Valvesprings seem to be one of the simplest components in an engine—but in fact are one of the most critical for engine performance. Seemingly just there to close the valves, the spring is actually the workhorse of the valvetrain system, providing the control forces upon which the system relies. Once spring control is lost, engine power production quickly deteriorates, with the power curve often clipped short of the engine combination’s true potential. It doesn’t matter if your engine’s cylinder head flow, camshaft, and internal parts are good for 7,000 rpm if spring control issues cut the rpm capacity cold at 6,500 rpm. At that point, your power production will be done. To get the most of the engine’s rpm capability, the springs must keep control of the valvetrain.
There are many aspects to spring selection, from the basic open and closed loads, rated in pounds of spring force, to the spring size, rate, material, and coil bind height. Many of these characteristics and specifications can be found in the manufacturer’s catalog, but there are details that go far beyond these basic stats. Professional racing teams go through extensive testing using sophisticated equipment to determine aspects such as spring harmonics and actual running limitations of a given spring design with a given cam and valvetrain combination. This kind of experimentation is beyond the capabilities of the average engine builder, but understanding the basic specifications and installation criteria is usually enough to ensure a successful installation. Here, we go through the relevant aspects of spring selection and the steps necessary to achieve a well-behaved valvetrain.
Starting at the beginning, the spring selection has to match the application. Most basically, this means the spring’s rate and installed load have to be matched to that required by the camshaft chosen. A light valvetrain or lower rpm will reduce the spring load requirement with the same camshaft, while a heavyweight valvetrain or more rpm will require more spring. What we are after here is the spring’s ability to control the motion of the valve and its actuation mechanism, while not overdoing the spring load to the point where the life of the camshaft and lifters is unduly compromised. It has to be a balance. For the most part, unless you’ve garnered a wealth of experience in selecting valvesprings, the safest bet is to consult the camshaft manufacturer’s recommendations. Recommendations can be had either through the cam manufacturer’s catalog or on its tech line if you’re running an unusual combination or have special requirements.
The minimal information you will need to provide is the installed height at which the springs will be run and the camshaft and valvetrain components used. For stock or mildly modified engines, most manufacturers have already done the homework, having specifications on the stock installed height and a recommended spring to suit a given cam. If your particular engine has been modified with longer valves, aftermarket heads, different retainers, or machined spring seats on the heads, etc., these changes have to be taken into account when the spring selection is made. The bottom line with non-factory combinations of parts is that the actual installed height must be measured. A valvespring micrometer makes checking the installed height an easy and precise task.
Commonly, street performance springs with cams of just over 0.500 inch of lift will be a single spring with a flat wound metal damper. The OEMs used this type of spring in many factory-performance applications. Beginners often mistake the spring with damper combination for a dual valvespring, which it isn’t. The damper’s purpose is to work as a friction brake, helping to control spring surge through the friction between the main spring and damper. This basic spring system is very effective for mild applications, but with higher rpm and more intense camshaft action, the spring requirements increase, and more load is often needed.
There are several ways to increase spring load (within the constraints of a given working height), including the use of a thicker spring wire, a larger spring diameter, or a higher quality material. Thicker means heavier, which in valvesprings causes problems, since one of the main masses the spring has to control at high rpm is its own. Further, the thicker the wire, the less clearance to coil bind with a given installed height, meaning you can only go so thick before you preclude the use of high cam lifts, ironically the main reason we need a thicker spring in the first place. Sometimes the available space for larger diameter springs is limited by constraints of the cylinder head and valvetrain layout. Trick springs are available in high-tech materials, which can provide a higher spring load from the same wire diameter as a conventional spring. But the most common and economical approach to higher spring loads in these applications is to go with a dual or even triple valvespring.
Dual valvesprings simply add another smaller diameter spring within the outer, typically with a surge damper in between. Triples, once popular in very fast classes of drag racing, just add a third even smaller spring inside the second. With multiple springs, special retainers are matched that have register steps to properly locate each of the springs as installed. Multiple spring assemblies have a much smaller ID than a single spring, often requiring the spring seat in the head to be machined to match. Also, since conventional umbrella-type valve stem seals don’t fit within the tighter confines of the multiple spring, the guide boss is typically machined to 0.500 or 0.530 inch to take the more compact positive-type seal.
