Understanding Valve Springs

Understanding Valve Springs

Technical information for differentiating valve springs by shape, as well as an analysis of many spring characteristics.

Valve springs are available in many configurations, but they are generally classified by four primary characteristics:

• Overall spring shape
• Wire cross-section
• Spring rate
• Number of springs in the spring assembly

Understanding these characteristics can help you select the correct valve spring for your engine’s required pressure, lift, RPM range, durability, and operating conditions.

Valve Spring Shape

What Are the Most Common Valve Spring Shapes?

Although valve springs can be manufactured in many shapes, the three most common are straight, beehive, and conical.

Straight Valve Springs

Straight valve springs are the most common design and maintain a consistent coil diameter throughout most of their height. The three primary advantages of a straight spring are:

• Straight springs are generally less expensive because they are easier to manufacture.
• When the cylinder head has a small spring pocket, a straight spring may be the only design that fits the available space.
• Their consistent inside diameter makes it easier to nest two or even three springs together in a dual- or triple-spring assembly.

Beehive Valve Springs

A beehive valve spring maintains a relatively consistent diameter through most of its height and then tapers inward near the top.

The smaller upper diameter reduces the moving mass of the valvetrain. Because the top of the spring and retainer experience the greatest movement, reducing mass in this area can provide a significant performance benefit. The smaller upper diameter also allows the use of a smaller retainer, which can be both lighter and stronger.

The cost of a beehive valve spring generally falls between that of a straight spring and a conical spring.

Conical Valve Springs

A conical valve spring is widest at the bottom and gradually decreases in diameter toward the top. Potential advantages include:

• Reduced dynamic mass
• Improved damping
• Greater resistance to spring surge
• Increased available valve travel
• Improved valvetrain stability
• Better control during valve-float events

Like a beehive spring, a conical spring reduces mass toward the top of the valvetrain. However, the conical shape also reduces the diameter and mass of the intermediate coils, further decreasing dynamic mass.

A conical spring can also provide built-in damping because its coils may become inactive progressively as the spring is compressed. Contact between adjacent coils dissipates unwanted vibrational energy as heat, helping control spring surge and valvetrain instability.

As the spring compresses, each upper coil stacks inside the coil below it. This arrangement can improve spring stability and reduce lateral movement, potentially increasing high-RPM stability and valve-guide life.

Valve Spring Wire Cross-Section

What Is the Difference Between Round and Ovate Valve-Spring Wire?

The two wire shapes commonly used in valve springs are round wire and ovate wire.

Round wire is generally less expensive than ovate wire and is suitable for many applications. Ovate wire is often used when an application requires additional spring travel or a more favorable stress distribution.

The cross-section of ovate wire may be oval or egg-shaped, making it wider than it is tall. This geometry can provide greater stiffness than round wire of the same installed height. This results in the ability to get a stiffer spring, with more allowable lift that fits in the same space as its round wire counterpart.

In addition to the above, a properly designed ovate wire can also distribute stress more evenly through the wire’s cross-section, reducing the peak stress concentrated near the inside diameter of the coil. A lower peak stress directly relates to increased spring life.

Valve Spring Count

What Is the Difference Between Single, Dual, and Triple Valve Springs?

Valve springs are also classified by the number of springs nested together. Depending on the application, a valve-spring assembly may contain one, two, or three springs. Potential advantages include:

• Higher total spring pressure
• Greater resistance to spring surge
• Increased damping
• Reduced likelihood that a single spring failure will immediately cause catastrophic engine damage

Higher Spring Pressure

Some high-performance applications require more spring pressure than one spring can reliably provide. In these cases, one or more inner springs can be added to increase the total force of the spring assembly.

Different Natural Frequencies

When the springs in a dual- or triple-spring assembly have different natural frequencies, they are less likely to experience peak surge at the same engine speed. If one spring begins to surge, the other springs may continue helping control the valvetrain.

Frictional Damping

When a spring assembly is designed with controlled interference between the inner and outer springs, the springs rub against each other during operation. This friction converts vibrational energy into heat and helps damp spring surge.

Additional Protection Following a Spring Failure

If one spring in a dual- or triple-spring assembly breaks, the remaining springs may temporarily retain enough control to reduce the likelihood of a valve or other catastrophic failure. The engine should still be shut down and repaired as soon as a broken spring is detected.

Valve Spring Pressure

What Is the Purpose of Valve Spring Pressure?

The primary purpose of a valve spring is to provide the force needed to keep the valvetrain components in contact with the camshaft and return the valve to its seat.

During valve opening, force travels from the camshaft through the valvetrain and into the valve and spring. When the valve is closed, the cylinder-head valve seat reacts against the valve while the spring applies force through the retainer and valve stem.

