Racing valve springs: Sound rules to remember

Racing valve springs: Sound rules to remember

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 evermore complicated until it reached a point where I doubted if I had any intuitive understanding of how these operations worked!

Dick Boyer of Erson Cams

Dick Boyer of Erson Cams

Happily, the accumulated reams of 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 speed. As engine speed increases so does inertia, which refers to valve train resistance to changes in speed and direction.

valve spring coil bind

Most engine builders arrange the valve spring to be within 0.050in to 0.060in of coil bound, which returns the spring to a uniform, stable shape on every closing cycle.

With regard to the valve when fully open—at maximum valve lift—most engine builders desire the valve spring to be within 0.050in to 0.060in of coil bound. 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 calms down—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

Valve-spring-illus

If insufficient clearance exists between pocket wall and spring, machine pocket with spring seat cutter to prevent binding. By contrast, excessive clearance between pocket wall and spring provokes unwanted, harmful movement. Correct by installing spring cup.

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 means how fast the valve is propelled off the valve seat but if it’s too aggressive at some point it will throw the lifter off the lobe and the assembly will go out of control which is particularly arduous, undermining 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.” Overly aggressive cam profiles can also have a nullifying effect, most notably when the air speed fails to move as fast as the valve events.

Seat pressures

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 is bouncing off 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 the 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 1,200lbs.

Kevin Stoa of KS Engineering

Kevin Stoa of KS Engineering

Leading circle track engine builder Kevin Stoa of KS Engineering says, “The worse 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.

“You could have the best valve in the world,” contends Stoa, “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 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 you 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.

Erson Cams, who is widely viewed as a leader in competition camshafts, has been providing proprietary racing valves springs since the nineteen-nineties. Designed for professional and sportsman racers, Erson’s FSP series is available for oval track, drag racing and endurance events as well as motorcycles. These competition valve springs are created from super-clean, ultra-strong, specifically blended steel alloy of the highest quality.

Source

Erson Cams
Louisville, Kentucky
(800) 641-7920

www.pbm-erson.com

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