Valve Events
Of the different valve events-intake valve opening, intake valve closing, exhaust valve opening, and exhaust valve closing-intake valve closing is the most important in terms of the rpm at which the engine will make peak power. "When you close the intake valve, air stops entering the cylinder," says Billy. "If you close the intake valve early, you will make more low-speed torque because less air escapes back into the manifold at low rpm, since the air has a slower intake-charge velocity and more time per degree of engine rotation to fill the cylinders. At high rpm, you need more time to fill the cylinder, so a later intake valve closing helps tune the engine more at high rpm, where peak power is made. The cam phasers found on L92 GM applications are great because they let you move the cam around to better tune at each rpm range."
Lobe Separation Angle
One of the most misunderstood aspects of camshafts is how LSA affects the power curve. Typically, wider-LSA cams have a wider powerband, reduced maximum cylinder pressure, decreased dynamic compression, and better idle quality and vacuum. On the other hand, tighter-LSA cams have a narrower powerband, increased cylinder pressure and dynamic compression, and reduced idle quality and vacuum. However, there are some exceptions to the rule. "As we found when testing our Thumpr cams, we can widen the power curve on a tight-LSA cam by increasing the exhaust duration," explains Billy. "The downside is that it will greatly decrease vacuum and is not a good path for EFI applications. Taking all these factors into account, it's not surprising that some cams in production motors today are on 120-degree-or-wider LSA."
Spring Pressure
Like choosing a camshaft, going with valvesprings that have produced good results in applications similar to yours will often suffice. On the other hand, if your combo isn't exactly mainstream, it's not a bad idea to seek expert advice to avert potentially catastrophic engine failure. "I know people want some sort of rule of thumb when selecting a set of springs, but unfortunately, it's not nearly that simple," says Billy. Load is just one of the factors that need to be considered when selecting valvesprings. "A lighter (mass) spring with less load is often far better than a fat spring with more load. There is also a tremendous consequence of the spring's natural frequency, the speed it vibrates at when struck by an outside force. This is another place where you should really trust the cam company to tell you what spring a certain lobe profile likes."