Back in the Eisenhower days of the late ‘50s, everyone thought by 2015 we’d be piloting flying cars, living on the moon and sending men to Mars. Clearly that was a bit optimistic, but if a writer had taken a shot at 21st-century performance engines he would have expected at the very least that hot rodders would be using pneumatic or electronic valve control by now. But as we plow midway through the century’s second decade, our engines still rely on valve springs and even pushrods just like our grandfathers’ did a century ago. The big difference is second-century engines spin a wee bit faster.
But do not be fooled – there’s plenty of science in valve springs.. The evolution of cam lobe profiles is really a study in the ability of the spring to control the valve. So this means as valve spring quality has improved, so have engine speeds and durability. As engine speeds increase, all this places even more demand on the valve train to maintain control over the valves.
Engine builders are forever referring to spring catalogs and each spring’s specs, so it’s best to know the terms that will be referenced in this story. Spring rate is expressed in pounds per inch (lbs/in) and is determined by a multitude of factors including wire diameter, overall spring diameter, and the spring’s height. A typical spring might have a rate of 500 lbs/in. As you can imagine, as the spring is compressed, the load increases. A spring’s installed height load is the amount of force created by the spring at a specific height. As an example, a spec of 120 pounds at 1.700 inches means when the spring is compressed to the height of 1.700 inches, it will require a force of more than 120 pounds to open the valve.
It’s important to emphasize that spring pressure, or load, is expressed in terms of pounds of force – which is not psi. The term pounds per square inch (psi) is used to express pressure exerted in all directions as in pressure in an air tank. When expressing spring load, the pounds of force we are talking about are exerted in a single, uniform direction.
Valve spring selection is first determined by the type of camshaft the engine will use. Flat tappet cams do not demand nearly as much valve spring pressure as roller cams, and hydraulic rollers are typically less radical than their mechanical cousins. Much of this demand for valve control is based on lifter velocity rates. The big issue is flat tappets are limited to a finite rate of lift (in terms of fractions of an inch of lift per degree of cam rotation) that is defined by the tappet diameter. Smaller diameter lifters have a lower velocity limit than larger lifters. This gives advantage to Ford lobes, which enjoy a 0.875-inch tappet over the Chevys that are only 0.842-inch. Chrysler engines are better yet with a 0.904-inch diameter tappet. Each larger tappet allows a higher lifter velocity, which increases the tappet’s maximum velocity potential. This can be viewed in terms of greater lift for the same amount of duration.
Most professional engine builders will agree that the installed height–dictated by the cylinder head and valve length–is the most important criteria to begin the process of selecting a spring. The installed height is defined as the distance from the spring seat in the head to the bottom of the valve spring retainer. When a spring is compressed to this height, it creates the seat load exerted on the valve to keep it closed.
Most enthusiasts think that valve float occurs when the spring loses control of the valve and launches the lifter off the nose of the lobe of the cam in a ballistic curve. This is referred to as lofting, which can and does occur. But Spintron research has proven that the most common loss of valve control begins when the valve bounces off the seat upon closing. This is a critical event, because when the intake valve bounces after the desired intake closing point, a certain amount of pressure already building in the cylinder is lost. If this loss of control continues – the valve bouncing several times – more cylinder pressure is vented back up into the intake tract and power drops dramatically. So based on this, seat pressure is a very important part of blueprinting valve springs to ensure that this loss of control does not occur within the engine’s intended power band.
Coil bind is another critical valve spring spec that helps define the total amount of valve lift possible with this spring. Coil bind is the height of the spring in its fully collapsed position. This is an important spec because the spring must be compatible with the overall lift created by the combination of the cam lobe and the rocker ratio.
As an example, take a big-block Chevy with an installed height of 1.900 inches. Our camshaft maximum lift is 0.65-inch. We found a spring in the COMP Cams catalog that seems to fit the requirement for the seat pressure (part no. 26094), but we need to know if it will handle the 0.650-inch valve lift. The coil bind figure for this spring is 1.200-inch. If we subtract the maximum valve lift from the installed height: 1.900- 0.650 = 1.250 inch. This tells us that we will have 0.050-inch of clearance at maximum valve lift before the spring goes into coil bind. COMP’s recommendation is 0.060inch, but many engine builders tell us that they will tighten this clearance for high RPM engines. They’ve found a shorter stack at peak lift tends to help dampen spring oscillations at high engine speeds. Generally speaking, a clearance of 0.050-inch ends up roughly 0.012-inch in between each of the active spring coils.
Retainer to Seal
Besides coil bind, selecting a valve spring also requires a dedicated retainer. Generally, the bottom of the retainer will come closest to the valve seal located on the guide. The best time to measure for retainer-to-seal clearance is when measuring for installed height. This is a simple procedure where the distance between the retainer and the seal should be at least 0.050-inch more than the total valve lift. This prevents the retainer from bottoming on the seal and causing damage to the seal or preventing the valve from opening fully. Otherwise the result could be major damage such as bent pushrods, damaged seals, cracked or broken valve guides and a host of other maladies that will be expensive to repair.
Retainers and Locks
It’s important to always follow the manufacturer’s recommendations for matching the retainer to the springs. This is important not only for the outer spring but also because the step in the retainer is used to locate the inner spring on dual- and triple-spring applications. We could probably do an entire story on just retainers and all the different variations and materials. A critical issue is retainer weight, especially when it comes to big-block engines with large diameter springs where the weight of the retainer is especially important.
A common misperception is that locks use the tang to prevent movement of the retainer. The reality is that the tang is only used to temporarily position the locks. Once load is applied, the taper angle (7 or 10 degrees) serves to nest into the matching angle in the retainer and the whole assembly binds itself in place. The more force applied to the locks, the more load is applied to retain their position.
The important consideration is to choose a lock intended for the size of the valve. With so many different valve stem diameters, from 5/16 to 3/8 and metric sizes like the 8mm LS valve stem size, a specific lock is required for each valve size. Retainers are also intended for a given valve stem size, so choosing these components is critical to ensuring your valve train will function as intended.
When it comes to installing a valve spring, you can run into all kinds of small issues that may require creative solutions. A common issue is a 0.100-inch taller valve is added to the heads, which now makes the installed height 0.100-inch too tall. While purchasing new valves with a lower lock position is one solution, this can be expensive. Another avenue is to use a spring seat. Spring seats are used to locate the spring, but they can also be used to decrease the installed height.
Spring seats are differentiated from basic shims in that seats also locate the spring either from the inside or outside diameter. Some engine builders may prefer one over the other, but the idea is to securely position the bottom of the spring to minimize the chance of it “dancing” or moving its seat location at high engine speeds.
So as you can see, there’s not a small amount of material to deal with when selecting and blueprinting valve springs. If you pay attention to these tips you will be leagues ahead of many other racers who don’t realize how important valve spring selection and installation is to engine performance. But now you know, which makes you powerful in your own right.