Note here that peak cylinder...
Note here that peak cylinder pressure occurs past TDC and (not shown) that this point is developed farther past TDC as engine speed increases ... something to keep in mind when either tailoring ignition spark curves, setting initial timing, or both.
Deeper Into The Valvetrain
Knowing the basics is not sufficient. Equally important is understanding what areas in power development are affected by the valve function. Without this knowledge, power may be lost in the quest for simply doing what's necessary to prolong valvetrain life. Depending upon the class of engine, the length of time between planned valvetrain maintenance (particularly valvespring inspection and replacement) may range from "after every race" to "not unless something fails." Regardless, the following information applies.
By their motion, valvesprings generate heat. It is therefore important that they receive adequate oil for cooling. Also, in the process of being compressed and extended, they never come to rest. Even when valves are seated, residual "harmonics" or compression and extension of spring coils within a given stack continue between times valves are being opened or closed. These movements are not always along the axis of a given spring, resulting in lateral distortion that accompanies axial motion. This produces components of spring motion that are counter to what might be called "pure" spring compression and extension.
Despite the efforts of camshaft designers to provide stable valve motion during low-lift opening and closing points, there remains a system of "flexible" parts between the valve tip and cam lobe (pushrods, springs, and rockers). These components contribute to valve motion that is not true to the cam's lobe profiles, thereby producing lift patterns less than what is designed into a given camshaft. During valve seating, reducing the elastic collision between valve heads and seats is an ongoing problem. Engine builders can help the situation by making certain manufacturer-recommended spring installation and pressures are applied throughout the life of a given spring.
Springs should be sufficiently stiff to control valve motion approaching maximum lift as well as during its acceleration to closing. As engine speed is increased, the more important spring forces required to control valve motion are during maximum valve acceleration. Boiled down to its essentials, this points to the importance of maintaining sufficient spring pressure just prior to maximum valve lift in order to maintain lobe/follower contact at and just beyond peak lift. Even though multiple springs can add net pressure to a given spring package, this approach is also important to controlling "surge" by combining different natural frequencies of individual springs. Spring design that includes so-called "beehive" or "tapered" shapes can also provide desirable harmonic damping.
Heading toward the conclusion of this series, we'll discuss induction systems (including manifolds, carburetors, and spacers) and oiling systems. In the last segment, we'll lay out the Cup-to-Saturday night approach to dynamometer testing, data analysis, and some on-track testing suggestions.