Performance Pushes, Technology Takes Cams To New Levels

Words: Cindy Bullion

Performance engine builders have long turned to the camshaft in their push for more power. While experimentation with duration, lobe separation angle, and profiles produced dyno results, the outer limits grew too aggressive in relation to other components. More experimenting ensued until, like in any scientific field, new technology emerged and the cycle began again. What was once considered aggressive no longer was. So is the case today.

“What a racer may have only dreamed of 20 years ago is now being successfully run at the track,” says Allan Bechtloff of Crane Cams. “It takes some time to do it, but racers will find a way to succeed using new designs and materials.”

A common definition of an “aggressive” camshaft is one that pushes the engine to maximum power potential, with its success depending on whether it reaches that potential without breaking anything.
In the early days, this “aggressive” evolution began with the flat tappet lifter trying to more aggressive lobe designs. When the design was changed to lift the tappet too fast or far, in a given amount of duration, the tappet dug into the lobe surface and caused failure. These more aggressive designs also made the love come to a peak without much radius of curvature at the top.

Engineers then turned to increasing the journal size of the camshaft for a smoother curve at the nose. That, however, required increasing the cam bearing size in the engine block, a risky move if webbing in the block was strong enough to allow for such a modification.

Bechtloff says the desired goal behind each of the camshaft changes was increasing flat tappet velocity, which is the measurement in one-thousandths of an inch that a tappet moves in one degree of camshaft rotation. The further the tappet travels in one degree of rotation, the more aggressive the lobe design.
To overcome the limitations a flat tappet camshaft presented in increasing that velocity, engineers increased rocker arm ratios.

“Let the rocker arm get you the valve lift you are looking for and not try to do it all with the cam lobe,” Bechtloff says of the thinking behind that decision.

The problem, he says, was an increased rocker ratio meant greater load on the cam lobe and lifter. Since flat tappet cams are usually made from an alloyed cast iron, they sometimes could not carry the increased load and failed. Enter the roller tappet design.

“The wheel on a roller tappet can follow a much more aggressive lobe design, with almost no limitation when compared to a flat tappet,” Bechtloff says. “There is only one tangent point of contact between the lobe and the roller wheel, and design can become as complex or as simple as the cam designer chooses.”

Modern roller cam lobes are designed from a series of curves put together to produce the valve motion most desired. Acceleration rates can be changed to get more performance from the engine and help stabilize the valve train, with valve spring characteristics taken into account to enhance both life and valve control. And, since the same laws of physics still apply, increasing the cam journal diameter for a bigger lobe and enlarging the cam barrel for strength are still options.

While these physical attribute changes alone have allowed for higher valve train performance in today’s engines, additional technological advancements provide for increased durability that tempts engine builders to continue pushing the limits.

The quickest way to increase a cam’s durability is to make it from a tougher material. Tool steel, actually available in several grades, has grown to be the most popular material among engine builders with high horsepower goals. S7 tool steel has a high alloy content and is through-hardened to better handle impact, though grooves can be worn in the cam face a bit more quickly than with more conventional materials like the case-hardened SAE 8620 or 9310. Special to COMP cams, ST4 tool steel has a higher carbon content and is less susceptible to wear than SAE 8620 and 9310. At the top of the material spectrum is PM M4, most commonly used in NASCAR-type applications.

COMP Cams valve train engineering group manager Billy Godbold says in addition to newer materials, surface preparations such as nitriding and micro-polishing can increase camshaft durability. Nitriding hardens flat tappet cams through the injection of nitrogen into lobe surfaces and journals, while micro-polishing removes microscopic imperfections in metal wear surfaces.

He adds that testing method and measurement advancements in recent years enable engineers to analyze early in development process how durable a camshaft will be under varying loads, as well as how it will perform in relation to other valve train components.

After all, how “aggressive” a cam is today really comes down to how well a profile fits inside an application given its rpm, rocker ratio, lash, valve train mass, and system stiffness.

“A lobe that might be extremely conservative and safe in a 5,500 rpm application would certainly produce motion that is too aggressive to control in a 7,500 rpm application, especially with flimsy pushrods and rockers or heavy valves,” Godbold illustrates. “In many ways, asking what is an aggressive cam is like asking what a heavy valve is, and will vary considerably from application to application. What would be an extremely light valve in a LS engine would be outstandingly heavy in a CBR600RR.”

The idea behind installing an aggressive camshaft is to increase the tappet velocity so the valve will more quickly open and close.

“We want to get the valve out of the way to induce airflow then slap it shut to stop the airflow, which in theory increases horsepower,” Bechtloff says. “But the valve that is sitting still has mass, and to overcome the inertia of the valve mass, a force will need to be applied to move it.”

That force comes through increased acceleration, by way of the camshaft. But running a camshaft that is too aggressive could end up causing pushrod bending, rocker arm deflection, excessive spring pressure, valve float, and negative harmonics in the valve train. Considering how a cam will relate to other valve train components is a must.

Before beefing up your cam, Bechtloff advises lightening mass elsewhere in the valve train.

“This can mean going to a smaller valve stem diameter, a hollow stem valve, changing to a Titanium material, or doing all three,” he says. “A lighter valve spring retainer is equally important.”

He cautions that in some applications a Titanium retainer can wear from contact with the spring, making a thin-wall too steel retainer that has been heat treated may be a better option.

“You can also reduce the weight of the rocker arm, especially the portion that sits on top of the valve stem,” Bechtloff adds. “One with less ‘nose weight’ is just like using a lighter retainer. But, be careful so you don’t increase its deflection or compromise its longevity.”

Valve springs also add to valve train mass and are likely due an upgrade when installing an aggressive cam. Consider using smaller diameter springs or switching to a beehive design. Then, says Bechtloff, you can look into reducing spring tension.

“Increased spring tension robs power due to the friction and energy it takes to open and close the valves,” he says. “Plus, it wears out parts like cams and lifters. You really shouldn’t run any more spring tension than it takes to keep the valve train stable and in control. Cam lobe design may be a key factor in reducing that tension.”

Your valve train upgrade has now come full circle, landing back at the camshaft. Just remember optimal valve train performance comes through careful matching of all components. Oh, and what may have been the best choices 10 years ago are likely not so today. That’s the nature of evolution.

Sources: COMP Cams, compcams.com; Crane Cams, cranecams.com