Instead, they design the lobe to slow the valve back down in the last few degrees of camshaft rotation and gently drop the valve back onto the seat. As you can imagine, stopping the inertia of the quickly moving valve just before it slams into the seat places a great deal of stress on the pushrod. This is compounded by the fact that the entire system unloads at maximum lift and then comes back together somewhere on the closing cycle. It gets even worse if the valve lifts beyond maximum lift and the spring slams it back against the rocker arm, sending a shock wave through the system. In many cases, the load on the pushrod just before valve closing is even greater than it is when the valve opens.

This results in a corruption of the valve timing events. If the pushrod flexes as the camshaft is trying to open the valve, it absorbs the energy from the cam and delays the opening. On the other side of the equation, if the pushrod flexes as the cam is trying to slow the valve just before it closes, it allows the valve to maintain too much velocity as it makes contact with the seat. In addition to the damage this does to the valve and seat, the extra speed can also cause the valve to bounce back open, reducing compression. You end up with a race engine that runs, but not to its full potential. All that money you spent on a state-of-the-art camshaft you have just flushed down the drain.

"The goal has gone from running as light a pushrod as you can, which you get with your 51/416-inch-diameter pushrod, to a more rigid piece with respect to bending," Griffin says. "Yes, a lighter pushrod is better, but if it is too light, it's too flimsy and it negates any advantage you gain from the lack of weight. A bigger pushrod with a thicker wall is much more resistant to buckling loads; the extra weight that comes from that is simply an unwanted-but unavoidable-by-product. Despite the added weight to the valvetrain system, the bigger pushrod almost always allows for greater rpm and more valve control."

Vibration
There is another factor to consider when it comes to pushrod size: the vibrations that are unavoidable in a valvetrain. Vibrations are generally worse at the higher rpm levels. Basically, vibration is the combined result of the components flexing and springing back to shape. Just like a spring, the metal components don't flex, spring back into shape, and then stop. They oscillate just like a spring. At certain rpm levels, the vibrations create a harmonic that throws everything into an out-of-control state. The problem is that this level is always changing, depending on components and conditions inside the engine, so it is impossible to predict where damaging harmonic conditions will show up.

One of the best ways Comp Cams engineers have found to control this is by using pushrods to essentially soak up the vibrations running through the valvetrain. "We are using the pushrod like a tuning fork," Griffin says. "When you run at any rpm level, the valvetrain becomes a vibrating system. What that means is it has frequencies where it likes to work and frequencies where it doesn't. What we've done with the pushrod by making it with a larger diameter and a thicker wall is allow it to have properties that are resistant to vibrations. As a result, your valve motion is a lot truer, and your valve is better controlled. It essentially protects the entire valvetrain."

Sized to Fit
No matter what diameter pushrod you are running, it's also critical to select the correct length. There are several factors that affect pushrod length. Deck height, rocker arm location, lifter size (roller lifters are typically longer), valve stem length, and cam base circle size are just a few of these factors. Because of this, pushrods generally aren't ordered until the engine is mocked up and the builder can see exactly what he needs.