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Camshaft Basics
Intake valves allow air/fuel into the combustion chamber. Exhaust valves allow the burned air/fuel to exit the combustion chamber and pass through to the exhaust. The opening and closing of the valves is controlled by the lobes (cams) on a camshaft. The two most important design specs for a camshaft are duration and lift.
Think of the valve as a door. Lift is how far the valve opens. The further a valve opens, the more intake or exhaust gas can pass through it. Lift figures can be measured at the camshaft lobe (lobe lift) or at the valve (gross lift). Lift at the valve may differ than lobe lift if there are rocker arms as rocker arms have a "ratio". On engines without rockers, lobe lift and valve lift will be the same.
Duration is the period of time a valve is open. Using the same door example, the longer a valve is open, the more intake or exhaust gas can pass through it. Duration is measured in degrees of crankshaft rotation. Duration can be established in several ways. Unfortunately a lack of standards can make camshaft comparisons confusing.
Some companies advertise duration as beginning the moment the valve begins to move and continues until its back on it's seat. Another method is to measure duration after the valve has moved a specific distance. Common distances are .006, .010, .012, and .050 inch. If you find this confusing, consider this - The numbers for "advertised duration" and "duration at .050-inch lifter rise" are different. For example, a camshaft can be advertised as having 290 degrees of duration. That same camshaft can have a duration of 240 degrees at .050, because it may take 25 degrees to get it from zero lift to .050 and another 25 degrees to bring it from .050 back to zero.
Duration significantly impacts performance. The more duration a cam has, the further up the rev range peak power will occur. That's because the more duration a cam has, the more "overlap" it has. Overlap is when the exhaust valves are closing, but not quite closed, and the intake valves for the same cylinder are opening. This is a point where both intake and exhaust valves are open at the same time. At low engine speeds, having both valves open at the same time causes something called reversion. Reversion causes gasses to travel in the opposite direction.
How?
On the intake side, with the engine at low speeds (like idle), and piston still traveling upward, the intake valve begins to open. Because the piston is traveling upward, it's pushing exhaust gases out. When that intake valve opens, exhaust is pushed into the intake tract.
A similar thing happens on the exhaust side. With the engine at low speeds (again at idle), and the piston having just past top dead center (TDC), it's on the intake stroke drawing vacuum with the exhaust valve open.
Right about now, those who are new at this may be thinking, "Why would anyone design a cam with overlap?" It's because although it allows reversion to occur at low speeds, the high speed benefits outweigh the low speed costs. As engine speed increases, gases develop more inertia. Think back to your early science classes - objects in motion tend to stay in motion. At higher speeds, the intake mixture develops enough inertia to keep flowing into the cylinder even though the piston is on it's way back up. This fills the the cylinder with more air & fuel. The exhaust works the same way. At high speeds, exhaust gases have enough inertia to continue down the exhaust port despite the vacuum created when the piston reverses direction. The result is higher flow and that's why cams with overlap are desirable - engines with higher flow produce more power.
Now you know why variable valve timing was invented. They allow the best of both worlds - no overlap at low speeds, and longer duration at higher revs.
Not all engines benefit from overlap. Supercharged and turbocharged engines are exceptions. Those engines perform better with less overlap than naturally aspirated engines. That's because under boost, the intake is under positive pressure. If the intake valve opens while the exhaust valve is still open, that intake stream will follow the path of least resistance - which is right through the open exhaust valve. This reduces power, wastes fuel, and causes the HC exhaust numbers to soar.