The intake opening (IO) typically occurs before top-dead center (BTDC) and intake closing (IC) occurs after bottom dead center (ABDC). It's also important to discuss the opening and closing points of the valves. But at lower rpms, the added duration opens the valves too long in relation to the piston's position in the cylinder, and the pumping pressure is lost. This is because the added time that the valves are open in high-rpm engines allows the airflow additional time to enter or exit the cylinder. Generally, added duration is helpful in high-rpm engines, but not typically in low-rpm engines. As an example, Howards Cams' small-block hydraulic flat-tappet Max Torque camshaft (PN 112021) Chevy measures 222/232-degree duration on the intake and exhaust, respectively at 0.050-inch lobe lift, whereas the advertised duration is 288/302 degrees. Consequently, the camshaft industry has established a common duration checking point of 0.050-inch lift. Determining the specific point that the lobe begins to move, the lifter off of the base circle (upwards) can be difficult-so most camshaft companies use a standardized checking point to reference lift. DurationД¬amshaft duration is the amount of crankshaft rotation that occurs as the cam lobe moves the lifter off of the base circle it is measured in crankshaft degrees to make it easier to degree (check) the cam to make sure it is positioned properly in the engine. Though these two gross lift examples are close to the upper end of a very high-performance small-block Chevy with a flat-tappet camshaft, the mathematical theory is the same for all engines. If we use the camshaft with the 0.370-inch lobe lift and match it to a rocker arm ratio of 1.7:1, we realize a gross lift of 0.629 inches. For most small-blocks, the standard rocker ratio is 1.5:1, so, by multiplying a theoretical lobe lift of 0.370-inch x 1.5 rocker arm ratio, we achieve a valve lift of 0.555-inch lift (0.370 x 1.5 = 0.555).Д«y replacing the standard rocker arm ratios with a higher-ratio rocker arm, you can add valve lift and some duration. The lift that is typically referred to in a camshaft catalog is the valve, or gross lift, and is achieved by multiplying the lobe lift by the rocker arm ratio. Just how far (mathematically) each lobe raises the lifter is called the lobe lift. This is accomplished on a typical Chevy V-8 with 16 lobes (an intake and an exhaust)-all of them ground to an eccentric shape called a lobe to allow each lifter to raise and fall above a base circle. One of the camshaft's most fundamental tasks is to transform its rotating motion into linear motion or lift. The better we understand how each of these terms affects an engine, the better armed we are to choose the best camshaft. So in this issue, we're going to do some heavy bench racing and delve into camshaft basics.

To allow more airflow into your engine, a camshaft is designed with a profile (or curve) that provides a specific amount lift to the valve. Just like that calculus class you wished you didn't have to take (or avoided altogether), the camshaft in your engine is largely based on the same mathematical rules.

In scientific terms, it's all about the area under the curve. Lift, duration, lobe center, overlap, and much more confer just how the camshaft will or will not alter an (often theoretical) engine's performance personality. And perhaps nothing else is discussed as emotionally as camshafts.

Whether we're talking about axle ratios, compression, carburetor size, timing, or e.t.'s, our hot-rod language sets us apart. It's Greek jargon, or street parlance to the rest of the world, but it often defines us. Camshaft Basics From the February, 2009 issue of Chevy High Performance By Bob Mehlhoff

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