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The pushrod deflects or flexes upon valve opening and then again just prior to valve closing. As the cam tries to move the valvetrain to open the valve, it has the weight of the valvetrain against it and in the case of the exhaust valve, cylinder pressure also. Upon valve closing, the cam is trying to slow the valvetrain down. So on valve opening and valve closing the pushrod flexes and as rpm increases it gets worse. Here’s how it effects rpm, power and durability;
Rpm is reduced when the pushrod flexes, and then rebounds like a pole vault. It throws the rocker and valve out of control. As rpm increases, the more the pushrod flexes, the more out of control the valvetrain becomes and the more valve "float" or "bounce" to the point that the engine rpm no longer increases.
Power is naturally affected by the loss of valvetrain control at rpm, but something else happens. Remember the engine responds to valve timing not cam timing. Even if the valvetrain stays in control. When the pushrod flexes upon valve opening and valve closing, the valve opens later and closes earlier. The engine "sees" less valve open time (like a smaller duration cam) and as rpm increases it gets worse. Loosing 6 to 8 degrees of valve timing is not unheard of!
Durability is affected when the valvetrain parts are "thrown" out of control by the rebounding pushrod. When this happens the valvetrain parts are jerked apart from each other, and then slammed back against each other by the valve spring pressure. This "slam" can break parts anyplace in the valvetrain and at the very least break the lubricating oil film causing excessive wear. All of the above can happen without a pushrod failure!