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Rocker Geometry 02

[falloutboy]

Der wohl beste Artikel den ich jemals über dieses Thema gelesen habe. Teil02

Setting Geometry

As mentioned, the rocker geometry has TWO considerations: The accuracy of how it is installed, which I have always referred to as the “installed geometry.” And second, how accurately it is designed, which I have always referred to as the “design geometry.” This second item relates to where the adjusting screw or cup is placed in the body, and at what angle.

As an engine builder, you can move the rocker arm up and down to the valve tip in setting that side of the rocker’s effect, but you can’t do too much about the pushrod side, and that is where the information comes from. That is up to the rocker arm manufacturer.

Of the two sides, however, the lesser of evils to shoot for is setting the VALVE side (or installed geometry) as closely as possible, because (a) this is where the motion is constrained by the valve guide, (b) this is the side that has the greatest motion, (c) this is the side that has the valve spring, where harmonics are generated and amplified, (d) and this is the side where all of the foregoing multiply into a measurable resistance value that generates heat, robs power and creates additional friction. The pushrod is, by comparison, free floating with the rocker body’s movement in and out as well as up and down, and it is moving less (the cam’s lift). But make no mistake about it, when the pushrod side is not performing to mid-lift geometry, it is losing information. The upside though, is that whatever is left, is getting through to the valve. When the geometry is off on BOTH sides of the rocker, because you didn’t install it accurately, you lose twice! Only 90% may go into the rocker from the cam, and of that, only 90% comes out, or 81% goes to the valve. I’m of course rounding things off for example, but the principle is the same.

I should add one other point here. Everything is “net.” So if you have those cute little “checker springs” laying around, find some other use for them, because outside of holding a valve in a head for display where someone can use their finger pressure to push the valve open, they have little use. You need the REAL running springs for any geometry setup. The same goes for checking flex or piston to valve clearance or anything else critical. Checking springs ADD about .040” (or more) NET valve lift to your engine. Or, another way to say it is you will LOSE .040” or more NET valve lift when you put the heads together with the running springs, compared to whatever you measured using the checking springs. This is true across the board, flat tappet cams, roller cams, aluminum rockers, steel rockers – it makes no appreciable difference.

Later in this article, I will offer installation and assembly tips but for now, here is the easiest of accurate ways to set INSTALLED rocker geometry:

The closed valve position is the easiest and the best. The cam must be in the closed position, on its base circle. Heads are assembled to the engine, with no pushrods in place. You must know what your “net” valve lift is supposed to be (we can get nitty-gritty later). You will subtract any valve lash so you have an accurate “net” lift.

For stud rockers, put it in place with an adjustable pushrod. You don’t need the poly locks; just let it set loosely on the stud. Knowing your net valve lift, DIVIDE it in half, and write it down on a piece of paper. Then, lengthen or shorten the adjustable pushrod to raise and lower the back of the rocker until you get the center of the trunnion exactly HALF of your net valve lift BELOW the center of the ROLLER TIP. If for example, you have .600” net valve lift, then this would be .300”. Keep in mind that I refer to “center” of the trunnion and roller pin. It is their axis that is what you are measuring. Some are easy to see and some are not. For those with flat machined surfaces, take a scribe, measure and mark these centers as best you can. But the main trick is that you want to be sure you measure this from a precise 90 degree reference to the valve centerline. To accomplish this, you are best served to use the top of the valve spring retainer. Simply lay a short machinist ruler (or something comparable) atop the valve spring retainer, and pass it along the side of the rocker arm.

When you have the height installed accurately, the trunnion will be exactly HALF of your net valve lift, below the roller tip centerline when the valve is CLOSED. As it opens, and moves to exactly mid-lift, the axis of the roller tip will have dropped down to be straight across from the trunnion and an imaginary line that runs between them (I call the motion line) will form a precise right angle with the valve centerline. The

roller is at its farthest point across the valve when this happens. When the valve continues to open the second half of its lift, to full lift, the roller will have moved exactly an equal amount BELOW the trunnion as it was above the trunnion when it was closed. And the roller will

be at its closest inside point on top of the valve. You will also have the ultimate least amount of roll across the valve.

For shaft mount rockers, it’s a little different, but the principle is the same. With shaft rockers you must use shims, or have a stand that has a surplus of metal that you are machining exactly what you need away. But you can take a measurement of the stand height without using a rocker arm. Just bolt the stand down to the head with a couple of bolts, lay the shaft in the stand’s bed-way, and use a machinist square along the side of the valve (or spring) and shoot the long end along the top of the shaft, so there is a gap beneath it and the top of the valve. Everything is about finding the centerlines, and being creative about doing this, while being accurate at the same time and measuring at an accurate right angle with the valve.

It makes no difference where the wear pattern is at on the top of the valve, when you have correct mid-lift geometry, and providing the pattern is “on top of the valve.” (Running off the edge of the valve is not good.)

