This is a chart showing pushrod...
This is a chart showing pushrod load traces. The 2,000 rpm load is what you would expect to see. By 5,000 rpm, the load over the nose is still the maximum. Peak opening load is roughly twice what it was at 2,000 rpm. The 7,000 and 9,000 rpm traces show how the loading of the pushrod has changed significantly. This is due to the dynamics of the system. The time the cam spends opening and closing the valve at 9,000 rpm is a fraction of what it was at 2,000 rpm, so the load required must increase.
The ideal pushrod will center the rocker arm over the tip of the valve stem with the correct lash dialed in. This means the tip of the rocker arm will be contacting the inner third of the tip of the valve stem as soon as the lash is taken up and valve lift begins. It rolls across the middle of the stem at mid-lift and reaches as far as the outer third of the stem at maximum lift. If the pushrod causes the rocker to roll over the edge of the valve stem at either end, it will cause undue wear on the rocker, the valve, and even the valveguide.
The best way to determine the correct valve length is with a pushrod checker, which is essentially an expandable pushrod. Screw the pushrod checker to its minimum length and insert it in the engine just as you would a normal pushrod. Extend it until you get the correct valvetrain geometry (the tip of the rocker arm stays on the tip of the valve stem). This will require turning the engine over several times by hand, so it's best to mock up only a single valve. Once you find the correct length, pull the pushrod checker.
A good pushrod checker will make it easy to determine the expanded length. For example, Comp Cams' Hi-Tech checking pushrods come in a number of minimum sizes. The checker pushrod is expanded by unthreading one of the ends. Each revolution equals 0.05 inch. Simply count the revolutions, convert to inches, and add that to the checker pushrod's minimum size. For example, if we were using an 8-inch checker pushrod and extended it six revolutions, the final length would be 8.3 inches. (You can also double-check your calculations if you have an extra-long dial caliper.) Subtract your lash from that number and you have the correct length. For more information on checking pushrod length, see "Determining the Proper Push" in the December '03 issue of Circle Track.
Proper valvetrain geometry...
Proper valvetrain geometry requires the end of the rocker arm to stay on the valve tip and be centered at mid-lift. Adjusting the pushrod length is the best way to achieve this.
Enter the Matrix
Just because the pushrod engine is nearly dead as far as over-the-road cars are concerned, don't think the performance aftermarket has forgotten about the racers. While working on this story, we heard whispers out of Comp Cams that the next generation of pushrod technology was on the table. Apparently, Comp Cams is working with other companies on a "graphite aluminum matrix pushrod."
No, we don't know what that means, either, but it sounds light and strong. The analogy for a graphite aluminum matrix pushrod is carbon fiber. Graphite strands are used much like carbon strands in carbon fiber, and aluminum is used as epoxy to form a bonding agent. The result is said to be an ultra-light material with incredible strength.
For the time being, there is no timetable to make these space-age pushrods a part of the catalog. Apparently, there is still plenty of R&D left to be done. Still, it's nice to know the future of racing technology is bright.