Valve/Rocker/Pushrod Geometry

[Ed-vancedEngines]

Not really,
My first thought when I read their preface I was thinking just like you are, but after reading and seeing all they had to say, in this one person's way of thinking, they seem to be mostly correct. My first thought was "Yea Right" What do they know about serious max lift engines. Their clients all run long duration circle track. But They seem to be closer than most of the cam companies about what will work.

T&D and Jesel use guages, that are used to check rocker stand ht vs valve stem ht wile valve is closed. I have checked the both of their methods against the Miller Mid Lift Theory, and to me it seems to offer the same results. I have checked just about every way I can think of and with shaft rockers using stands, the ones I just named are using different checking methods with the same ending results. It looks as if Reed Cams is mostly refering to the Miller Mid Lift Theories, but are wording the method different.

Maybe I was not understanding what is written on their webpage as well as you did, or maybe at least saw it different.

Ed

[idunno]

Look at it like this, the rocker can rotate a full 360 degrees. Now obviously the cam cannot make the rocker do it but it can provide a portion of that full circle capability. So the actual movement of the rocker arm is determined by how much lobe lift the cam has and thats what determines the arc of the rocker, think of a pie shape. So if you split that pie shape right thru the center (mid lift) the rocker contact point on the valve tip is going to be the farthest point away from the rocker center. At the valve closed position and max lift position the contact point on the valve tip will be at its closest point to the center. What they are trying to say is alot of instances the contact pattern will not be dead center of the valve , because of variables of the parts being used. I have seen on some Dart pro one BBC heads where this is the case, it really needs a longer rocker arm to get the contact pattern dead center to the ex. valve when the geometry is correct. Short of building your own rockers to fit the combination of valve angles, valve heights and then add the changing distance from rocker stud to valve tip because of intersecting angles, you have to just buy what someone else is making and bolt it on. I have seen many people get the geometry way out just to get the contact pattern in the center of the valve, when the contact pattern would not be perfectly centered of the valve tip if the geometry was corrected.
Now heres the deal with the set up tools from Jesel, their tool will not set up this way. Their theory is that the rocker will be stronger if the contact point will not pull back in as much at max lift. So it sets up like a stud mounted pattern would be if the pushrod is shorter than it should be. They only pull back in a little at max lift because thats where the maximum spring pressures are.
Its not too hard to figure out how to achieve the maximum amount of travel produced on one end from the other end of a pivot point. Thats basically what a rocker arm is, a pivot point.

[James]

Usually the push rod in the loaded position has to be at the very least .025 longer, and I always shoot for .050 longer when running conditions are in place. I find that with the little longer than shorter method or even right on buy the gauge, are not long enought when the engine is at 7000-9000 rpm because of deflection and just the way everything moves around.

Awesomebill

Do you reconfigure your geometry with the longer pushrods for minimum sweep or do you reset the lash adjustment on the rocker? Please correct me if my thinking is off. But with the geometry of the rocker staying the same from when you checked it with the shorter push rod, the longer push rod will sweep at max lift further towards the intake then in the closed position. Will the deflection of the push rod correct this to maintain minimum sweep? Thanks

[Erland Cox]

Some years ago I put together a Prostock 500ci engine. I did the heads and I also calculated the rocker arm geometry for minimum sweep. What happened when we ran the engine was that the valve adjusters broke (jesel) almost every run. So I figured that it was impossible to run a correct geometry with this much lift and valve pressure. When opening the valve the adjusters can take a lot of side load because the spring pressure is "only" 500lbs. Aproaching max lift with spring pressures above 1000lbs it doesn't take a lot of side load to break the adjuster. Lowering the rocker stand pads.100 inches and shortening the push rods cured the problem. The devil is in the detail. Erland.

[larrycoyle]

Well, I have noticed that if you set your rocker arm geometry according to either T&D or Jesel and then also check it with the Miller theories that they will all hold to the same settings. I think that Miller was plagurized by Lunati Cams possibly but the Comp Cams instructions will not work on any serious engine. I do not understand why cam companies keep on giving bogus tech info but they do.
Ed

Ed, just to clarify one thing about this statement regarding Lunati and his theory of valve train geometry. Jim Miller was actually working for Lunati when all of this came about. Jim had actually been making rockers for Boss 429 Fords for several years prior to his time at Lunati Cams and he probably had the theory in his mind at the time but he was an employee when he drew this up for the catalog. To give Jim his due, he is a pretty smart guy but a little eccentric like the rest of us techies. Also, and I am not defending Comp Cams but their instructions were probably written like they are for the street motor guy (their core business) or as you say the ones who do not build "serious engines" for a living.

Larry