but depends on too many things already pre-determined before assembly.

What the article is talking about is easy to do if you have an adjustable push rod. Finished push rods are available in .050" inch increments, and besides, the major cam mfrs. will al make custom pushrods of any desire length for a nominal charge; there's little or no excuse for a performance build ever to have the wrong length, except for ignorance or laziness on the part of the builder, or excessive attempts at economy on the part of the engine's owner.

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"So many Mustangs, so little time..."
ICON Motorsports

jcb999

maybe its just my setup but even with the comp adjustable pushrod checker, the rocker tip moves across probably almost 80 percent of the valve stem tip (while being centered at mid lift). Based on the explaination in the text, i get the impression that the writer is saying that about anything more than 50 percent of the tip (figuring .16 movement over the .3472 valve stem dia) is not good.

Based on what i am seeing, that can't be fixed by pushrod length adjustment.

[This message has been edited by jcb999 (edited July 31, 2001).]