Camshaft Profiling - Performance Profiling

The info you need to pick the right 'stick

By John Nelson

Consider the camshaft. On the one hand, it's a simple thing, a stick with bumps on it. It sits above the crankshaft, linked to it by a timing chain, and spins at half the crankshaft's speed. The camshaft's rotary motion is converted, through a lifter, pushrod, and rocker arm, to linear motion. Voila, the valves open and the engine operates. If only it were that easy. The configuration of those bumps--more accurately referred to as lobes--defines the performance personality of an internal-combustion engine. By determining when an engine's valves will be open, and how long they will stay open, the camshaft determines how much power an engine will make. It also dictates where this power will be made throughout the engine's rpm range. And that's just for starters. The camshaft is often called the brains of the engine, and that brain determines what kind of powerplant you will have. In the next few pages, we'll give you the information you need to pick a camshaft that will create a powerplant you can live with.

What we can't do is give you a handy-dandy formula for choosing the right cam, because there isn't one. Camshaft selection depends on a host of variables. Engine displacement, compression ratio, type and size of cylinder heads, carb size (or are you running EFI?), and exhaust-system specs are all crucial to picking a cam that optimizes engine performance. But unless you're building an engine that will only see dyno duty, that's just the start. How much does your Chevy weigh? On top of that, transmission type, torque-converter stall speed, and rear gear ratio are all critical when it comes to choosing a camshaft. Accounting for all these variables could drive a gearhead nuts, yet it's something more basic that trips most of us up.

The first step in choosing a cam is, quite simply, honesty--brutal honesty. "Motor specifics are important, but so is setting out your purposes and realistic goals," says Lunati's James Humphrey. "The most common issue we see is that people don't think about where they're heading," chimes in Comp Cams engineer Billy Godbold. We all like to think we can make use of a car that makes 500 hp at 6,000 rpm, but is this really the case? Is the vehicle in question meant for bracket-race duty, or is it a Pro Touring creation for a cross-country trip? If it's a street/strip car, how much of its time will be spent on the strip and how much on the street? Or are you in the performance-street category, just looking for a bit more kick in your daily driver?

With intent in mind, take it a step further and ask yourself what your goals are. Do you need low-speed torque, or do you really need high-rpm power? Is idle quality important? Custom cam grinder Ed Curtis of FlowTech Industries sums it up better than we could. "You need the power where you're gonna drive it," Curtis tells us. "You want to give up the peak for a nice power curve." In other words, pick a cam that will put the power where you can use it.

Once armed with this information, you can go on to make a selection, but again, a little honesty goes a long way. "The biggest mistake we see is mismatched combinations," says Isky Racing Cams' Nolan Jamora. "Someone reads an article, and hears from a friend, and finds a manifold on sale... They think they have the best combo, but it doesn't work." This statement strikes to the heart of the issue. The camshaft is important, but the overall combination-including the camshaft--is even more important. Here's the good part: Every cam company we talked with wants to talk to you about your combo and picking the right cam for it. "There are too many resources for people to have to do this alone," declares Comp's Godbold. "Get advice," echoes Lunati's Humphrey. "You can't have too much information." We agree, and in that spirit, here's our camshaft primer.

How Long is too Long?
This is a subjective question, of course, based on the requirements for a specific combo and its intended usage. That being said, duration--the amount of time the valves are open--has a dramatic affect on engine performance. Stock camshafts usually offer relatively short duration and lift figures; for a small-block, we're talking less than 200 degrees duration at 0.050 and less than 0.450 inch lift, done in the interests of a smooth idle and optimal part-throttle response. What happens when we change this? Let's focus, for a moment, on the intake side. Longer intake duration opens the valve sooner in the cycle and closes it later. At lower engine speeds, this means the intake valve is still open as the piston begins to move upward, pushing fresh air/fuel mixture out of the cylinder. On top of that, cylinder pressure can't build until the intake valve closes. The result is an engine with poor low- and midrange response. But the late-closing intake valve becomes an advantage at higher engine speeds, when the higher air-inlet speeds keep filling the cylinders, even with the late-closing intake valve. In other words, increasing duration shifts an engine's torque curve higher in the rpm range. Choosing a performance cam becomes a big compromise; when it comes to power, you have to give some to get some. Each engine combo will work best with a particular amount of lift and duration, so the cam swapper's goal is to create that combination. Taking the conservative route with duration figures may not yield the most peak horsepower, but it will improve and maintain an engine's torque band, improving acceleration and throttle response across a wider rpm range.

