There are many factors to consider in choosing the correct valve spring for your engine.  Physical fit such as diameter and installed height, having enough travel in the spring for the valve lift, and proper closed and open pressures to control the valvetrain at the desired rpm. 

Physical fit is determined by your cylinder heads and valve length.  Many heads are restricted as to how large of a diameter spring can be used.  The cylinder head (spring pocket diameter and depth) and valve length (along with spring retainers and locks) will determine your valve spring installed height.

Spring travel is designed into the spring.  This is how much actual travel is in the spring from the installed height to coil bind (spring totally collapsed).  This travel, minus .060, is the safe amount of valve lift the spring will accept.  For example: .660 spring travel, minus .060 = .600 maximum safe valve lift.

Spring Pressures are determined by spring design and installed height.  The spring design would be wire diameter, number of coils, spring diameter, number of springs (single, dual, or triple) and type of steel.  The installed height is how much the spring is compressed when the valve is closed.  The recommended open and closed pressures are best obtained from your cam spec card or by calling your cam grinder.  One thing to remember is that the cam profile, valvetrain weights, rocker ratio and rpm vary these recommendations.  An example would be two engines with the same parts running different rpm would need different spring pressures.  A combination turning 9000 rpm needs considerably more spring pressure than the same combination turning 7500 rpm.  You could use the same pressure at 7500 rpm as 9000 rpm but using excessive spring pressures shortens the life of the valvetrain components and increases heat.  

 Why do prices vary so much for the same size and pressure valve spring?

The quality of the steel used in the spring (and damper if used), the inspection processes and the surface treatments of the finished spring all add costs to the finished product.  It is not uncommon to see only a 4-to-6-pound load loss after use.  Lesser priced springs may skip a process or use a lower quality steel.  The bottom line here is, the valve spring is the most highly stressed part in your engine, using a premium valve spring is money well spent on power and durability. 

 What is valve spring installed height?  

    Installed height (or assembled height) is the measurement of what the valve spring itself measures when it is installed on the cylinder head with the valve closed.  This means from the bottom to the top of the spring only.  Do not include the retainer or spring seat in this measurement.

 What if I change the installed height?

Obviously, a valve spring is a coil spring. If you put a light coil spring in your hand and squeeze, the more you squeeze the higher the tension. The same with the valve spring. The shorter the installed height (squeezed more) the higher the pressures, both when the valve is closed and valve open. Naturally, the opposite happens when taller installed height is used. In addition, when the installed height is changed, the amount of travel (valve lift) that the spring will accept changes. The shorter the installed height, the closer the spring is to coil bind resulting in less travel (less lift).

 When checking the pressure of a valve spring should a retainer be used? 

A single spring with a damper, a dual spring or a triple spring should be checked with a retainer.  The steps on the underside of the retainer will change the pressure and coil bind numbers. When checking the spring pressure, you will have to add the thickness of the retainer (from where the outer spring sits to the highest portion on the top of the retainer) to the installed spring height to get your checking distance.

 A word of caution:

As I said earlier, the valve spring is the highest stressed part in your engine. The specialized surface finish of the spring is very important to its durability.  Any damage to the spring surface can cause it to fail in that location.  This includes scratches from mishandling, marring from incorrectly fitting retainers or spring seats and pitting from rust.