by Chirag Asaravala

The average enthusiast fails to give adequate consideration to valve springs. This is in part due to the abundance of fully-assembled aftermarket cylinder heads which give an enthusiast a false sense of security by the fact that they come preassembled with new valves, springs, and retainers. The fact is the valvespring is the single most critical assembly in the "success" of a well built and well tuned engine combination. By success I mean the likelihood that the engine will produce all of the power that is represented by your carefully selected combination of displacement, camshaft, cylinder head flow and induction. It is quite often the valvetrain that is the culprit when an engine fails to meet its projected power levels.

How Valve Springs Effect the Engine

Thomas Griffin leads the R&D group at Comp Cams. He has a Masters degree in Mechanical Engineering and spent nine years in Comp's valvespring program, performing a variety of valvetrain dynamics testing..

Most gearheads are well versed on the relationship of cam specifications to engine rpm range and consequently to horsepower. As a result they are able to deduce that the faster you spin a motor, and the greater the valves are opened, the stiffer a valve spring needs to be to control that valve. The understanding quite often ends there - at the confirmation that open and closed spring pressures are adequate for the camshaft they have selected. But this is not enough, and if we are to limit our understanding of valvetrain function to only the idea that the spring must control the valve without "valve float" then we are surely leaving power on the table. There is much more to a valve system, and with the expertise of Thomas Griffin, head of R&D at Comp Cams, we're able to convey those ideas to you in this article.

As with many other areas of engine and vehicle technology, much of the understanding and innovation begins on the race track and then is extrapolated and adapted for the general consumer. Valvetrain understanding is no different. In racing classes where camshafts are bound to a maximum lift rule, engine builders are forced to find ways to make power. As a result camshafts are designed with very aggressive lobe profiles that essentially launch the valve open over the nose of the cam lobe. This can yield measurable increases in dynamic lift and duration. However, the tradeoff is that a stiffer valve spring is needed to bring the valve back to the seat.

Builders quickly realized that while they could create lobe ramps to aggressively move the tappet and valve, there was a point of diminishing returns due to the weight of the spring resulting from the need for more spring pressure. R&D engineers at companies like Comp Cams were of course inspired to help meet their customers' requirements, and conducted research of their own. What they learned was an understanding of the snowball effect that results from increasing spring pressure. Going to a stiffer spring usually means a larger diameter spring. This, in turn, necessitates a larger retainer. The stiffer spring will likely also mandate a stiffer pushrod. So now not only has spring pressure increased, but so has weight and resultant force required to move the valve. At 5000 rpms the effects are drastic. The force required to move the valve off the seat and to compress the spring are so great that pushrod and camshaft bending occur. As a result the camshaft must rotate further to get the valve off the seat - resulting in reduced lift and duration. The end result- increasing cam lift in this way does not result in a linear increase in power. After a point you need to make proportionally larger increases in lift to get the same amount of gain. Another way to think of it; opening the valve another .020" may net 20 horsepower, but the next .020" may be just half that gain because of the parasitic losses from the stiffer (and heavier) spring.

It's just a few grams!
It's easy to dismiss all this because of the fact we're talking about a small amount of weight. After all, relative to a 500 lb. motor, something that weighs as much as a nut or a bolt seems so insignificant. However in the context of a valvetrain that is moving thousands of times per minute, a few grams exerts an exponential amount of force and inertia. While it takes crankshaft power to overcome these forces, the losses don't end there. The heavy spring pressure can collapse the lifter at high rpms, resulting in an uncontrolled valve - at best this limits engine rpm, at worst the pistons hit the valves and mayhem ensues.
There are other downsides as well, in the form of heat and metallurgical stress on all components between, and including, the rocker arm, spring, pushrod, tappet and camshaft. It is not at all uncommon for any of these components to fail simply because of spring pressure.

A Better Design
The limitation, until recently, has been linear-rate coil springs. These are springs that have a consistent rate throughout their compression - determined by the thickness of the wire, the number of coils, and the overall mean diameter of the spring. When a racer needs more spring pressure this means slightly thicker wire, usually an increase in diameter, and sometimes an increase in height in order to maintain adequate distance between the wires to prevent coil bind. In all cases the spring becomes heavier, adding to the snowball effect.

To end this vicious cycle, engineers at Comp began experimenting with progressive rate springs. Such springs vary in their pressure as a function of compression. Whereas a linear-rate spring will exert, for instance, 200lbs of pressure for every .100" compression, a progressive rate spring may exert 200lbs the first .100" of and 250lbs the second .100". This is achieved by altering the spring diameter over the length of the spring, the spacing between coils, and even the shape of the wire (ovate versus traditional round wire). Continue