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Secrets Of Camshaft Power

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Another method of improving cam performance is to increase the amount of lobe lift. Designing a cam profile with more lift results in increased duration in the high-lift regions where cylinder heads flow the most air. Short duration cams with relatively high valve lift can provide excellent responsiveness, great torque, and good power. But high lift cams are less dependable. You need the right valvesprings to handle the increased lift, and the heads must be set up to accommodate the extra lift. There are a few examples where increased lift won't improve performance due to decreased velocity through the port; these typically occur in the race engine world (0.650-1.00-inch valve lift). Some late model engines with restrictive throttle-body, intake, cylinder head runner, and exhaust flow simply can't flow enough air to support higher lift.

Besides grinding a lobe with more lift, you can increase the lift of an existing cam profile by going to a higher rocker arm ratio. For example, small-block Chevys where the cylinder head runners are not maxed out may benefit from moving up from the stock 1.5:1 ratio to 1.6:1 rockers. But going up more than one tenth in rocker ratio can lead to trouble; there's a limit to how fast you can move and accelerate the valve before the valvespring can no longer control the system. If a profile was a good design with 1.6:1 rockers, it'll probably be unstable with 1.8:1 rockers. The correct solution is to design the profile from the ground up for use with high-ratio rocker arms.

Duration, lift, and LDA combine to produce an "overlap triangle." The greater the duration and lift the more overlap area, LDAs remaining equal. Given the same duration, LDA and overlap are inversely proportional: Increased LDA decreases overlap (and vice versa). More overlap decreases low-rpm vacuum and response, but in the midrange overlap improves the signal provided by the fast-moving exhaust to the incoming intake charge. This increased signal typically provides a noticeable engine acceleration improvement.

Less overlap increases efficiency by reducing the amount of raw fuel that escapes through the exhaust, while improving low-end response due to less reversion of the exhaust gases back up the intake port; the result is better idle, a stronger vacuum signal, and improved fuel economy.

Due to the differences in cylinder head, intake, and exhaust configuration, different engine combos are extremely sensitive to the camshaft's overlap region. Not only is the duration and area of the overlap triangle important but also its overall shape. Much recent progress in cam design has been due to careful tailoring of the shape of the overlap triangle. According to Comp, the most critical engine factors for optimizing overlap include intake system efficiency, exhaust system efficiency, and how well the heads flow from the intake toward the exhaust with both valves slightly open.

Asymmetric Lobes
In the past, both the opening and closing sides of a cam lobe were identical. More recently, designers developed asymmetric lobes, wherein the shape of the opening and closing sides differ. Asymmetry helps optimize the dynamics of a valvetrain system by producing a lobe with the shortest seat timing and the most area. The designer wants to open the valve as fast as possible without overcoming the spring's ability to absorb the valvetrain's kinetic energy, then close the valve as fast as possible without resulting in valve bounce. There are many different theories about how to design the most aggressive, stable profile.

Asymmetric lobes can better tailor the cam to specific cylinder head idiosyncrasies. To optimize airflow, some heads may need a slow opening intake, or a slower-closing exhaust.

Hydraulic lifters can provide quiet valvetrain operation only if the closing velocity is kept below a certain threshold. However, the opening velocity can be higher and still provide quiet operation. Almost all modern hydraulic profiles have some asymmetry.

Dual Pattern Cams
If an engine has equal airflow potential on the intake and exhaust sides, a single-pattern cam is sufficient. When the airflow differs markedly between the intake and exhaust, a dual-pattern cam should be used to balance flow through the engine. In street applications, they help compensate for a full exhaust system. The amount of difference between the intake and exhaust lobes is based on the cylinder head's characteristics, the intake and exhaust system design, and whether the engine is normally-aspirated, blown, or nitrous-injected.

A recent trend in dual-pattern street cams is to use unique-profile intake and exhaust lobes. Not only does the duration and/or lift of each lobe differ, but the overall lobe shape is specifically optimized for use on the intake or exhaust side. Comp's Xtreme Energy series is an example of this approach. The intake profiles minimize seat timing and maximize area, and the exhaust profiles promote excellent scavenging, increased signal, and maximum airflow.

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