In last month's Motor Series installment, we discussed maximizing engine efficiency by optimizing an engine's compression ratio, combustion process, and cylinder sealing. This month, we'll focus on camshafts because they can increase an engine's efficiency by improving its ability to breathe. Camshafts are a fundamental ingredient to engine performance for two reasons: First, they improve airflow through the engine by optimizing the induction and exhaust cycles. Second, cams act as the engine's brain and personality because they control the timing and duration of most major events during engine operation. Regardless of whether your engine is small or large in displacement, camshafts are the principal means by which you can tune the engine's horsepower and torque curve to give it the personality you desire. If you want to increase low-end torque to launch a heavy bike from a stop, or top-end horsepower for greater top speed, choosing the appropriate cam configuration will help you achieve the desired performance results. But keep in mind that the bigger-is-better theory does not apply to camshafts as it typically does to displacement.
Big Twin Evos, Shovels, Pans, and Knuckles use a single camshaft with four lobes, one lobe for each valve. This makes for a tidy package but requires the pushrods be at a steep angle, which reduces valve-timing accuracy. The cam on the left is a high-performance bump stick with broader lobes, which increases duration. The cam on the right is a stock Evo
Big Twin Evos, Shovels, Pans, and Knuckles use a single camshaft with four lobes, one lobe
The primary factors for cam selection are engine displacement, mechanical compression ratio, and the most important: rpm range. However, other factors such as induction and exhaust airflow, stroke, rod length, rod-to-stroke ratio, gearing and total bike weight also come into play. Once these factors are known, camshafts with appropriate variables can be selected. Common camshaft variables include valve lift, duration, overlap, and the timing of the valve events (points at which the valves open and close). In the case of racing camshafts, rate of lift is also a factor. Nevertheless, these variables are critical in determining not only an engine's maximum power output but also the shape of its power curve.
FundamentalsEach camshaft includes one or more elliptical lobes welded or machined to the shaft. The purpose of a lobe is to open and close an intake or exhaust valve. The shape and location of the lobe on the shaft determines when the valve opens, how high it lifts, when it closes, and how rapidly the valve movement takes place. For Harley V-Twin engines, which have four valves (two per cylinder), there are four cam lobes: two intakes and two exhausts. The Sportster engine has four camshafts, each with one lobe, while the Twin Cam engine has two camshafts with two lobes each. Conversely, the Big Twin Evo, Shovelhead, Panhead, and Knucklehead engines have one camshaft with four lobes. Although the V-Rod engine is somewhat similar to the Sportster in that it uses four camshafts, it requires eight lobes (two per shaft) because it has four valves per cylinder or eight valves total.
As an aside to this discussion, it is interesting to note that the V-Rod engine uses overhead camshafts, which places the cams in the rocker box area located above the head and valves. In contrast, all other previously mentioned Harley V-Twin engines are pushrod engines where the camshaft(s) is placed near the crankcase area, which is below the valves and cylinder head. Pushrod engines include a valvetrain, consisting of tappet (also called lifter), pushrod, and rocker arm, to transmit camshaft lobe action to each valve.