Words And Photos: Richard Holdener

Ask an engine builder about the heart of any performance motor, and likely as not, they will tell you it’s the camshaft. Short of the intake manifold, no other single component affects the shape of the power curve as much as the ole bumpstick.

In testing on our big-block 454 Chevy, replacing the stock cam can improve power production by 50 hp or more. That represents a pretty big change in power output from a single component.

For some, camshaft technology continues to be shrouded in apparent mystery, as improper camming is second only to over-carburetion on the list of performance blunders. While it is true that there is plenty of science and technology involved in designing (and selecting) the ideal camshaft for a given engine combination, answers to all your technical (and not so technical) questions are but a phone call away. While most enthusiasts may not be qualified to select the perfect cam for their combination, the cam experts at COMP Cams certainly are. Rather than select a cam based on a hunch or what their brother’s friend might be running (in a different combination no less), some simple information to the right ears can save you plenty of frustration.

To demonstrate the effect of cam timing, we selected a big-block Chevy test motor fresh from the junkyard. This test revolved around the effect of lift, as in is more lift really better? The test compared a pair of camshafts with otherwise identical duration figures, but sizable changes in lift. The tested high-lift cam offered .080 more lift, a significant amount and one that required not only the right cylinder heads but the right valve spring package.

It seems like common sense if you have cylinder heads that continue to flow well at .600 lift, then the motor should respond well to a .600-lift cam. If, on the other hand, the flow drops off after .550 lift, then maybe a .600 lift cam is not only a waste of time, but might actually be detrimental to power production. Understanding the relationship between cam lift and head flow, we chose the extra components accordingly for our test motor. If we planned to run a .600-lift cam, we better have cylinder heads that flow well at (and beyond) .600 lift, right?

Optimum performance only comes when all of the components of motor were designed (or engineered) to work together as a team in a desired rpm range. Adding what might be considered a wild cam profile to an otherwise stock motor will usually result in less than desirable results. The stock heads and intake start to sign off well before the motor can take advantage of the wilder cam timing.

As luck would have it, we had no such problem with our 454, as the available piston-to-valve clearance with the stock pistons (insufficient valve reliefs) limited us to reasonable cam profiles. In fact, the duration figures (224 intake/230 exhaust) were right what we would chose for a mild, street BBC application. The P-V clearance issue was compounded with the installation of the Speedmaster as-cast, oval-port, aluminum heads. Despite the large combustion chambers, the heads featured a 2.25/1.88 valve package that further reduced available clearance. What the heads did provide was considerable flow compared to the stock iron heads. Peak flow (at .700 lift) checked in at over 330 cfm, meaning these heads could support well over 650 hp on the right application. Tested at .600 lift, the heads still flowed 315 cfm, so they looked to be a good match for our pair of cams.

After installation of the Speedmaster aluminum heads (with Fel Pro head gaskets and ARP head bolts), the 454 was completed with a Speedmaster dual-plane Eliminator intake, Holley 650 XP carb, and Hooker long-tube headers. Of course, the lead characters in this test were the pair of Xtreme Energy cam profiles. First up was the low-lift, XR274HR. This cam combined just .510 lift (intake and exhaust) with the mandatory (for this test) 224/230-degree duration split and a 110-degree lsa. The cam was run with the stock hydraulic roller lifters, COMP pushrods, and 1.7-ratio High Energy roller rockers. Equipped with the low-lift cam, the BBC produced 435 hp at 4,900 rpm and 515 lb-ft of torque at 3,700 rpm. After installation of the high-lift XR271HR cam (.591/.610 lift split and wider 112-degree lsa), the peak numbers rose to 446 hp at 5,300 rpm and 511 lb-ft of torque at 3,700 rpm.

The 454 definitely benefited from the high-lift cam, but a close look at the dyno curves (see graph) reveals that the low-lift cam actually offered (slightly) more power up to 4,300 rpm. We suspect some of the change came from the difference in lsa, but we know that the test motor also responded well to the increased lift. The rule of thumb is, if the head flows well at the intended lift, chances are better that the motor will respond to the increase.

Not surprisingly, given identical duration figures, the power curves offered by the two COMP hydraulic roller cams were similar. The one wild card in this test was a change in lobe separation angle, as the high-lift cam also offered a slightly wider lsa. This tends to increase power production higher in the rev range, while trading low-speed torque, though a power peak below 5,500 rpm can hardly be called “high.” Equipped with the COMP XR274HR cam (.510 lift, 224/230-degree duration split, and 110 lsa), the 454 produced 435 hp at 4,900 rpm and 515 lb-ft of torque at 3,700 rpm. After installation of the high-lift, XR271HR cam (.591/.610 lift split, 224/230-degree duration split, and 112 lsa), the aluminum-headed, Gen VI 454 produced 446 hp at 5,300 rpm and 511 lb-ft of torque at 3,700 rpm. The high-lift cam with wider lsa offered improved peak and top-end power, but the low-lift cam offered slightly (4-6 hp) more power up to 4,300 rpm. The difference at the top of the rev range was 15 hp.

Sources: COMP Cams, compcams.com; Holley/Hooker/NOS, holley.com; LKQ-Pic Your Part, lkqonline.com; MSD, Msdignition.com; Speedmaster, Speedmaster79.com