Performance is all about pushing the envelope. As camshafts become more aggressive in the search for more power, these lobes make life increasingly difficult for valvesprings. This may seem like a problem only for drag racers spinning stratospheric-rpm small-blocks, but this situation applies even to everyday street engines. Valve float and loss of valve control can happen even at conservative engine speeds.
Mass Hysteria
This is the new PN 26120 beehive spring from Comp Cams that we used in the 454 H.O. Rat test. Note that the top of the spring is smaller, which reduces spring weight, and drastically reduces the weight of the retainer. This allows more of the spring load to control the rest of the valvetrain as opposed to using that load to control the spring.
The valvespring's only job is to control the valve. This means that the valve should open and close only when the camshaft signals the valve to do so. Roller cams drastically increase acceleration rates, especially compared to flat-tappet lifters, which means that the valvespring must now control a mass that is now moving much quicker at the same rpm. That might seem like a simple thing, but keep in mind that the rocker-arm ratio also multiplies the acceleration rate of the valve. On the opening side of the lobe, the valve accelerates up to its greatest speed and then must start decelerating back to zero velocity at maximum valve lift. Then, the valve must begin to close, accelerating up to a given speed and then back to zero again as the valve closes. All of this has to happen very quickly, especially at high rpm.
All of this sounds fairly simple--and for stock engines, it is. But as we add variables like increased valvetrain weight and engine speed, the situation changes drastically. As engine speed increases, the valve must open and close in a shorter period of time. What is easily accomplished at 3,000 rpm must be completed in half the time at 6,000 rpm. At 6,000 rpm, an intake valve must open and close 50 times a second! Again, if the valvespring is designed, installed, and used properly, this is usually not a problem. But consider what happens when we add weight into this equation with a larger valve. Simple physics tells us that heavier objects require more force to accelerate, just like a heavier car requires more horsepower to accelerate. Adding larger valves means the valvespring must control additional weight, more than likely at higher engine speeds.
Here's the rub. Bump the valvespring pressure up to increase control, and that additional load pushes down on that little piston inside the hydraulic lifter. At higher engine speeds, the acceleration loads are so great that the lifter is no longer able to maintain the oil in the lifter cavity, pumping the lifter down. This creates excessive lash and lost valve lift that quickly kills power. So, there is a limit to the amount of valvespring pressure we can apply to control this heavy valvetrain. Of course, we could bolt in a new mechanical roller camshaft, but that's an expensive solution. Since we can't increase the spring load with a hydraulic lifter, the more elegant solution is to reduce valvetrain weight.
This is a trace generated by Comp Cams' Spintron valvespring test machine, which accurately measures forces on a spring. Loss of valve control (called valve float) generally occurs as the valve bounces off the seat on the closing side as shown in the two traces. Valve bounce on the intake side causes loss of power, because the cylinder is not sealed when the valve is off the seat.
Weight Watchers
Racing-engine builders have known for decades that lighter valvetrain components allow higher engine speeds without loss of valve control. The key is to reduce weight on the valve side of the rocker arm, because the rocker multiplies the lobe acceleration rate by the rocker ratio. This is the idea behind titanium retainers, valvesprings, and ultra-lightweight titanium valves. All of these are excellent ways to reduce the mass of the valvetrain that must be accelerated, but they are also very expensive, and for the most part, race-only solutions. But let us consider the valvespring itself.
For several years, GM has employed a beehive-shaped spring in the LS1 engine. This spring's ovate or oval wire provides more lift without increasing the spring height. It also has a variable rate, meaning that when first compressed, the spring operates with a lower spring rate and progressively becomes stiffer as the smaller-diameter windings compress into the larger coils. Most importantly, the beehive shape reduces the weight of the spring. This is a critical point, because coil springs must use a certain amount of their pressure to control themselves. So, if we reduce the mass of the spring, more of the spring pressure should be available to maintain control of the valve, and it may be able to accommodate a higher engine speed. The beehive shape also uses a radically smaller and lighter retainer. This is important because we're reducing the mass at the top of the spring, which reduces lateral leverage as well. Keep in mind that the top of the spring travels much greater distances and is subjected to greater accelerative forces than the bottom of the spring.