Related to the valvesprings is the retainer-to-guide clearance. While with modest valve lifts this is not an issue, with high lift, the retainer can actually crash into the top of the guide boss, or at least squash the valve stem seal. When you’re in the range of lifts where this becomes an issue, you’re probably going to be machining the guide boss and spring seat for a dual spring and positive seal. Making the check for retainer clearance is as simple as installing a valve with a checking spring, rigging a dial indicator, and testing to determine how far open the valve can be pushed before the retainer makes contact. Generally, a minimum clearance of 0.050 inch should be available before contact with the seal, but of course, more is better. The guide boss can easily be shortened by most machine shops when the related machining is done for dual springs or positive valve seals.
Getting a handle on the installed height is easily accomplished during the assembly phase of the heads. My preference is to use a height micrometer, which gives a highly accurate reading. The machine work on the valves and seats must be completed first, since this will affect the final position of the valve. The other components can also alter the installed height, so the check should be made with the retainers and keepers that will be used in the finished engine. In fact, it is not uncommon for older OEM factory steel retainers to vary by 0.015 to 0.020 inch in where they sit from the highest to the lowest, depending on minute variations in the machining of any particular retainer.
This variation is not a highly critical amount in mild applications, but in more demanding applications, it doesn’t hurt to check each retainer and select a consistent set. Generally, quality aftermarket machined steel or titanium retainers are more accurate, but it still pays to measure each valve location at the time of head assembly since there’s no guarantee that every valve seat was machined to exactly the same height, or even that the heads’ spring seats are consistent in height. A final note about the measured installed height is that it will generally grow some 0.010 to 0.020 inch after the engine is run, as the keepers wedge up tighter to the valve stem and retainer. This is more the case with stock-angle seven-degree retainers and locks and less of an issue with aftermarket 10-degree locks such as Comp Cam’s Super Locks and matching 10-degree retainers.
Think about how the tolerances can stack up against you if the installed height was checked on only one of the valves. Say one valve seat was machined slightly lower, adding 0.010 inch, a spring seat dug a little deeper at the factory adding 0.015 inch, a looser retainer adding another 0.010 inch. Now let’s say the one spring you checked was at 1.875 inches instead of the recommended 1.850 inches, and you figured that was close enough. That one unlucky valvespring could well be installed 0.060 inch too high going in and may be as much as 0.080 inch off by the time the keepers wedge up in running. That’s enough to cost serious spring load. If the springs are on the marginal side, it can add up to a difficult-to-diagnose problem, even though you’d swear the installed height was checked.
Tightening up the installed height is easily accomplished with shims under the springs. The only thing to remember is to get shims that match the outside diameter required of the spring or spring pocket and/or have an inside diameter that securely registers with the machined boss surrounding the valveguide. Shims are readily available for virtually any production applications from supply companies such as Goodson, while aftermarket companies such as Competition Cams carry spring shims with IDs to match custom-machined spring seats. If more installed height is required, special retainers as well as keepers are available which can add 0.050 inch, 0.100 inch, or more to the installed height, but clearance to the rocker may become a problem with some parts combinations. To a small degree, the spring seat on the head may also be machined to gain installed height, but the practice is discouraged, as thinning the material in this part of the head may cause reliability problems.
Getting measurements of the installed height is just sound engine-building practice and isn’t at all difficult to do, even if the engine is already assembled in the car. Minimally, a dial caliper can be used with the spring still in place, measuring from the top of the retainer, down straight to the machined flat area of the spring seat, or shim if they have been installed. Subtract out the thickness of the retainer, and you have the installed height. This will get you in the ballpark and is a useful way to diagnose a valvespring problem and discover if the installed height is way off. A much more accurate check involves removing the spring and then checking the installed height with a valvespring micrometer, providing a highly accurate read of the installed height. Removing the valvespring is accomplished by running a compressed air line into the cylinder (rockers removed) to hold the valves shut, and then using an on-the-car spring compressor such as those available from Competition Cams to remove the spring. Now the height mic can be installed and the installed height verified and corrected with shims if required.