We now know that the intake valve doesn't close at the bottom of the intake stroke, but stays open partly into the compression stroke. This means that as the piston is traveling upward on its compression stroke the intake valve is still open! The later we close the intake valve the less compression we'll have because we're letting air back out. Dynamic compression ratio is the compression ratio taking into account the closing of the intake valve event. We know that a higher compression ratio yields more power due to higher combustion pressure on the power stroke. So now if we close the intake valve later, we reduce our dynamic compression ratio and also hurt power. That doesn't sound good, especially since most performance camshafts close the intake valve later than stock cams. There's some truth to that. Since the dynamic compression ratio goes down most of the time you'll see a loss of power at lower rpm with aggressive camshafts. At the same time though, the power at higher rpm will increase. Since the dynamic compression is lower, why isn't it losing power everywhere? Grab some caffeine and do some mental stretching because here comes the big one.
Air is a compressible fluid and as it travels at different velocities and pressures through an engine it has momentum. This concept is why a camshaft that holds the intake valve open longer can make more power at higher rpm. At low engine speeds the air has less momentum and the cylinder fills in with all the air it can get and some of it might even be pushed back out as the piston starts going up on its compression stroke. At high engine speeds where the piston and air is moving much faster, we can actually have air still cramming in after the piston is going back up on its compression stroke. A good analogy would be to imagine a train crashing into a solid wall with the back of the train keeping momentum and piling up on the wreckage. I mentioned the intake valve is opening before the piston is all the way at TDC and that the exhaust valve doesn't close until after TDC, that means that both valves are open at the same time! This is called overlap, and the same concept of momentum applies here. The idea is that the air going out the exhaust port will help pull in fresh intake charge as the intake valve is opening. Too much overlap can hurt idle and low speed running. Not enough overlap and it can hurt top end horsepower. It's a complex interaction that is determined by various factors throughout the engine.
The intake manifold design, head port design, valve size, valve lift profile, rod-to-stroke ratio, exhaust port design, and exhaust manifold design are all key components in determining the power output of an engine and how it produces power across the rpm range. This subject is very complex and auto manufacturers and top-level race programs have teams of engineers working on designing better engines. To keep it simple, let's just stick to the concept that holding the intake valve open longer will generally reduce low-rpm power and increase high-rpm power. This leads into the cam duration and valve timing events.
Cam Duration
Cam duration refers to how many degrees of crankshaft rotation that the valve is open. Looking at a lift curve, which shows valve lift in reference to crankshaft degrees, you can see that it resembles a bell curve.