Ramp It Up
Increasing valve lift allows more air/fuel mixture to enter the cylinders and more exhaust to exit, which generally leads to an increase in engine performance. Theoretically, doing this without increasing duration would create more power without altering the characteristics of the power curve. In reality, an increase in lift almost always leads to an increase in duration, because increasing lobe lift requires more distance to create the opening and closing ramps. And as we've noted elsewhere, more duration is not necessarily a good thing. This paradigm is being altered, however, by today's computer-designed, asymmetrical-profile cams. "Our asymmetrical cams have a high-acceleration opening ramp, while the closing side sets the valve down softly, helping longevity," says Isky's Jamora. "The idea is to have instant opening, dwell at the top, set it down." How is this accomplished? A cam's opening or closing ramp may look like it's a single section, but computer programs enable designers to get much more sophisticated. "With software, we can design a ramp with six or seven sections," observes Jamora.

Camp Cams' Godbold echoes these ideas. "Before, we had to make everything on the lobe faster," he tells us. "Now we look at every section of the lobe. We can be quick off the seat and create a lot of area under the curve. It makes a cam little and big at the same time." Crane had similar ideas in mind with its Z-cams. In addition to getting the valves open as quickly as possible, David Bly discusses Crane's goals when it comes to duration. "We want to keep our advertised duration number as small as possible, and our 0.050-inch duration figure as large as possible. The idea is to have a camshaft that 'acts' big but also improves low- and midrange power."

Open & Closed
A camshaft's function can actually be reduced to four points: intake opening (IO), intake closing (IC), exhaust opening (EO), and exhaust closing (EC). Intake closing is crucial, since it does the most to establish where peak torque occurs. An early IC improves low-speed torque, but limits high-rpm power since it also limits time for cylinder filling. On the other hand, a later IC allows more time for a cylinder to fill at high rpm but limits low-end torque, since cylinder pressure is pushed back through the intake port. Intake opening plays a big part in establishing overlap (when both intake and exhaust valves are open). An early IO increases overlap and can lead to a sluggish engine, since the intake charge is contaminated with exhaust gasses. A later IO reduces overlap, improves idle quality, and increases low-speed torque. Exhaust opening ranks second only to intake closing in affecting engine performance. An early EO can limit low- and midrange power by allowing torque-creating cylinder pressure to escape but help high-rpm performance by creating more time for exhaust gas to be expelled. Exhaust closing also affects overlap. An early EC reduces overlap. The time that both the intake and exhaust valves are open is reduced, improving idle but limiting midrange power. A late EC increases overlap, which hurts idle but helps high-rpm power. You'll find all these figures on a typical cam card, which can usually be downloaded from the manufacturers' Web sites before you buy. After that, it's all about deciding what characteristics you want your ride to have and choosing accordingly.

Take Your Pick
It's the lifter dilemma: Flat-tappet or roller, solid or hydraulic? All have their places in today's performance world, and price is often the deciding factor. Here's a quick rundown, with comparative price samples from Summit Racing.
* Hydraulic flat-tappet: This type of lifter is self-adjusting, thanks to a valve-controlled plunger inside its lifter body. Engine oil pressure maintains preload against the pushrod, and since no lash allowance is needed, they run quieter than mechanical lifters. On the other hand, they can perform poorly at high rpm due to an inability to bleed down excessive oil pressure. Plan on paying about $170 for a cam and lifters.
* Mechanical flat-tappet: This type of lifter is also called a solid, since that's essentially what it is--a solid link between the cam lob and the pushrod. They allow greater rpm potential, since bleed-down is not a concern, but do require that lash, or clearance, be set between the valve and rocker arm to allow for expansion as the engine gets hotter. You're looking at around $200 for a cam-and-lifter set.
* Mechanical roller-tappet: This type of tappet allows for the most-aggressive lobe designs, due to the roller follower, and lends itself to high-rpm operation, since it's a solid body design. Like its solid flat-tappet brother, it requires a lash setting that must be readjusted over time. A retrofit mechanical-roller cam-and-lifter kit costs about $550.
* Hydraulic roller-tappet: This type of tappet, used in OEM small-blocks since 1987, allows for aggressive lobe designs along with self-adjustability and quiet operation. They can, however, also suffer from bleed-down at high rpm. The tab jumps to almost $700 for a retrofit cam-and-lifter set.

One of a roller cam's greatest advantages is that rolling frictional forces are less than those caused by the "sliding" of a flat-tappet cam, which frees up some horsepower. In general, roller-tappet profiles can be more aggressive; in other words, more lift can be employed for a given duration. On the other hand, a flat-tappet cam actually accelerates the lifter more quickly in the initial portion of the lift curve. With a short-duration cam, a flat-tappet cam can actually get to a higher lift faster than a roller cam. Again, it all comes down to defining priorities and determining what will work best with your combo.