BUZZ PHRASE: AREA UNDER THE CURVE
If you've indulged in any bit of camshaft research or bench racing, chances are you've heard the term "area under the curve." It's the Holy Grail of cam design, but what does it mean? The term itself describes what a valve-lift cycle would look like if plotted on a graph, with time in crank degrees running horizontally and valve lift in thousandths of an inch running vertically. The more quickly the valve opens and dwells in that position, the greater the space from opening to closing, or beneath the curve. The payoff, according to Comp's Godbold, is lots of high-lift duration without low-lift duration, making for better performance at all rpm.
ESSENTIAL CAMSHAFT TERMS
The intake centerline is the point of highest lift on the intake lobe, expressed in degrees after top dead center (ATDC). The exhaust centerline is that lobe's highest point of lift, expressed in degrees before top dead center (BTDC). The centerline is used to tie the valve timing to the crankshaft's rotation.
As lift refers to how far a valve opens, duration refers to how long it stays open. The opening time is expressed in degrees of crankshaft rotation, so a cam spec'd out at 230 degrees duration means the valve is being held open for 230 degrees of crankshaft rotation.
Cam makers usually refer to two types of duration:
* Advertised duration is the number of crankshaft degrees that the cam follower is lifted more than a predetermined amount off of its seat. The SAE standard is 0.006 inch. So advertised duration is the number of crankshaft degrees that the valve is open at least 0.006 inch. Not all manufacturers, however, use the SAE standard.
* Duration at 0.050 inch, on the other hand, measures the movement of the cam follower, in crankshaft degrees, from the point where it is first lifted 0.050 inch off the base circle on the opening side of the lobe to the point where it descends to 0.050 inch off the base circle on the closing side. This industry standard is especially valuable for comparing cams from different makers.
A cam lifter, also known as a follower or a tappet, makes direct contact with the cam lobes and follows the contour, or profile, of the cam. We'll discuss this in greater detail later, but there are four types of lifters: hydraulic flat-tappet, solid flat-tappet, mechanical roller-tappet, and hydraulic roller-tappet.
An eight-cylinder Chevy camshaft features 16 eccentrics, or lobes, that are based on a circle (thus its name, "base circle"). The height of the lobe above this radius constitutes the cam's lobe lift.
Lobe-Separation Angle (LSA)
This measurement indicates the angle, in camshaft degrees, between the maximum lift points on the intake lobe and the exhaust lobe. Lobe separation is usually calculated by adding the intake centerline and the exhaust centerline, then dividing by two. For example, a cam with a 106-degree intake centerline and a 114-degree exhaust centerline has an LSA of 110 degrees [(106 + 114) / 2 = 110].
Overlap is the amount of time, measured in crankshaft degrees, that both the intake valve and the exhaust valve are open. This situation happens at the end of the exhaust stroke and the beginning of the intake stroke. Increasing duration or decreasing lobe-separation angle--or doing both--increases overlap. A cam with an exhaust closing point of 4 degrees ATDC and an intake opening point of 8 degrees BTDC has 12 degrees of overlap.
single & dual
* Single pattern: In this type of cam, the intake and exhaust lobe profiles are identical, employing equal amounts of duration and lift. For example, a Comp Cams 270 Magnum (PN 12-211-2) cam has 270 degrees advertised duration, 224 degrees duration at 0.050 inch, and 0.470 inch lift on both the intake and exhaust sides.
* Dual pattern: This type of camshaft has different intake and exhaust profiles. In general, the exhaust lobe will employ more lift and duration to help evacuate exhaust gas from the cylinders. For example, Lunati Voodoo cam (PN 60130) has 261/267 degrees advertised duration (intake/ exhaust), 0.231/0.237 duration at 0.050, and 0.555/0.566 inch lift.
