Correctly viewed, a pushrod engine's valvetrain assembly stands at the gateway of improved power. It is not a collection of components only intended to time and provide the correct valve motion. Rather, its proper operation should be considered satisfactory when it is characterized by minimal deflection and an ability to allow all components to functionally reflect the mechanical and design intentions of the camshaft. In fact, valvetrain performance is often a limiting factor in circle track engine power output.
These statements are as true for engines in the NASCAR Sprint Cup (NSC) as they are for those at your local Saturday night track. Small-block V-8 engines that are the foundation for the majority of circle track racing today were originally designed in the mid-50s to the early-60s, even though aftermarket parts designed for racing purposes have provided much-needed improvements. Keep in mind that the primary objective of engine designers at the time was to develop a low-cost package with competitive power, at the production engine level.
In reality, the small-block Chevrolet and small-block Ford V-8 engines are outstanding designs that have stood the test of time. But, by today's standards, they are not state-of-the-art in terms of what constitutes a competitive circle track engine. To a large measure, these are the same engines that Smokey worked with when he was racing in NASCAR, during the mid-to-late-50s part of his career.
A cross-section of a NASCAR Sprint Cup engine's valvetrain.
Today, newer pushrod V-8 racing engine design features include raised cams which shorten, and therefore allow for, stiffening of the pushrods. This provides for larger diameter camshafts which enable more aggressive cam designs. Current NSC engines have significantly raised centerline camshafts that are 60 mm in diameter. NASCAR now regulates both the height of the cam above the crankshaft centerline and the diameter of the camshaft journals themselves. The newest NASCAR engines (the GM R07.2 and Toyota Cup engines) are presently at the limit of these regulations. As you would expect and without regulation, cylinder block and camshaft designers would likely continue incrementally increasing the cam diameter and height (in the block) until packaging limits were reached.
Pushrod engines have inherent design compromises which were made to produce cost-effective power for production vehicles. This is a clear result of the ongoing challenge facing OEM engine designers; e.g., production costs vs. potential power output. Such compromise designs have significantly more component mass and less overall stiffness than overhead cam engines. These aforementioned compromises all combine to cause valves to follow the cam profile less precisely at high speed, certainly when compared to valvetrains characterized by higher stiffness.
One way to mathematically model a valvetrain is to consider it as an engineering problem that addresses vibrations. In this type of model, each component is represented as a spring, a mass and a damper. Because each component is essentially a "spring" (based on the fact it is compressible and elastic with regard to compression loads), this is an important concept to keep in mind.
The spring portion of the model represents the stiffness of the component. Theoretically, you would like to keep the mass of all the components as low as possible, while increasing stiffness. This would result in a valvetrain which follows the cam precisely (absent of any "flex" or distortion and loss of effectiveness). The pushrod valvetrain, as raced in most circle track engines, cannot accomplish this. Over the years, specialty parts manufacturers and engine builders have optimized the small-block valvetrain to perform exceptionally well in racing. For example, engine builders use the (flexing or compressible) pushrod to store energy like a spring, which effectively extends over the nose of the cam. Some have called this "loft."