When you get right down to it, all the valvespring essentially does is pull the valve closed after the cam has pushed it open. Unfortunately, if that is all you care about when it comes to valvesprings, then you are on a quick road to broken parts and poor finishes.
Valvesprings are among the most stressed parts on any race engine. They have to be strong enough to handle extreme cam lift characteristics at high rpm, yet light enough to reduce the amount of mass in the valvetrain. They are constantly pushed to the limits of valve float (and often beyond it) in the search for more rpm. They live in an environment of extreme heat. And to top it off, there are 16 of them in your race engine, so you can multiply the probability of a broken spring ending your night early by that factor right there.
Fortunately, the technology behind valvesprings has made great advancements in the last 10 years, and the possibility of a broken spring is actually quite low if they are correctly installed and monitored. Part of that responsibility falls on your engine builder, but much of it, for example monitoring your springs, falls on you. This is especially true if you are racing a stock engine or plan to change out your own springs.
It's All About PressureAs we mentioned, the spring's role in the engine is to close the valve. More truthfully, the spring has to keep the entire valvetrain connected so that the valve (either intake or exhaust) closes at the rate determined by the back side of the cam lobe. This means it has to be strong enough to keep the valve from lofting, or continuing to open past the point where the lifter reaches the crown of the cam lobe. If the valve continues to open because of its own momentum, it creates a break in the valvetrain. A healthy valvetrain is one in which all the components are in continuous contact. When they aren't and the valve separates from the end of the rocker, the spring eventually regains control and sends the valve crashing back into the seat and rocker, sending a shockwave through the system. Too much of this ends in broken parts. You have to make sure you have enough spring pressure when the valve is fully open (the spring is at its greatest compression). This is referred to as pressure over the nose.
If you've heard the phrase over the nose, then you've probably also heard of pressure on the seat. This is the amount of pressure that the spring is exerting on the valve when it is fully closed. A valvespring should never be in a totally relaxed state. Instead, it should always exert some pressure on the valve, even when the lifter is on the base circle of the cam for that valve. This is necessary to keep the valve seated tightly against the head to promote a good seal as well as eliminate valve bounce. Bounce is the tendency of the valve to rebound or bounce back up off the seat after it closes. This can become a problem at high rpm, so the faster the motor, the more seat pressure you are normally going to need.
Finding the right mix between spring pressures is one way an engine builder earns his keep. Too little spring pressure results in poor valve control and limits engine rpm. Too much pressure causes the pushrods to flex and shortens camshaft and lifter life. One important factor to remember if you're running a stock class is that high seat pressures can pull pressed-in rocker studs from the head. If the rule book allows you to use screw-in studs, this is always a good idea for a racing engine.
Installation and Coil BindA spring's seat pressure is adjustable by how much it is compressed between the spring seat on the cylinder head and the bottom of the spring retainer, which is attached to the top of the valve stem. This is called the installed height and is critical to proper valvetrain geometry. Increasing the installed height reduces the seat pressure, and to an extent the open pressure.