Cam Terminology

Lift

The word lift refers to actual distance that the cam lobe actually "lifts" the lifter and pushrod, pushing the valve open. This is how much the valve is "lifted" off its seat at the cam lobe's highest point. If the valves don't open wide enough, they will cause a restriction to the air trying to enter or exit the cylinder. However, opening the valve past a certain point will not increase the flow in or out of the cylinder. A good way to demonstrate this is with the garden hose in your backyard. When you first start to turn the water on, the flow increases, but after a turn or so, opening the valve more has no effect on how fast the water comes out of the hose.

Duration

This measurement is taken in degrees of crankshaft rotation. Duration correlates to the amount of time (duration) that the valve stays off of the seat during the lifting cycle of the cam lobe. This measurement is taken when the cam lobe actually starts raising the lifter, until it finishes putting the lifter back at its start position. If the lobe separation is a constant, a longer duration will produce more peak power, a rougher idle, less torque at low rpm, and peak power at a higher rpm. As strange as this may sound, more duration can be helpful in engines that run at a high rpm. The extra degrees of time that the valve is open at high rpm gives the air more time to get into (or out of) the cylinder in spite of the piston's stroke.

Lobe Separation

Lobe Separation Angle (LSA) is the measured angle, as measured in camshaft degrees, between the maximum lift points of the intake and exhaust lobes. If you look at the end of a camshaft, the complete distance around the lifter lobe is 360 degrees. Keep in mind that lobe separation angle is said to be measured in degrees of the camshaft. Lobe separation is what affects valve overlap. This overlap affects the nature of the engine's power curve, idle quality, idle vacuum, etc. If we keep duration at a constant, a wider LSA will give an engine more peak power, a rougher idle, more torque at lower rpm, and peak power also occurring at a lower rpm.

Not only will a wider LSA give you the aforementioned qualities, but it also results in less cylinder pressure, which is great for higher compression street engines.

Overlap

This is also known as Lobe Center Angle, and is the crankshaft degrees that are measured when the intake and exhaust valves are both open. This "overlap" occurs at the end of the exhaust stroke and at the beginning of the intake stroke. Increasing lift duration and/or decreasing lobe separation increases overlap. When an engine is running, the exhaust valve needs to stay open slightly after the piston passes top dead center. This helps keep the momentum of the exiting exhaust gases to maximize the amount of exhaust gas pulled out (scavenged) from the cylinder. The intake valve on the other hand, opens before top dead center, and uses the momentum of the exiting exhaust gas to start pulling the intake charge into the cylinder. More is not always better, as too large of a lobe center angle can result in too little overlap to make good power. Too little overlap causes the lack of complete expulsion of the exhaust gases, and less intake charge filling the cylinder. Smaller amounts of overlap produce a smoother idle, and a slight benefit in top end horsepower. This effect on performance is directly linked to engine rpm. Higher engine speed causes greater exhaust-gas velocity, which relates to greater momentum of the exiting exhaust gases. This is why a longer-duration camshaft produces power higher in the rpm range. It also causes the loss of low rpm power and economy that larger cams experience. But at higher engine speeds these conditions are minimized due to the slight lag time it takes to get the intake charge moving into the cylinder.

Centerline

The cam's centerline is used to correlate valve timing to the crankshaft's rotation. This is again measured in crankshaft degrees. Centerline is explained as the number of degrees that the crankshaft must rotate from top dead center until the cam has rotated to the peak (or centerline) of a given intake or exhaust lobe. For the engine to run at peak performance, the valves must open and close at the correct time in relation to the piston's position and the crankshaft's speed. The intake centerline is the point of highest lift on the intake lobe. The exhaust centerline is the point of highest lift on the exhaust lobe. The cam centerline is the point halfway between the intake and exhaust centerlines.

SOURCE
Competition Cams
3406 Democrat Road
Memphis
TN  38118
800-999-0853
www.compcams.com