This measurement refers to the distance, measured in thousandths of an inch, that the valve is lifted off its seat at the cam lobe's highest point. Remember, the cam's rotary motion is converted to linear motion through a lifter, pushrod, and finally a rocker arm. Therefore, valve lift equals lobe lift multiplied by the rocker-arm ratio. For example, if we have a lobe that measures 0.334 inch running a 1.5:1 rocker-arm ratio, 0.334 x 1.5 = 0.501 inch valve lift. Running a 1.6:1 rocker arm creates 0.534 inch valve lift.
Q&A WITH NOLAN JAMORA OF ISKY RACING CAMS
CHP: Isky is known as one of the real old-time hot rod outfits. That said, what's new?
Nolan Jamora: Isky still has its hot rod heritage, but in the last five years we've moved to the forefront of tech. CNC, Spintron, cam-design software--in order to stay current, you have invest in technology.
CHP: Where's most of Isky's business coming from these days?
NJ: I'd say 60 percent is hard-core racing and 40 percent is street, and three fourths of that is still traditional small-block and big-block applications. In the last year we've introduced 200 new grinds for the SBC, both hydraulic- and solid-roller. We're now on the LSX stuff.
CHP: What are you seeing in the LSX market?
NJ: Street/strip and drag is where we're seeing growth. There are blocks out there, and cars are coming out of warranty... There's potential.
CHP: What drove Isky to introduce all those new grinds for traditional small-blocks?
NJ: The availability of good-flowing heads and manifolds has never been better. Some of the out-of-the-box heads are fantastic. You need a cam that cam utilize that flow. You can't use a 15-year-old cam. It's the same with big-blocks.
CHP: What else has Isky been working on?
NJ: We found that we have to supply the whole package: lifters, springs, and pushrods. Things have to match. Doing one at a time creates a weak link.
CHP: What information would you like the public to know?
NJ: It's the combination that's important. If you improve your heads, you can go with more cam. But the best tip is to call us. Very rarely will a head guy know the combos, but a camshaft guy has to know what works.
In general, what the OEM does drives the aftermarket. One example of this is the increased cam-core diameter GM employs on the LSX engines. A 55mm core's advantages include its strength over traditional cams, and it is mainly used in more-extreme, high-horsepower drag-racing applications. The deflection under the torsional stress a camshaft incurs can actually change duration values--this happens less with the larger-diameter cam. But is also has a more functional benefit, according to Crane's David Bly. "The whole cam is scaled up, and it allows for more modern lobe profiles." Isky's Nolan Jamora agrees: "We get a constant stream of requests. It's part of the trickle-down effect from newer technology." The larger-core cams won't fit in a traditional small-block, but they will fit in aftermarket blocks. As these pieces become the basis for more builds, more of them will take advantage of the benefits of a 55mm core."
Q&A WITH ED CURTIS OF FLOWTECH INDUCTION
CHP: So, tell us about FlowTech Induction.
Ed Curtis: Most of what I concentrate my business on is working with select dealers that want a valvetrain and heads package.
CHP: And part of that is creating custom cams?
EC: If you call one of the big companies, you're not gonna get a tech who's the designer on the phone. That's where I come in.
CHP: How do you go about creating a custom cam?
EC: One thing is the term "custom cam." If it's in a catalog, it's not custom. The big companies are locked into their lobe designs. I'll go to whoever gives me what I want. Multiple vendors give me multiple choices, and most will also tweak things for me.
CHP: What are people looking for when they come to you for a custom cam or a package?
EC: People want everything. The Power Tour made a big impact, so they want the wheels, the suspension, and a 450hp motor that gets 20 mpg. I handle the valvetrain end of that.
CHP: Besides the multiple lobe choices, what makes your cams different?
EC: Many people only know what happens at 0.050. What I value is area under the curve, though that's an overused term.
CHP: What area makes up the bulk of your Chevy business?
EC: LSX packages are the top movers. The Gen I stuff, most people just pick up a book. They're not looking for that last 10-15 percent. But the high-end guy, the guy who spends $30-40K, does.
CHP: And that's where you come in?
EC: My grinds cost a bit more, though not a lot. I put in a little more time, and it works. It's amazing how many people want a source, and I can work with them one on one to give them a true custom cam.