Test Results
When considering the equation of Force = Mass x Acceleration, some of these results may be intuitive. What is not intuitive is where the results occur-you can only determine the problem areas by testing (though some very advanced and very expensive software can predict some of this). The alarming finding in this scenario is the bounce profile of the Inconel valve at 6,900 rpm. For an LS7 this is in the operating range of the engine, below redline. It is apparent the valve is out of control at this point-so much so at 0.030-inch of bounce off the seat that failures will result if you operate in this range periodically.
Figure 1 clearly shows the heavier solid Inconel valve was out of control at 6,900 rpm, while the hollow-stem valve was stable up to 7,600 rpm. These two valves had the same geometry, except the solid valve is 27g heavier than the hollow-stem valve. The benefit of the solid Inconel valve being able to sustain high temperature is restraining the valvetrain from running at high speed.
Going back to our equation, one solution may be to use a higher load valvespring, another may be to change the deceleration ramp of the cam lobe. Most often a heavier load valvespring will provide the necessary solution, but it may have a different natural frequency or different undesired characteristics. We find that often for hydraulic systems a dual spring may not be the solution either, because the standard diameter spring has a more defined natural frequency. The solution we will test next is a combination of different springs and different lobe profiles. In this application we need the high-temperature benefits of Inconel, so a lighter valve is not an option.
The difference in loft performance between the two valves was not as dramatic. However, the heavier, solid valve had more loft at lower rpm than the hollow-stem valve simply because of its higher inertia load against the spring load.
Loft is the term used when the inertia force of the valve being opened exceeds the load of the spring holding the system against the cam lobe. Loft is not as distinctly easy to define as problematic. Often loft improves performance because you are effectively increasing the lift and total valve open area. Where we find problems are when the loft starts to be excessive and approach coil bind of the spring. Another problem is when the valve is lofted and the spring returns it to the cam profile on top of the nose-often causing another loft or bounce at or near peak lift. When loft motion simply looks like a larger version of the lobe, doesn't get too close to coil bind, and is smooth throughout its motion, we call it "controlled loft"-which can be very beneficial.