In selecting valvesprings, the load must be sufficient to provide control for the application, preferably with a margin of extra capacity, while not overwhelming the limits of the mechanical parts. Just where these limits are depends upon the type of camshaft, and just as importantly, the quality of the valvetrain parts. Roller camshafts-such as the one in our 509 big-block here-can tolerate far more spring load than their flat-tappet counterparts, and most manufacturers will base the spring recommendation upon the type of camshaft and the requirements of the lobe design and intended rpm range. Spring loads are given in terms of seat load and open load, which provide the range of spring force over the operating range of the camshaft. Since a spring's force is dependent upon the amount of compression, it is important to pay attention to the actual installed height of the spring.
The installed height is the foundation for correctly setting up the valvesprings. Generally, the manufacturer will provide a specification of spring load at a given installed height, but it is up to the engine builder to confirm the installed height by direct measurement. Once the installed height is determined, the spring can be checked on a spring testing fixture to confirm the spring load at that height, and the noted load can be compared to the requirements of the application. The seat load can be adjusted up by adding shims to reduce the installed height, or the installed height is sometimes increased via retainer and/or valve lock changes or cylinder head machining to reduce the load.
Seat load is only half the picture, however, with open load being the other half. Open load is the amount of force applied by the spring when it is fully compressed at peak lift. Clearly, the first bit of information needed here is just how much lift is being delivered. While the camshaft specifications and rocker ratio can be used to determine the maximum valve lift, the most accurate means is through direct measurement. This is accomplished via mocking up the valvetrain and recording max lift with a dial indicator at the retainer. Subtracting the amount of valve lift from the installed height will provide the open height of the valvespring. The spring can be tested on a valvespring testing fixture at this height to determine the true open load. The seat and open load provided by the spring can then be compared to the requirements of the camshaft. Often, the expertise of the engine builder plays a major role in determining how much seat and open load is appropriate for a given engine combination.
Another critical aspect to be aware of when contemplating a spring combination is the coil bind clearance. The coil bind height simply refers to the height at which the valvespring coils are stacked solid. This specification is listed for most springs in the manufacturer's catalog, or it can be directly measured. Clearly, this relates the spring's physical design to the installed height and the valve lift at which it will be used. As noted, the installed height minus the maximum lift at the valve gives the open height specification. This open spring height has to be above the coil bind height of the spring, plus a margin of safety. Allowing 0.060 inch to coil bind is generally considered a safe margin.