Introduction:
Some have theorized intake valves in supercharged engines require greater seat pressure (valve closed) then naturally aspirated (NA) engines therefore stronger valve springs are required. This article discusses a theory regarding valve spring requirements and documents dyno results as realized in the supercharged Ford F150 Lightning.
Valve Spring Requirement:
Valve springs return the valve head to its seat after being opened by the cam and holds the valve head against the seat until the camshaft opens the valve again on the next cycle. The valve spring needs to be strong enough to overcome the inertia of the valve train so the valve follows the contour of the camshaft for proper valve action. As RPM increases the inertia increases at the square of rpm requiring much stronger valve springs in high RPM engines. The Valve Train in a push rod engine consists of the valve, valve spring, lifter, push rod and rocker arm. In an overhead cam engine only the valve, valve spring and cam follower are involved. Since there is less mass to overcome in an overhead cam engine the valve springs can be weaker. Engines manufacturers typically install the weakest valve spring possible that will properly close the valve throughout the desired RPM range of the engine. Engine manufacturers use valve spring strength to limit valvetrain wear and useful RPM power band thus designing in a RPM governor all of which improves reliability.
Additional Valve Spring Requirements:
There are two distinct additional requirements a supercharged engine places on valve springs. These requirements are not recognized by leading valve spring manufacturers or most race engine builders and are therefore a new concept to most people looking for increased performance. Firstly supercharged engines require stronger valve springs then NA engines do to a force applied to the valve head that is associated with manifold pressure and the engine’s volumetric efficiency (VE). If for instance a supercharged engine running 20-psi manifold pressure has a volumetric efficiency of 100% when the piston reaches the bottom of the intake stroke and the intake valve closes there will be 20-psi on both sides of the valve head, i.e., 20-psi on the intake manifold side and 20-psi on the cylinder side because the cylinder fully filled with air during the intake cycle. Since the air pressure in this case are the same on both sides of the valve no force is applied to the valve head. If however the engine has a volumetric efficiency of only 50% there will be 20-psi pressure on the manifold side but only 10-psi on the cylinder side when the valve closes, because the cylinder did not completely fill, a net pressure of 10-psi will be present across the valve head. If the intake valve has a surface area of 2.5 square inches a force of 25 pounds (2.5x10) will be generated that will be trying to open the valve thus reducing the effective valve spring pressure by 25 pounds. Since engine manufacturers install valve springs that are only strong enough to support proper valve action to their RPM limit any reduction in effective valve spring strength will lower the useful power band RPM limit. If the valve spring does not have enough strength to keep the valve following the camshaft profile the valve is said to “float”.
The second additional requirement arises during the exhaust cycle. During the exhaust cycle the cylinder pressure drops to zero or can even go negative do to exhaust header-savaging effects. During the exhaust cycle the intake valve spring must be strong enough to hold the intake valve closed against manifold pressure. If the manifold pressure is 20-psi and the valve head has 2.5 square inches of surface area the force attempting to open the valve will be 50 pounds (2.5x20). If the intake valve opens during the exhaust cycle the air/fuel intake charge will flow through the intake valve and out the exhaust valve during the exhaust cycle and will not be used by the engine to develop power.