Camshafts- Three-Way Cam Lobe Shootout

Each cam required a spring... 

   

  read full caption

Each cam required a spring change to allow us to get the most performance out of each camshaft. Westech's Steve Brul kept the valves in place with compressed air and used a Moroso stud mount valvespring compressor tool to swap the springs.

Three Springs for Three Cams
It would be nice if all the moons and stars aligned in the engine-building world so that one valvespring would work for all applications. Perhaps back in the '20s that was the case, but not now. Because we have three completely different cam designs in a flat-tappet, a hydraulic roller, and a mechanical roller, all three require their own design valvespring. Spring pressure is critical to ensure that the valve is always controlled by the camshaft. Valve float is a common term referring to a loss of control, but for most engines the first sign of trouble is when the intake valve bounces off the seat on the closing portion of the lift curve. This allows cylinder pressure to escape back into the intake manifold, reducing power. Eventually the engine will begin to pop and bang, sounding like an ignition misfire, when in reality it is the valvesprings that have failed. Increasing the spring rate is the most popular solution to this problem, but another fix is to either reduce the rocker arm ratio or reduce the weight of the rocker arm side of the valvetrain, as with titanium retainers. Another excellent investment is thicker-walled pushrods. For small-block Chevys, 0.080-inch-wall-thickness, 51/416-inch-diameter pushrods are very common, but they do cost more. Stronger pushrods tend to deflect less, which reduces the pole-vault effect that can occur at high rpm when the pushrod bends and then launches the lifter over the nose of the cam.

The problem with increasing spring pressure with a flat-tappet camshaft is that too much pressure can literally wipe the lobe right off the cam. This is especially critical during camshaft break-in. For our engine, using the 928 dual springs required us to remove the inner spring for break-in and then install the inner springs after the cam had established its wear pattern. Even then we added extra insurance by using Comp Cams' break-in lubricant, which offers a higher zinc additive package to reduce initial wear on the new cam. When it comes to longer-duration hydraulic, flat- tappet, and hydraulic roller camshafts, the valvespring question becomes a delicate balancing act between maintaining sufficient spring pressure to control the valves at higher engine speeds and avoiding excessive spring pressures that can cause problems.

We also checked pushrod length... 

   

  read full caption

We also checked pushrod length with each cam change to maintain valvetrain geometry accuracy. Because the Dart heads use 0.200-inch-longer valves to fit stronger springs, this required custom-length pushrods for each camshaft. We set the pushrod length to create this position (arrow) on the valve tip with the lifter on the base circle of the lobe.

The beauty of a mechanical roller cam is that it allows the luxury of higher spring pressures, but there are difficulties here as well. Increased spring pressures place higher loads on the valvetrain, causing increased wear, not to mention abuse on those tiny roller bearings in the lifters. One reason for increased spring pressure is the higher engine speed that is part of the equation for a long-duration mechanical roller camshaft. We've included a short spring-pressure chart created with help from Westech's Steve Brul that we used to help us determine the best springs for each of the three different camshafts. These are numbers that Brul has found works for him.

An interesting question surfaced during this testing relative to how much spring pressure a hydraulic roller cam combination could withstand. Keeping this explanation short and simple, too much spring pressure does not really force the hydraulic lifter piston down, as is commonly thought. What really happens is that higher spring pressures tend to deflect the pushrod, which causes valvetrain separation at higher engine speeds when the pushrod pole vaults the valve past the nose of the cam. This clearance in the valvetrain allows the lifter piston to pump up. When the cam lobe returns to the base circle, the pumped-up lifter holds the intake valve open and causes the engine to lose power. Reducing hydraulic roller valvespring pressures to more manageable levels reduces pushrod flex and minimizes lifter pump-up.

*After cam break-in. It is advisable to remove the inner spring on any dual-spring package when breaking in a new flat-tappet camshaft. This is not necessary with roller cams.