This distinction is important because spring force is carried through different components at different points in the lift cycle.

It is also important to note that valve spring force always increases as the spring is compressed.

Seat Pressure

What Is Valve Spring Seat Pressure?

Seat pressure is the force the valve spring applies to the retainer when the valve is closed at its installed height.

When the valve is fully seated, the valve seat supports the spring load and prevents the spring from expanding farther. Consequently, a moderate increase in seat pressure does not load the moving part of a valvetrain system any more. (Disclaimer: The increase in seat pressure does have a direct effect on the pressures seen by the moving parts of the valvetrain once the valve starts to open.)

However, excessive seat pressure can still increase loads and wear in components such as the valve, valve seat, retainer, and locks.

How Much Seat Pressure Does an Engine Need?

The primary purpose of seat pressure is to keep the valve closed whenever the camshaft is not opening it. Factors that may affect this force include:

• Spring surge and valvetrain vibration
• Intake-manifold boost pressure
• Exhaust-manifold backpressure
• Combustion-chamber pressure

The main ones of these to consider are the boost pressure and the exhaust back pressure, especially in high boost applications like sled pulling.

The force created by gas pressure can be estimated by multiplying the relevant pressure differential by the valve’s effective area. Then this gas pressure force should be subtracted from your seat pressure to find the effective seat pressure:

Gas Pressure Force = Pressure × Valve Area

Effective Seat Pressure = Seat Pressure - Gas Pressure Force

The above is a conservative estimate as it doesn’t account for chamber pressure. This is done because it is safer to lean towards a stiffer seat pressure than one that is too low.

Pressure at Full Lift

What Is Valve Spring Open Pressure?

The pressure at full lift is the force the spring applies to the underside of the retainer when the valve is fully open. It is also commonly called open pressure or over-the-nose pressure.

At this point, the spring is at its highest operating load. Unlike seat pressure, open pressure is held up through the active valvetrain—including the valve tip, rocker arm, pushrod, lifter, and camshaft. Excessive open pressure can therefore accelerate valvetrain wear and increase frictional losses.

With this said, an unstable valvetrain caused by insufficient spring pressure will damage components much more quickly than the increased wear that results from an excessively stiff spring.

Increased open pressure gives the spring more force to decelerate the valvetrain as it approaches maximum lift and accelerate it as the valve begins closing. Proper open pressure helps the lifter remain in contact with the camshaft and allows the valve to follow the intended cam profile.

“Minimum Gap”

What Is “Minimum Gap” in a Valvetrain?

Spring performance involves more than seat pressure and open pressure. Spring force increases continuously as the valve opens, while the force required to control the valvetrain changes throughout the lift cycle.

If the available spring force and the force required to keep the valvetrain following the cam profile are plotted against cam or valve position, the distance between the two curves represents the available force margin or “Minimum Gap.” The point at which this margin is smallest is the minimum gap.

This point represents the portion of the lift curve where the valvetrain is closest to losing contact with the camshaft, sometimes called lofting.

There are a few things to consider when thinking about Minimum Gap:

• As engine speed increases, valvetrain acceleration and inertial force increase. This reduces the Minimum Gap until the system can no longer follow the cam profile.
• An unnecessarily large Minimum Gap at peak RPM may add load and friction without providing a meaningful performance benefit. The system needs sufficient—not excessive—spring force to maintain the intended motion.
• Spring surge, component deflection, manufacturing variation, heat, and other dynamic effects consume part of the available force margin. A system with a very small Minimum Gap may become unstable when one of these disturbances occurs.

Valve Spring Rate

What Is Valve Spring Rate?

Spring rate describes how much additional force is produced for each unit of spring compression. It is commonly expressed in pounds per inch.

Spring Rate = Change in Spring Force ÷ Change in Spring Height

A valve spring does not operate only at installed height and full lift. Its force follows a path between these two points, and that path is determined by the spring’s rate.

What Is a Progressive-Rate Valve Spring?

A progressive-rate spring develops an increasing spring rate as it is compressed. This often occurs when selected coils progressively contact adjacent coils and become inactive. With fewer active coils available to deflect, the spring rate increases.

Progressive coil contact can also provide damping by dissipating vibrational energy as heat. This is one reason conical and some beehive springs can provide favorable dynamic behavior.

Valve Spring Resonance and Damping

What Is Valve Spring Surge?

Every time a valve opens, the spring is rapidly compressed and released. This motion creates stress waves that travel through the spring after the primary valve motion has occurred. This behavior is commonly called valve-spring surge.

The stress wave can temporarily increase or decrease the force applied by the spring, potentially allowing the valvetrain to lose control or enter valve float.

