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Restricting the flow rate of air to the intake manifold is a convenient and popular method used by several motor sport disciplines to regulate engine performance. This principle is applied in the Formula SAE and Formula Student competitions, the rules of which stipulate that all the air entering the engine must pass though a 20mm diameter orifice. The restriction acts as a partially closed throttle which generates a vacuum in the inlet plenum. During the valve overlap period of the cycle, which may be as much as 100 degrees crank angle in the motorcycle engines used by most FSAE competitors, this vacuum causes reverse flow of exhaust gas into the intake runners. This, in turn, reduces the amount of fresh air entering the cylinder during the subsequent intake stroke and therefore reduces the torque produced. This effect is particularly noticeable at medium engine speeds when the time available for reverse flow is greater than at the peak torque speed.

Historically, when valve springs were wound with round wire and the coils were nominally equally spaced, it was relatively easy for the engineer to calculate the virtually-linear load carrying capacity, the almost non-varying stiffness and the relatively-constant natural frequency of the spring. This was the design data that was required then for some simplistic but effective calculations of the valvetrain dynamic stability. In recent times, valve springs have come to be commonly wound with other wire sections such as ovate and with coil-coil spacings that are unequal, giving the spring a variable load carrying capacity, variable stiffness and a variable natural frequency with deflection. Such springs are known as progressive wound springs. The computation of these spring characteristics is no longer a simple matter and neither is their incorporation within the calculation of the dynamic stability of the entire valvetrain. The technical literature is very sparse on these topics.