Closing Time

The Ups and Downs of Valvesprings

By Bob Mehlhoff

Although the basic purpose of a valvespring is to close the valve while also ensuring that the valvetrain stays in contact with the cam lobe, it must perform this feat under grueling conditions that vary tremendously several times every minute. The expected rpm range, camshaft profile, and cylinder-head design just begin the list of criteria for choosing the right valvespring. Yes, like almost everything in the automotive arena, parts need to be matched to the performance level of the engine to perform their best. Use old valvesprings, ones with the wrong pressure, or install them improperly, and you'll wonder why your engine doesn't perform as well as your buddy's. For this month's "How It Works" segment, we're going to look into the world of valvesprings as they pertain to a conventional Chevrolet engine. As we often mention, volumes could be written on this ever-evolving topic (and have been), but in these next few pages we'll cover the overall qualities of valvesprings and some important things to consider when purchasing or installing your next set.

It's a Spring Thing
In general, a valvespring produces a load based on how much it is squeezed. The valvespring's rate is dependent on the number of coils, wire diameter, shape (not all valvesprings are uniform), and outer diameter (OD), and its pressure can vary depending on how it is installed. A valvespring's rate is stated in pounds per inch (lb/in), not psi as is often mistakenly applied. But most importantly, so that the valvespring works in direct harmony with the camshaft, it is critical that the valvesprings are installed and matched to the camshaft design (profile) and intended engine operation.Each valvespring (fitted with a retainer and keeper) acts to produce constant pressure to draw the valve closed so that it fits tightly against the seat in the combustion chamber. As the engine operates, the valve lifter moves over the nose of the cam lobe, and that movement transfers up to the rocker arm to push the valve open against valvespring pressure. As the lifter travels off of the nose of the cam lobe, the valvespring closes the valve. As the engine speed increases, so does the inertia. Here the weight of the valvetrain begins to play a big role, and if the valvespring is not properly matched and installed to the application, engine power and valvetrain life may suffer. Valve float may occur (even from 3,000 rpm and beyond) as the valve bounces off of the seat upon closing. Because of this, high-dollar race engines often utilize valvetrain components made from weight-saving titanium. To avoid this pitfall, do your homework, choose the right valve springs (typically recom-mended by the cam manufacturer), and install them correctly. Your engine will reap the benefits.

The Spring Opening
The valvespring's open pressure is the pressure measured against the retainer when the valve is at its maximum open point, and it is a function of seat pressure, net valve lift, and spring rate. We can determine the open load pressure if we know the rate and the installed height load. To demonstrate this, we'll use a seat pressure of 115 pounds, a rate of 400 lb/in, and 0.500-inch lift. Now, we add one-half of the spring rate to the seat pressure (200 + 115 = 315 pounds of open pressure). The proper amount of open pressure is required to control the valve lifter as it runs up the opening ramp of the camshaft lobe and quickly travels over the nose of the cam, which causes the valve to move in the opposite direction. If valvespring pressure is insufficient here, valve float may occur and reduce camshaft life as well as upper-rpm performance. Also, excess open pressure may reduce the life of the camshaft. Additionally, as valvesprings age (even if the engine is not being operated) their pressure and performance may be lost.

A stock Chevy small-block valvespring typically uses a valvespring that measures 1.250 inches across. Most performance small-block Chevy heads use a valve-spring with an OD of 1.450 or 1.550 inches. Another benefit of a larger valvespring diameter is that it may allow for larger wire diameters and the added space needed to run an inner valvespring (dual springs). Some single-valvespring setups (typically found on low-performance applications) incorporate an inner valvespring (ribbon), a flat wire that serves to dampen the valvespring's natural frequency that develops at certain engine speeds.

Installed Height
A valvespring's installed height is the dis-tance (measured with the valve closed) from the bottom of the outer edge of the valvespring retainer (where the outer valvespring locates to the spring pocket in the cylinder head). Two simple methods of shortening a valvespring's installed height are a shim in the spring pocket below the valvespring, or using a different type valvespring retainer. Retainers that incorporate a deeper dish will provide added installed height; a shallower dish, less installed height. Another option is to use a valve lock made to change where the retainer is held on the valve stem. The installed height affects what the valvespring tension will be. To see what your valvespring's installed height should be, consult your cam card. And remember that all valvesprings should be checked at a given height to be certain that the spring pressures are equal.

After the valvesprings have been installed, it is essential to check for coil bind. This occurs when the valvespring is compressed so far that its coils touch one another and the spring bottoms out. To measure for coil bind, install the retainer in the valvespring, compress the spring until it coil binds, and then measure from the bottom side of the retainer to the bottom of the spring. This dimension is the coil-bind height and can be measured on the cylinder head with a spring compression tool. With the valve fully open there must be at least 0.050-inch clearance between the coils of the inner and outer springs. This equates to about 0.010 inch between each of the active coils.

Another way to perform this test is by taking the valvespring (off the engine) and compressing it in a vise until it bottoms out. Now measure the height of the valvespring in the vise. The minimum clearance will be just enough to insert a 0.010-inch feeler gauge between each of the active coils. Assume we have an installed height of 1.700 inches with a valvespring that coil binds at 1.100 inches. If we subtract the coil-bind height from the installed height we have 0.600 inch (1.700 - 1.100 = 0.600 inch). Now subtract our 0.050 minimum clearance to get a maximum lift allowance of 0.550 inch. However, running a valvespring this close will be hard on it, so in this example we might even want to add another 0.050-inch clearance limiting our maximum cam lift to 0.500 inch (0.550 - 0.050 = 0.500). If we wanted to run more lift (above 0.500 inch) we could replace either the retainer or the valve to gain more installed height, change to a valvespring that will allow more lift, or machine the spring seat for extra depth.

Conclusion
If you want to make sure that your next engine is making all the power you paid for, don't forget to spend the time to check your valvetrain. Equally as important is that you select and install the valvesprings per the cam manufacturer's recommen-dations. A little time spent here will pay big dividends in the power game.

Valve Float causes* Valvetrain too heavy for the valvesprings
* Worn or weak valve-springs
* Valvesprings mismatched to camshaft profile
* A high rocker-arm ratio that produces excessive valve accelerations