This spring surge, if not quickly dissipated through damping, will turn into lateral motion, increasing wear on the valve, valve guide, retainer, and other valvetrain components.

What Is Valve Spring Resonance?

If the excitation created by the valvetrain aligns with one of the spring’s natural vibration modes, successive energy inputs can reinforce one another. This condition is resonance, and it can rapidly amplify spring motion and cause valvetrain instability.

A child on a swing provides a useful comparison. Each correctly timed push adds energy to the existing motion. Similarly, repeated valvetrain excitation can amplify spring motion when its frequency coincides with a natural frequency or a significant harmonic of the spring-and-valvetrain system.

Every valvetrain component has natural frequencies. If multiple components experience resonance within the same operating range, the resulting instability can be especially severe. High-RPM valvetrains should therefore be designed so that critical component frequencies do not reinforce one another within the intended operating range.

How Is Valve Spring Surge Controlled?

1. Controlled friction between inner and outer springs
2. Friction between a spring and a dedicated damper
3. Progressive coil contact
4. Conical or beehive spring geometry
5. Carefully controlled operation near, but not at, coil bind
6. Lower seating velocities

How Do Multiple Springs Affect Resonance?

In a dual- or triple-spring assembly, the individual springs will generally have different natural frequencies. This makes them less likely to experience peak surge simultaneously. A spring that is not resonating can continue helping control the valve while another spring passes through a resonant condition.

A multiple-spring assembly may encounter more individual resonant points because it contains more springs, but each event may be less severe than the resonance of a single spring acting alone. Controlled interference between the springs can provide additional frictional damping.

How Does a Conical Spring Reduce Surge?

Conical springs reduce mass in the fastest-moving upper coils, decreasing the energy available to excite the spring. Their changing coil diameter also produces a broader range of local natural frequencies rather than one dominant frequency. Progressive coil contact may provide additional damping as the spring approaches full lift. There is also increased stability when excessive spring surge is present. As pointed out in the “Conical Valve Springs” section, the coils stack in each other when fully compressed. This makes it so that the spring surge has a harder time turning into side-to-side movement, reducing the damage it can cause to the valve and guide.

Valve Spring Stress and Fatigue Life

Why Do Valve Springs Fail?

Few engine components experience as many severe load cycles as a valve spring. A valve spring must deform and return to its original shape thousands of times per minute, quickly accumulating millions of fatigue cycles.

Valve-spring durability depends on controlling stress and heat while selecting materials and manufacturing processes that resist fatigue initiation and crack growth.

Matching Valve Springs to the Application

At its simplest, a valve spring is a coil of wire that stores and releases energy as it is compressed. Although the spring appears to bend, the wire in a helical compression spring is loaded primarily in torsional shear.

Every spring material has a stress limit. As operating stress approaches that limit, the number of cycles the spring can survive generally decreases. This creates an unavoidable tradeoff between maximum performance and service life.

For example, a spring capable of surviving approximately 10,000 street miles might provide unacceptable durability in a road car but exceptional service life in a drag-racing engine. Installing a race-only spring in a street engine can therefore produce disappointing durability, while designing every racing spring for conservative street use would sacrifice performance.

Application requirements extend beyond the number of operating cycles. Spring temperature, engine speed, lift, acceleration, spring pressure, lubrication, and maintenance intervals all affect service life.

How Does Wire Shape Affect Valve Spring Stress?

Stress is not distributed evenly through a valve-spring wire. The highest corrected torsional stress generally occurs near the inside diameter of the coil.

Wire geometry can be designed to place more material where stress is highest and less where it is lower. This may produce an egg-shaped or asymmetrical cross-section with the thicker portion oriented toward the inside of the spring. This type of wire is commonly included within the broader category of ovate wire.

By distributing stress more evenly, properly designed ovate wire can carry a greater total load without overstressing the inside portion of the wire.

Why Is Valve-Spring Wire Quality Important?

Wire quality is a critical factor in valve-spring fatigue life. Even a small inclusion, seam, or surface imperfection can concentrate stress and provide a location for a fatigue crack to begin.

Manton uses premium-quality spring wire selected for demanding valvetrain applications. The material is processed to minimize harmful inclusions near the highly stressed wire surface. The wire is also inspected for abnormalities so that detectable defects can be removed before the springs are manufactured.

Why Is the Surface Finish of a Valve Spring Important?

The surface of a valve-spring wire is critical because fatigue cracks commonly begin at or near the surface.

When a material is cyclically loaded, portions of its surface experience alternating tensile and compressive stresses. A microscopic surface defect can grow incrementally during each load cycle until the remaining material can no longer support the applied load. Improving the surface of a spring makes it so a crack has a harder time starting, improving the life of the spring.

