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Secrets Of Camshaft Power

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Lobe Centerline
Tailoring the valve opening and closing points on an actual camshaft is accomplished by varying the lobe centerline location, changing the LDA, and refining the profile shape itself. We'll consider changing the centerline location first. Advancing the cam moves both the intake and exhaust centerlines an equal amount, resulting in earlier valve timing events. Engines typically respond better with a few degrees of advance, probably due to the importance of the intake closing point on performance. For racing, advanced cams benefit torque converter stall, improve off-the-line drag-race launches, and help circle track cars come off the corner.

Cam companies often grind their street cams advanced (4 degrees is typical), which allows the end-user to receive the benefits of increased cylinder pressure, yet still install the cam using the standard timing marks. One exception is Crane's CompuCam series, which varies because of the vacuum signal requirements of the ECMs it's designed to operate with.

Lobe Displacement Angle
Although the installer can advance and retard the lobe centerlines, the displacement angle between the centerlines is ground into the cam at the time of manufacture and cannot be changed by the end-user. Narrow LDAs tend to increase midrange torque and result in faster revving engines, while wide LDAs result in wider power bands and more peak power at the price of somewhat lazier initial response.

A street engine with a wide LDA has higher vacuum and a smoother idle. On the street, LDA should be tailored to the induction system in use. According to Comp Cams, typical carbureted, dual-plane manifold applications like 110-112 LDAs, while fuel-injected combos want slightly wider 112- to 114-degree LDAs. Fuel-injection doesn't require the signal during overlap that carburetors need to provide correct fuel atomization, and most computer controllers require the additional idle vacuum that results from decreased overlap.

Bracket racers with higher stall-speed converters, high compression, single-plane intakes, and large carbs usually want 106-110-degree LDAs.

Engines equipped with blowers or turbos, or used primarily with nitrous oxide, typically work best with wider 110- to 116-degree separations. Race engine speeds have increased over the years causing a corresponding upward creep in LDA and duration.

Duration
Duration has a marked effect on a cam's power band and driveability. Higher durations increase the top-end at the expense of the low-end. A cam's "advertised duration" has been a popular sales tool, but to compare two different cams using these numbers is dicey because there's no set tappet rise for measuring advertised duration. Measuring duration at 0.050-inch tappet lift has become standard with most high-performance cams. Most engine builders feel that 0.050 duration is closely related to the rpm range where the engine makes its best power. Typical daily driven, under-10.25:1-compression ratio street machines with standard-size carbs, aftermarket intakes, headers, and recurved ignitions, like cams with 0.050-inch durations in the 215- to 230-degree range if using a hydraulic grind, or 230- to 240 degrees with a solid.

When comparing two different cams, if both profiles rate the advertised duration at the same lift, the cam with the shorter advertised duration in comparison to the 0.050 duration has more aggressive rmp. Providing it maintains stable valve motion, the aggressive profile yields better vacuum, increased responsiveness, a broader torque range, and other driveability improvements because it effectively has the opening and closing points of a smaller cam combined with the area under the lift curve of a larger cam.

Engines with significant airflow or compression restrictions like aggressive profiles. This is due to the increased signal that gets more of the charge through the restriction and/or the decreased seat timing that results in earlier intake closing and more cylinder pressure.

Big cams with more duration and overlap allow octane-limited engines to run higher compression without detonating in the low- to mid-range. Conversely, running too big a cam with too low a compression ratio leads to sluggish response below 3,000 rpm. Follow the cam grinder's recommendations on proper cam profile-to-compression ratio match-up.

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