Have you ever noticed that if you make a quick "puff" at a candle from a short distance, that the air, even though you are not blowing anymore, still makes it to the candle and blows it out? That's because air has "mass", and when in motion, it must be acted upon to stop it. Have you ever noticed that as a kid when blowing bubbles, if you try to blow a bubble to keep it aloft, it seemed that sometimes the bubble would be passed by the air, only to wiggle around, and then "follow" the air you just blew? These little things also apply in an engine.
As the air is going through the intake, and through the throttle body, there is a bit of turbulence created by the throttle plate. As it continues on to the intake runners, the air is straightened out into a nice laminar flow, as the intake valves open, and the piston is rapidly moveing downward, the air rushes in to fill the combustion chamber, flowing down the sides of the cylinder wall, hitting the top of the piston and moving up the other side and thru the middle, and swirling. as the intake valves close very fast, its like a door slamming shut. The air that was following in to the combustion chamber is now suddenly stopped. This sends a "shock wave" backwards through the intake runners, and if timed properly, this "shockwave" or "Pulse" is now helping to "push" more air into another cylinder.
Meanwhile, as the air in the first cylinder is being ignited, and producing power on the downward stroke, the piston now begins its upward stroke. The exhaust stroke.This is where scavenging comes into play. Scavenging is when the exhaust valves open and the exhaust exits the cylinder leaving a vacuum behind it.
At that point the piston moves the exhaust out rapidly in a "puff". This puff, once again has mass
. There is a high pressure area leading the way in front of the exhaust pulse, and when the exhaust valve closes, it happens abruptly, creating a low pressure area behind the exhaust pulse. This pulse travels down the exhaust, and past another cylinder. The low pressure of the pulse "pulls" the exhaust gasses from the next cylinder, when doing this, it creates a vacuum in the combustion chamber helping to "pull" more air into the cylinder as the intake valve is starting to open while the exhaust valve is still open, this is called overlap.
All these pulses need to keep up their velocity to remain efficient. They do this by staying hot, and by the size of the exhaust pipes. Newer vehicles with variable valve timing (VVT) keep these pulses timed throughout the rpm range. There is a "Goldie Locks" effect taking place. Too large of a pipe, and the pulses slow, exhaust cools too fast, and the engine looses efficiency. Too small of a pipe, and the exhaust is sped up beyond how the timming of the valves was designed, the exhaust temps rise, and the engine looses efficiency. Any change in the system, be it a larger exhaust, or whatever, has an effect on it. Sometimes positive, as the auto makers have to abide by emissions standards, but many times negative, as some aftermarket manufactures just want it to "sound cool".
When installing a Turbo, or Supercharger, the scavenging effect is not as important, as the higher pressure "pushes" everything through the system, in which case, a larger free flowing exhaust would be the most beneficial.
I have oversimplified this just a bit, as I don't have time to go into EVERY thing, or break down the physics involved, but thought it would be a good read for those thinking about changeing their exhaust system and wondering how the new exhaust may affect performance.