How Does Shot Peening Improve Valve-Spring Life?

Shot peening is commonly used to improve fatigue resistance. During this process, the spring surface is impacted with small, round media called shot. These impacts plastically deform the outermost layer and introduce beneficial residual compressive stress.

This compressive stress helps counteract the tensile stress that causes fatigue cracks to form and grow. Shot-peening intensity and coverage must be carefully controlled. An inadequate process may not provide the desired benefit, while an overly aggressive or poorly controlled process can damage the surface.

Valve-Lift Limitations

What Is Valve-Spring Coil Bind?

One of the most apparent limits on valve lift is coil bind. As a spring is compressed, its coils move closer together until they contact one another and the spring effectively becomes solid.

Actual coil bind during engine operation must be avoided. Attempting to compress a spring beyond its solid height will cause severe engine damage, including a broken spring, bent pushrod, damaged camshaft or lifter, pulled rocker-arm fastener, or broken retainer.

The required coil-bind clearance depends on the spring design, application, operating speed, and spring manufacturer’s specifications.

Checking Valvetrain Interference

The spring or retainer may contact other valvetrain components before the spring reaches coil bind. Clearance should therefore be checked throughout the valvetrain’s complete range of motion during every engine build.

Some interference points include:

1. Contact between the bottom of the retainer and the valve-stem seal
2. Contact between the retainer and the underside of the rocker arm
3. Contact between the inner coils of a conical spring and the valve-guide boss or spring locator at full lift

The last condition can be difficult to detect because the spring moves dynamically during operation. Adequate static clearance while rotating the engine by hand does not always guarantee that contact will not occur at operating speed.

Rocker-Arm Lift Limitations

Shoe-Style Rocker Arms

Although it is not a valve-spring limitation, edge riding can limit the usable lift of a shoe-style rocker arm.

A shoe-style rocker arm is designed to operate within a specific lift and geometry range. If valve lift or geometry causes the contact point between the valve and the rocker to move beyond the rocker arm’s smoothly finished contact surface, this will quickly damage the valve tip or lash cap.

Roller Rocker Arms

A roller-style rocker arm does not experience edge riding in the same manner and is therefore commonly used in higher-lift applications. However, it still has a finite operating range.

As a roller rocker arm actuates the valve, its roller moves across the valve tip. Increasing valve lift or using incorrect geometry can increase this travel. The roller’s sweep pattern must be checked to ensure that it remains safely on the valve tip throughout the complete lift cycle.

Valve Spring Temperature and Lubrication

Why Is Valve Spring Temperature Important?

Heat is a major factor in valve-spring life. Valve springs generate heat through coils impacting each other, friction between spring components, and contact between coils or dampers. Any energy dissipated while controlling unwanted vibration is ultimately converted into heat.

Some heat is conducted through the spring seat and retainer into surrounding components. In many applications, however, engine oil provides a majority of spring cooling.

This raises an important question: Is every valve spring receiving enough oil to control its operating temperature? The answer depends heavily on the engine and application.

Sprint-Car Valve-Spring Cooling

A sprint-car engine subjects its valve springs to high loads for a sustained period. Under some conditions, normal oil splash may provide sufficient cooling; otherwise, dedicated spring oilers may be beneficial.

A sprint car also experiences substantial lateral acceleration. Oil can collect in the outside valve cover, providing abundant cooling to the outside cylinder bank while moving away from the springs on the inside bank. The inside bank may therefore receive less spring-cooling oil even when the engine contains an adequate total oil volume.

Blown-Alcohol Hemi Valve-Spring Cooling

A blown-alcohol Hemi drag-racing engine may use triple valve springs with dampers, producing substantial frictional heat. However, the engine operates under full load for only a short period, sustaining relatively cool oil.

Hemi cylinder-head geometry creates another consideration. The intake valve springs are positioned higher in the cylinder head than the exhaust springs. The lower exhaust springs will receive more splash oil, while the upper intake springs will receive nearly none.

For this reason, intake-side spring oilers are often especially valuable in racing Hemi applications. Exhaust-side oiling requirements should still be evaluated based on the specific cylinder head, oil drainback arrangement, and operating data.

Street Hemi Valve-Spring Cooling

A street-driven 426 Hemi may use less aggressive springs than a racing engine, but it can remain at elevated operating temperature for extended periods.

When the vehicle is idling in traffic or operating in high ambient temperatures, the engine may be heat-soaked while producing little oil movement inside the valve cover. Because the intake springs are positioned high in the cylinder head, they also will receive limited splash oil under these conditions. Dedicated intake-spring oilers can therefore be beneficial in some street Hemi applications.