Graphing the Cam at the Valve

To see what is really happening at the valve, you need to check your rocker’s motion by measuring it at the valve. The best way is to essentially treat this like you’re degreeing your cam, but you’re measuring motion at the valve. Only instead of just picking up points of lift

to compare to the crank, as you would with the .050” tappet lift measurements on a cam card, you will be creating a graph all the way through the entire cycle of valve lift, opening and closing. If you are fortunate to have a CAM PRO or CAM DOCTOR, or something similar, life is good. If not, you can do the old fashioned way. You will need graph paper that can be found at art stores, engineering supplies and many science or school supply providers. You need a dial indicator that you’ll mount directly above the valve spring retainer, nearly fully compressed so you follow the valve’s stroke fully – and you want to be sure the indicator stem is lined up parallel with the valve. As with setting your cam, you need to have a degree wheel in place on the crank and zeroed accurately to TDC of the piston.

With the above in place and ready, you have TWO choices to how you measure this; which are merely opposite perspectives. You can choose an even number of crank degrees you move through to measure valve lift, or you can choose an even number of valve lift to measure degrees of crank rotation. It doesn’t really matter which you use, because it is the comparison against other like tests that is important, and

both need to be the same. You don’t have to be too crazy about fine increments here, just choose valve lift jumps of maybe .020” and write down the crank movement; or choose 5 or 10 degree crank measurements and write down the valve lift.

Personally, I like the second method of using a fixed crank stepping, and then noting the valve lift. This goes directly to the points I make about “area-underthe-curve” that you are looking for. For those new to this term, area-underthe-curve refers to the VALVE LIFT CURVE as charted across a graph of time (meaning crank degrees), and what most engine builders agree, is that lifting the valve as quickly as possible and as soon as possible, while setting it down quickly after it has hung open for as broad a period of time as needed, but without being too fast to damage the valve train from excessive “bounce” is what everyone wants. So, when you want to see the gains and losses in this area from inaccurate rocker geometry, you’re really looking for wasted time when the crank is moving more than it needs to lift the valve the same amount. So, if you standardize your testing to the same valve lift measurements, the gains and losses in the crank are readily seen.

Once you’ve charted one rocker geometry setup, perhaps the one you’ve been using, now make the changes with pushrod length and/or stand height (for shaft systems) to meet with what I hope I have informed about earlier in this article. You will often see that PEAK valve lift is very close to the same, but much less at other points in the curve. That is the lost information. The degree of this will depend on many factors that take another story to itemize. But the bottom line is; you will appreciate how important it is keeping the same geometry, for making sure your cam changes show results that are directly accredited to them and only them. Otherwise, your information is tainted.

Shoe Tip Rocker Geometry

As with aluminum rocker arms, there are different design geometry shoe tip rocker arms, but the priority for adjusting the valve tip side is still there for the same reasons. However, you don’t easily have the same accuracy as you do with picking up precise center points on the roller and trunnion of a needle bearing roller tip rocker arm. Even finding the axis is not easy. Setting it by the same rules is best simply for standardizing one cam to another. Only your reference is the actual contact point of the pad itself. When it is at mid-lift, you will have a 90 degree relationship between the TIP of the valve and the center of rotation for the rocker arm. But finding that center is tricky, because these are usually ball fulcrum rockers, and they are surrounded by the stamped metal that has no clear axis to it. One solution is to put a stud upside down in a vice and rotate it carefully while observing the fixed point on its side that most closely represents where the center is, then making a little felt tip pen mark. This would then be set exactly half of your net valve lift below the valve tip in the closed position. It’s not as accurate

as fixed points to set calipers against, but it will get you very close if you have patience and a sharp eye; and with shoe tip rockers, the amount of error you might be off will have no measurable effect in cam efficiency as it would with needle bearing roller tip rockers.

Twisted Rockers

Unfortunately, engine builders are led into a false security by stud mount rockers sold for heads that shouldn’t use them. These are aftermarket heads with pushrod offsets that require an offset pushrod cup, or adjusting screw. Shaft systems usually have this adequately

fixed, but when heads are sold with studs in them, that clearly need to be removed for a shaft system, this is the false sense of security one gets. The two rockers shown here is exactly what you DON’T want to have (Figure 4).

When you have this much rotation, the roller tip does NOT lay flat on the valve tip, and as it opens the valve its energy is making a cross sword slash across the valve tip that rounds off the top of the valve tips, side loads the guide on the X axis (length of head), and shifts side loads to the bearings in a way that often push or break one prematurely. It’s bad news, costs horsepower and breaks parts.

Ratio & Geometry

If the rocker geometry is off, then so too is the ratio. There’s some good news though: Don’t worry about it, because very few of the rocker arm makers did. The history of rocker design didn’t have much accuracy involved. There was no standard, because there was no need for such accuracy in the old days. Rockers always were, and to a great degree, still are designed in the closed valve position. But as long as you stay focused on what you need to do, you won’t try juggling things around outside of the real priorities, based on a false idea of what YOUR rocker ratio ought to be. It can be all over the place from .05 less, up to maybe close to what it is supposed to be, simply because many manufacturers began in the closed position, and then started moving specs around from a hit and miss until it got close. Once it did, they left it there. Over the years, more consistent manufacturers with the least amount of broken parts have been the model other newer companies would copy. But the mistakes get copied, too. You have to always check things for yourself, forget the advertising. If you do, then you can’t blame anyone but yourself.