By Archie Bosman:
Before starting this valve spring story, I thought I had a grasp of its mechanics, but then as it developed it became ever more complicated until it reached a point where I doubted if I had any intuitive understanding of how valve springs and their attendant valve gear function!
Happily, the accumulated reams of research data were simplified when Dick Boyer entered the picture. Here, courtesy of Erson Cams, are several sound rules to remember. It’s a brief insight developed for those interested in high-performance engine technology that explains the severity of the environment in which the valve spring operates, and some of its relationships with the various functions of the valve train.
Erson suggests that the primary factors considered when selecting valve springs in a racing engine are, first, the amount of valve lift and, second, engine speeds. As engine speed increases so does inertia, which refers to valve train resistance to changes in speed and direction.
With regard to the valve when fully open—at maximum valve lift—most engine builders desire the valve spring to be almost coil bound. Usually, the spring’s top and bottom coils will be touching with a few coils in the middle presenting a tiny gap that cumulatively amounts to 0.050in to 0.060in. This almost coil-bound condition returns the coil spring to a uniform, stable shape on every closing cycle.
If not, the spring exhibits excessive space between the coils and it never relaxes—it constantly shakes and wiggles. Therefore, it could be argued that a valve spring operating at moderate lift that doesn’t close properly is more inclined to ail with premature weakness or breakage than one operating with higher lift that does close properly.
Aggressive camshafts and when they undermine the combination
Beyond these two parameters there is the aggressiveness of the camshaft profile to consider. Race engine builders often refer to an aggressive lobe as a “square lobe” which conveys a somewhat exaggerated mental picture. Aggressiveness in this context means how fast the valve is propelled off the valve seat. If it’s too aggressive, at some point in the engine rev cycle it will loft the lifter off the cam lobe and the valve train assembly will lose control. This is debilitating as it undermines valve spring longevity.
Race engine builder Jon Kaase warns, “When the valve-to-lifter assembly leaves the cam lobe on the opening side and comes crashing down on the closing side, this affects the entire valve train particularly valve springs. It weakens them and can break them. It also injures solid roller lifter wheels as it bangs them against the camshaft lobes.” Consequently, overly aggressive cam profiles have a nullifying effect, most notably when piston speeds outrun the valve events.
Springs are available as singles, doubles, or triples and they are selected by the amount of pressure required for the lifter to follow the cam lobe. Naturally, you use the lightest spring to control the valve—that is, to keep it closed and not allow it to chatter—because the higher the spring pressure, the more power is absorbed by the engine to operate it. If a single spring can control the valve, adding more spring pressure will not generate more power. On the other hand, cylinder pressure will be lost if the valve chatters. Valve chatter or valve bounce means the valve continues to bounce on the seat when it closes. Chatter is often caused by insufficient valve spring pressure or by the valve closing too aggressively—despite, sometimes, the lifter following the cam lobe faithfully.
Seat pressures used with flat-tappet camshafts are usually in the 120lb to 140lb range. However, by employing careful running-in procedures with expensive tool steel flat tappets and camshafts, some engine builders adopt 200lb seat pressures and beyond.
But on Kaase’s Boss Nine hot rod engines, which use a hydraulic roller camshaft and operate with single valve springs, the seat pressure is around 160lb. On their P-51 race engines, which run a solid roller cam and double springs, the seat pressures are 220lb to 230lb. And on their Mountain Motor Pro Stock race engines, seat pressures are generated by triple springs and maintained in the 450lb range. Valve spring open pressures on these engines operate at around 1,200lb.
Erson’s Dick Boyer agrees that the worst thing you can do is to lose valve train control due to insufficient spring pressure. The horsepower loss caused by adding a little more spring pressure is negligible compared to the effects of insufficient spring pressure, which will lead to failure.
Engine builder Kevin Stoa adds, “You could have the best valve in the world, but if it floats it can act like a jack hammer and break.” Often the good name of the valve maker is blemished when the fault lies in inadequate spring pressure or the valve train going out of control. As a consequence the valve can be hammered, as Stoa indicates, until it breaks.
What’s valve float?
Valve float occurs when the valve train is out of control. It’s when the lifters have lost contact with the lobes—when they no longer follow the cam. “If the valve train loses control during a dynamometer pull it is audible”, says Stoa. Instead of the air flow increasing or remaining linear it decreases precipitously. The loss is also apparent in the numbers. “The airflow might be 800cfm but if it encounters valve float it will instantly drop to, say, 500cfm or 600cfm. The fix is not always simple but initially we might experiment by increasing valve spring pressure or reducing rocker arm ratio. Let’s say your rocker is 1.6:1 we might reduce it to 1.5:1.”
Limited Dirt Modified oval track race cars are powered by 360cu in engines that run flat-tappet camshafts with valve lift of around 0.540in to 0.560in and a stock diameter spring. On solid roller cam small-block engines, the valve lift is around 0.700in. These classes, which are often limited to 8,000rpm or 8,400rpm, run valve springs for a season which amounts to approximately 1,500 laps.
Compare these statistics with the aggressive ramps and high lifts of Pro Stock drag race engines, which are checked after every run and when their resilience falls from 450lbs to between 360lbs to 300lb they are renewed. Pro Stock intake valve springs are usually replaced every sixth or seventh run.
Designed for professional and sportsman racers, Erson’s FSP series is available for oval track, drag racing, and endurance events as well as motorcycles. According to Boyer, the use of high quality materials is key to creating successful competition valve springs. “They must be produced from pristinely clean, ultra strong and specifically blended steel alloy wire or else…”