The force on the pushrod can exceed 4,000 pounds. That's a lot of force on a slender metal
When you think about it, everything in the valvetrain between the camshaft and the valve is little more than a high-tech telephone line. All the parts and pieces-the springs, the retainers, the rockers, the pushrods, and the lifters-are there simply to transmit the commands of the camshaft to the valves. Of course, each part adds a little to the equation: The springs help the valves close and accurately follow the back half of the cam lobe, and the rocker arms magnify the commands of the valve, allowing valve lift over 0.5 inch without requiring a lobe the size of your wrist. You get the idea. The pushrods, however, are often ignored and thought to be an extension to get from the lifters to the rocker arms.
If this is how you think of pushrods, you are missing out. Engineers at Competition Cams have spent countless hours studying every link in the valvetrain. They recently revealed that the desire of many engine builders for lightweight components was harming overall performance when it comes to pushrod selection. Given the trend of aggressive camshaft grinds coupled with ever-increasing rpm levels, the standard 51/416-inch diameter pushrod that was perfectly fine several years ago can now easily be the weakest link in the system.
It turns out, size does matter. Because racing engines are taking advantage of more aggres
"It's not the compression that's hurting these things," Thomas Griffin of Comp Cams explains. "It's the buckling loads, or the tendency of the pushrod to bend under pressure. Somebody might say, 'I've only got 500 pounds of spring pressure on the seat with a 1.8 rocker ratio,' which gives you 900 pounds on the pushrod over the nose. But that's cranking the engine over by hand. But when the engine is running, it is spinning much faster and inertial loads become a factor. And now that builders are pushing the rpm levels even higher, that just multiplies the load. We've had all kinds of valvetrain combos on our Spintron machine and put strain gauges on the pushrods, and we are seeing over 4,000 pounds of force exerted on the pushrods. That's a couple of Volkswagen Bugs balanced on the end of a slender metal tube."
It turns out that the pushrods are under the greatest loads at two of the worst possible times in the valve cycle: just as it opens and just before it closes. Why the pushrod is under load just as the valve opens is easy to understand. It is trying to accelerate the mass of the valve from a stationary position, which requires a great deal of energy. Radical opening ramps on racing camshafts make the load even greater.
To eliminate the chance of accidentally installing the wrong rods, the Comp Cams laser etc
It's a little more difficult to understand why a pushrod would be under its maximum load just before valve closing. After all, since the spring does the work of closing the valve, at this point, isn't the pushrod simply following the lifter back onto the cam's base circle? Not exactly.
In a perfect world, an engine would work most efficiently if the intake and exhaust valves popped open at the right time and were instantly at maximum lift. Then they would stay at maximum lift until time to close, and then drop back instantly against the seat. That, of course, isn't possible. So cam designers create steep opening and closing ramps to move the valves as quickly as possible when working on racing engines. They can't allow the valve to simply slam against the seat at maximum velocity. That would cause valve bounce, which reduces compression and destroys both the valve and the head.