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Hib may be able to give a better explaination, but I'll give it a shot.Stallion said:
It's also easier to describe with drawings and arrows and whatnot...
Where to start....? This is going to be very round-about to get to your answer.
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OK, here's the short answer, if I understand this stuff correctly myself. The air filter collects and stabilizes the air before it enters the carburator on it's way to be atomized into a homogeneous mixture where is will be burned in the combustion chamber.
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First, there's ALWAYS air in the filter. It's never a sealed vacuum. The carb could never suck enough air on my 14" diameter x 4" tall filter to create a loss of air (or create a sealed vacuum) within the filter. (I really probably have more filter than I theoriectically need, but that's irrelavent right now.) The carb will never 'run out of air', so just keep that in mind.
The entire intake system is designed to get the right amount of fuel-air mixture ("f/a") into the combustion chamber. (I believe the ideal ratio is 14.1:1 or 14.4:1) Now, in order for the best combustion to take place, the f/a mixture needs to be as close to a homogenous mixture as it can be. Meaning the more thoroughly it's mixed, the better it will burn.
The air filter, as stated above, helps collect and stabilize (or direct) the air into the carburator. This stabilized, or directed, air can be better atomized on it's way through the system. As it passes through the carb, the gas (fuel) is added. Now, you have this f/a mixture running down the carb, through the upper intake manifold, to the cylinder heads ("heads"), and then finally into the cylinders ("combustion chamber"). Throughout this journey, the f/a mixture is being mixed more and more as it tumbles through the system.
Air that starts out stabile is better able to be mixed and atomized on it's way to the cylinders. You get this from the air inside the filter. Contrary to simple logic, air that is already swirling like crazy above the carburator does not actually mix better. Getting to an explaination beyond this would require someone more versed in physics (and again, drawings would help). So sufice to say, when a carburator has to work to suck in turbulent air above it because of no filter, the f/a mixture is not as good, the atomization of the f/a mixture is not as good, and you end up with a loss in power due to poor combustion.
Here's an analogy, albeit a silly one.
Imagine you have a shop vac and you are trying to catch flys with it by holding up the hose in place. Waiting for the flies to get close enough to the suction area could take a long time. That's not very efficient.
Now imagine you have something hovering above the end of the hose and that once the flys flew into this area, they couldn't get out as easily. You're going to capture a lot more flies. Air filters work kinda the same way I guess. It just collects the air into a central location and makes it easier for the carb to suck it in.
Did that make any sense at all?
I haven't been through this site in a long time, but I know a lot of people could learn a thing or two by checking it out. It's actually kinda fun!
http://auto.howstuffworks.com/inside-engines-roundup.htm
