Posted from another board, thought I'd share the good read.

"
Rocker "geometry" is "angles" of motion.

This Illustration shows OVER-ARCING, whereby the roller tip side of the rocker arm is moving outward and "OVER" its path as it pushes downward upon the valve. NOTE that "C" (ROLLER SWEEP), is .184". The valve tip width is only .342" (11/32), so here more than 1/2 of the tip's width sees roll across action, which induces side loads on the valve tip, valve guide and valve stem. The roller does NOT "roll," much to the argument of many. It merely provides a "pivotal" axis for converting the linear motion into radial motion. In fact, anyone who's ever seen high speed photography of the roller working atop the valve, will confirm that is does more "skipping" than anything (kind of like an airplane tire first hitting the runway). Note the "Motion Lines" (in YELLOW) angle between the trunnion and roller axis in relation to the valve.

Of the various points of information shown above, note "A" illustrates that 30 degrees of rocker ROTATION is occurring to initiate "B", a valve lift of .685". (See: MID-LIFT-ARC) This is wasted motion, compared to the valve's INSTALLED Geometry being set for MID-LIFT, which yields .709" VALVE LIFT for the SAME 30 degrees of rocker rotation!

As with the UNDER-ARCING illustration, which is purposely shown as the exact opposite perspective, both extremes illustrate how the "symptom" of a NET valve lift can be the same, while opposite circumstances prevail. The main thing to understand from this, aside from the obvious increase in roller SWEEP over the MID-LIFT example, is that the valve lift speed is inverted between over-arcing and under-arcing. As stated in the drawing, this example has the valve starting off the seat slower, then increasing as the roller begins to leave the linear path pushing down upon the valve, begins it is reaching a 90 degree relationship. When it begins to follow over its axis, as shown above, the speed of the valve's opening increases proportionately until reaching this 90 degree point. This is measured at the crankshaft as additional crank degrees of rotation to turn open the valve a specific amount, compared to MID-LIFT geometry. Velocity will always be at its peak at 90 degrees to the rotational axis.

OVER-ARCING is the MOST COMMON symptom of rocker arm geometry found on most engines. There are many reasons for this, but simply, it's because few people understand how much and why the rocker needs to be RAISED to adjust its pivot points. The junk plastic pushrod checkers that have been sold for years, do NOTHING to set geometry accurately. In some cases, only making it worse. At the very least, they give the engine builder (and his or her customer) a very false sense of security."

 

Yes you see this all the time as few people really run a sweep on the tips at all and reshim stands and re check.

 

Interesting post. Thanks for posting.

 

Any chance of getting the Under-Arcing illustration posted up? I'm a visual person, even though I understand what's going on, it would be a bit better.

 

Indirectly related question, where is there a chart with the dimensions of the different times of rockers: LS1, SBC, BBC, SBF, Cleveland, 385, etc.

Thanks,

David

 

There is no real chart of that but the standard top to trunion measurements are in some basic groups.

 

so basicly when the valve is closed you want the roller directy in the middle of the valve tip right? or do you want it a tad bit ahead? same goes when the valve is full open...

 

Here is a good read on rocker geometry.

http://www.mid-lift.com/MASTER-INDEX.htm

 

Umm...Miller actually makes a lot of sense. Your drawing above is only half the story. You could fix valve over/under arching my adjusting the rocker pivot and pushrod length. He goes on to say that the rocker pushrod side also has to be configured to handle under/over arching. He says his stud and shaft rockers are made for each head/block combination with the correct angles.

Is that also what the Crane Quick Lift rockers do?

 

https://ls1tech.com/forums/showthrea...light=geometry

Same thoughts, different day and forum.

mid-lift actually didn't like the quick lift stuff:

http://www.mid-lift.com/Cranes_Coup.htm

 

Interesting... how do you check for this?

I was thinking maybe if you saw what part of the stem has wear you could guess if you had this problm.

 

Mark the valve tip with a black marker and rotate the engine by hand a couple of times to see the 'swipe' that the rocker leaves.

 

an adjustable pushrod and a dial indicator is all you need
there are lots of varying theories on rocker-arm geometry
and its effect on valve events pertaining to piston speed
IMHO reliability is not worth sacrificing for such small gains

standard practice puts the center of the roller over the center
of the valve at half max valve lift

lash caps can help rocker to retainer interference instead of
grinding or using too long pushrods

 

Just don't use the adjustable pushrod with strong springs. I bent mine learning that lesson...

I ended up just using my inner spring for checking purposes - and I had to order longer length Comp Cams pushrods from Summit. They mark up perfect now - the same distance on either side of the valvestem centerline.

Jim

 

Hey all, Lunati has a straight forward PDF for setting up proper valve train geometry. Its a two pager. Easy to print and reference.

http://www.holley.com/data/TechServi...20Geometry.pdf

 

From my post above:

Quote from Buddy Rawls:

"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."

 

Does anyone sell a "solid roller" that is equal to the length and general configuration of the factory hydraulic roller for checking purposes?

 

Well, what we have here is two (actually three) issues that can only be right if the rocker manufacturer designed the rocker to be right AND the guides and rocker studs are located with high accuracy.

The goals are to maximize lift and lift under the curve, and produce the least wear on the valve guides.

To keep the lift at the maximum the line from the center of the trunnion to the contact face of the valve stem should be perpendicular at mid-lift. Anything else will reduce overall lift.

The MEI feature is recognizing that this issue also occurs on the pushrod side. So that the line from the top of the pushrod socket to the center of the trunnion should also be perpendicular at mid-lift. It appears only MEI does this intentionally.

The other issue is valve guide where. Applying and off-set force on the top of the stem requires it to be resisted by the guide. However, this misalignment can only be small (the valve stem is only .313 diameter, the guide is 10x or so longer).

Ideally, the rocker arm will have the roller and pushrod at the right lengths and angles to account for the total lift and pushrod and stud geometry to allow for everything to be accounted for.

Otherwise, shouldn't setup for mid-lift, check the contact pattern on the valve stem, and if it is incorrect fix it by swapping rockers?

This is complicated by possible machining irregularities with the head. Not all rocker studs, especially for a production engine, will have the accuracy needed for an 8000 rpm race engine out of the box!

 

I noticed this "wide" sweep on my buddys SBC
23degree aluminum heads were nice at one time, but shaved to death
.100" long valves and lash caps but shorter than stock pushrods
.630 roller mandated the long valves
it was more a matter of making it work at all with his particular combination
than wishing for perfect geometry (or a new set of heads)

 

ive skimmed through the thread and didnt find my answer so.. how would the rockers arc say a stocker rocker compared to a crane quick lift 1.7 in terms of power/tq? would the curve change slightly because of the change in geometry?