“Questions three, six, and seven are more about lift. After three and six let you figure out where you ‘want’ to run the lift, question seven sets where you ‘can’ run the lift with available componets,” he adds. “This often shows you how much emphasis you need to place on finding or developing new valve springs and assorted bits.
“After you have a grasp of the lobe families you can safely run and the general lift range, then you go back and start to nail down duration and centerlines to optimize the valve timing points for a given application. That brings you back mainly to questions three and four. Always remember, the greater the cylinder head flow relative to displacement, the less valve duration you need. Also, it’s important to note the relationship between rocker ratio and valve duration. A general rule of thumb is that point of rocker ratio added results in about +2 degrees duration at the valve. Hence, a 260 at 0.050 tappet duration cam acts about 4 degrees larger with a 1.8:1 ratio rocker arm than it would with a 1.6:1 ratio. Lash goes the opposite direction. Every time you loosen the lash 0.004-inch, the duration will shrink about 2 to 3 degrees, depending on how quick the cam is off the seat -- more aggressive cams change less with lash because they have steeper ramps.”
Rocker Ratio and Lobe Design
Speaking of rocker arm ratios, there is a lot that can be done there to manipulate how the camshaft activates the valves.
“The valve doesn’t know what caused the motion,” Bechtloff says. “It opens and closes, but it doesn’t know what the rocker arm is responsible for versus the cam. In many cases you can use either or both to accomplish what you’re wanting to do, but if you are trying to increase valve movement by increasing the rocker arm ratio, it speeds the valve up out-of-the-way real fast and then closes it fast. The motor usually likes that; it wants the valve to be closed and then when it’s time for it to open to disappear up out-of-the-way instantly so it can flow more air. Then it wants to have the valve magically appear back on the seat when it’s time for it to be closed. It’s that time where the valve is moving up or down from full lift to on the seat that flow through the ports is not as efficient as possible. Using a high rocker arm ratio is good in that it makes the valve move very fast, but whatever it gives you in terms of speed on the opening side, it also does the same thing on the closing side. That means it slams the valves shut which dynamically can cause a lot of issues. It puts a lot of jerk in the spring and can bounce the valve on the seat.
“The rocker arm is just like a teeter totter. It’s a lever, so whatever speed you get opening the valve up you have to live with on the closing side as well. But the cam lobe can be designed in such a way, and most of them are, so that it’s asymmetrical. This means that it can have a quick opening and a gentle closing. If the designer of the cam knows that the intention of the engine builder is to run a rocker with a really high ratio he can design around that so that it sets the valve down very gently. The net speed is a combination of the cam and the rocker arm ratio.”
Running high ratio rocker arms works to achieve the concept Godbold spoke about earlier for maximizing engine performance by minimizing the amount of time the valve is off the seat. At first, that seems counterintuitive, but a camshaft that slowly opens the valves means that the intake closing event has to be backed up all the way well into the compression stroke. Being able to open and close the valves very quickly means you can minimize reversion as well as lost air and fuel to overlap. But ripping the valves open and slamming them closed is extremely hard on the valvetrain and you soon find yourself trying to reach a very delicate balance between power and longevity.
The Camshaft as a System
It’s no secret that in most applications you can make more power by dialing up a more aggressive camshaft, but the downside is you will quickly reach a point where things like springs and valves start breaking. Valve bounce, which releases valuable compression back into the ports, is also an issue when the cam becomes more aggressive than the spring can handle. This is why every cam designer we talked to said you cannot build a great cam in a vacuum.
Instead, you must consider the cam as part of the entire valvetrain system. If the cam is going to chew through valvesprings like a puppy with a new toy, then it isn’t the springs that are bad, you just designed a camshaft that is too strong for the application. Now you need to either dial back on the cam’s aggressiveness or -- ideally -- find a spring that can hold up to the punishment.
Finding or developing new components that can stand up to the pounding delivered by an ultra-aggressive camshaft has become a big part of the winning-camshaft equation. For example, Isky has developed and patented its own high-performance roller lifter that completely does away with needle bearings. Instead, Isky’s EZ-Roll lifters use a solid one-piece bearing that significantly increases the size of the loading area between the roller axle and the bearing. With a more traditional needle bearing lifter Isky was having trouble with the needle bearings giving up when the lifters were mated with very aggressive roller camshafts. But they say the issue is now a nonfactor with their new lifter technology, allowing super aggressive cams be used in many more applications.
The same holds true for Comp Cams’ new 5/16-inch pushrods with a thicker 0.105-inch wall. Thanks to Spintron testing we’ve known for years that stiffer pushrods can improve valve control in practically any application. But the answer has usually been to go with a bigger pushrod. But for many stock car racing classes that require an untouched head or even stock diameter pushrods, a 5/16 stick was as big as you could get. Comp’s new 0.105 thick wall pushrods are nearly 20 percent stiffer than the standard 0.080 wall pushrods allowing much greater options when it comes to camshaft choice.
“When I recommend a stiffer pushrod I’ve had engine builders resist and tell me, ‘Well, I’ve never failed a pushrod,’” Godbold says with a laugh. “But that’s not the issue. The stiffer pushrod cuts down on valvetrain deflection and helps us be more accurate controlling the valve timing events.”
Godbold says that when Comp began encouraging engine builders to use stiffer pushrods they actually got reports back that the engine builders didn’t like the pushrods because they were losing power on the engine dyno. After doing some research they discovered that over the years those engine builders had actually found cams that masked the effects of pushrod flex almost by accident. When a pushrod bends or flexes it delays valve opening which effectively shortens the cam’s duration. So when stiffer pushrods were used with the same cam, the engine saw for the first time cam’s true duration, which turned out to be too much for the application. The engine builders were able to gain back all the power they lost -- and actually make more on top of that -- by switching to a smaller camshaft.