Camshafts Variables - Hot Bike Magazine

Although true performance differences between two cams can only be determined by dyno or trackside testing, a cam's duration and valve opening and closing points can offer some hints about its performance characteristics. With that thought in mind, let's review some key cam terminology along with some supporting information you should know when selecting a cam. To simplify the following discussion, we will assume we have a pushrod engine with one camshaft.

A set of four Sportster cams (two intakes and two exhausts) is shown on the right. Each cam has one lobe. On the left is a high-performance Big Twin Evo cam with four lobes. Although more complex and costly, the four Sportster cams allow for straighter pushrod angles, thus more accurate valve timing. They also spread the load of the valve springs over four shafts instead of one, which is important in a high-rpm race engine with high valve lifts and aggressive lobes.

A set of four Sportster cams (two intakes and two exhausts) is shown on the right. Each ca

Camshaft Variables
* Valve lift
* Duration
* Lobe Separation Angle (LSA)
* Overlap
* Single or Dual Pattern
* TDC Lift
* Rocker Arm Ratio
* Opening/Closing Ramp Aggressiveness
* Solid or Hydraulic Lifters

Table 1 A single-pattern cam has the same amount of duration and lift for the intake and exhaust valves. In contrast, a dual-pattern cam has a different amount of duration and/or lift for the intake and exhaust sides.

Duration
Duration is the amount of time a valve is opened and is measured in crankshaft degrees. You can determine the duration for the intake or exhaust valve by adding together the opening and closing values, and then adding another 180 degrees for the distance the crank moves between TDC and BDC. For example, if a cam lobe opens an intake valve at 26 degrees and closes it at 52 degrees, the valve would be opened a total of 258 degrees (26 + 52 + 180 = 258).

Keep in mind, however, to be consistent and accurate in calculating valve timing, you have to use the same measuring point in all calculations. Unfortunately, some cam manufactures use different measuring points, so accurate comparisons may be difficult. Here's what you need to know to keep out of trouble when comparing cam timings: To minimize cam lobe wear and noise, subtle opening and closing ramps are ground onto the lobes to slowly and gently start the opening and closing process. Valve opening starts slowly when the tappet contacts the ramp, and then accelerates. Movement slows again as the valve reaches full open then accelerates again before the closing ramp slows movement down.

Since it is difficult to accurately measure valve opening and closing points when the tappet is on the slow moving ramp, manufacturers take measurements when the tappet is beyond the ramp (or most of it) and the tappet is moving quickly. In the Harley world, valve timing is most commonly taken when the cam lobe raises the tappet 0.053-inch. Some cam manufacturers measure at 0.020-inch tappet lift. Regardless, when comparing cam specs, be sure the numbers were taken at the same tappet lift.

Duration determines the location of the engine's power band and its maximum potential rpm. Generally, duration is a function of engine displacement and operating rpm. All else being equal, increasing duration moves the points at which peak torque and horsepower occur to a higher rpm. Basically, as duration is increased, high-rpm horsepower increases at the expense of low-speed torque. Of course, the reverse is also true when decreasing duration.