The inherent problem with an overhead valve engine, especially those pushed to performance limits, is the intricate group of components known as the valvetrain. The assembly of rapidly “up and down” moving pieces demands the utmost attention to detail to survive significant compression, power adders, and shocking RPM demands.
The goal of proper pushrod length is to center the rocker arm over the valve stem. As the rocker travels in an arched motion, it will be slightly outward on the valve face at the maximum lift and inward when the cam is at the base circle. The key is to have these two extremes centered over your valve face.
Let’s say that just before you shift, you’re spinning your engine at 7,000 rpm, a conservative number in many cases. That engine RPM calculates to all valvetrain pieces cycling up and downward approximately 54 times per second. We pulled our Howards Cams selection of tools from our toolbox to get our geometry exact on our current big-block Chevy engine.
If you view the motion of your rocker arms as the engine rotates, it travels in an arc with the fulcrum as its pivot point. Since our big-block is now equipped with new 24-degree aftermarket aluminum cylinder heads, plus a new cam and roller lifters, this accumulation of parts from different manufacturers will typically demand a new pushrod length.
We pulled our Howards Cams degree wheel, threaded pushrods, and lightweight valvesprings out of our toolbox. We sealed the bottom end of our block after degreeing our cam and put our degree wheel back in place. We can use the wheel to find the maximum lift point with each cam lobe using our cam card.
Getting the exact geometry for the valvetrain relies heavily on the pushrod length. If the pushrod is too long, the rocker will travel across the valve stem too far outward towards the exhaust side of your cylinder head. Too short, and the contact pattern will ride more towards the intake side of the valve.
Option One: Using A Rocker Arm…
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