Big-Block Bumpstick Blowout

Testing cam profiles in a 454

Steve Dulcich Dec 10, 2004 0 Comment(s)
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01. We had on hand a GM Performance Parts 454 HO crate engine, a mill that is mild in compression ratio at 8.6:1, but features large rectangular port cylinder heads. The basic combo is similar to the 454s used in big-block Corvettes in the early '70s. What differences could be found in cam selection?

Ah, the camshaft: that magic stick that offers the promise of so much extra horsepower, while at the same time creating the fear of transforming a tractable and smooth powerplant into a snorting, lumping, and bumping beast. For some, the mechanical melody of a stout stick is sweet music, with the menacing rat-tat-tat of a long-duration lope being just the prescription for boulevard presence. Others prefer a subtler scheme, stepping up the specification for a gain in mechanical muscle, but not at the expense of down-the-road practicality. The choices in camshafts are virtually endless. With so many possible permutations, where does one begin in selecting a camshaft--be it for a cruiser or bruiser? It helps to know the specifications, and how they play in the engine's operation.

It's no secret that the camshaft profile has a key effect on engine output. An engine produces power largely by virtue of airflow, and flow into and out of the cylinders is via the valves. The camshaft's duty is to provide for the opening and closing of the valves, a function critical to power production. Stripped to its basics, the camshaft's specifications relate to valve action in terms of duration and lift. "Lift" is simple enough to understand, referring to how far the valve is opened off the seat, with the specification provided as a fraction of an inch.

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02. Topping the big-block was a Performer RPM intake manifold from Edelbrock, with an 850 Speed Demon carburetor.

Duration, the measure of how long the valves are held open, needs a reference in order to be quantified. To express this value of "how long," camshaft duration is given in degrees of crankshaft rotation, essentially measuring how far around the crank turns in degrees during the period a cam lobe is lifting the lifter. Duration is checked at a given specification, with "advertised" duration numbers recording the open time from a nominal lift value, such as .008 inch, while another standard is the commonly used duration at .050 inch.Why is it so important to know about duration when selecting a cam? Duration has a dramatic effect on the idle quality and rpm range of the engine. In general, longer-duration cams will raise the rpm at which peak torque will occur, and raise peak horsepower. The drawbacks are losses in low-rpm cylinder pressure, torque, and idle quality. Therein lies the tradeoff in cam selection: go bigger for more bite up top, but too big and the engine will become increasingly hard to live with.

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03. A set of Hooker 2-inch headers was utilized for testing purposes, run with collector extensions and open exhaust.

Revisiting lift: As with duration, stepping it up will generally increase output, and, to a lesser extent rpm capabilities, provided the cylinder heads, intake, and exhaust system are up to the task. However, higher peak lift doesn't come with as pronounced a penalty in low-engine speed operation. Here lies an opportunity to gain a performance advantage.

If the camshaft is designed to open the valves at a more aggressive rate, the lobe can provide more lift at a given duration level. Competition Cams sought to exploit these possibilities with its aggressive Xtreme Energy line of camshafts. These faster than traditional lobe profiles accelerate more quickly and at a higher velocity. The result is the power-enhancing benefit of higher lift while mitigating the effects of excessive duration. Bottom line: It means more power and low-end torque for a given cam-duration "size."




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