The air filter for your engine is just as important as the oil filter. If you get your vehicle serviced frequently at the quick lube places you probably have bought quite a few of them. The power that your engine produces requires 15 pounds of air for every 1 pound of fuel it ingests. If you can imagine 15 pounds of air you get the picture. That air filter has its work cut out for it.
The air filter has to remove airborne particles that vary in size all the while letting particle free air easily pass into the engine. Air filters are rated by efficiency and life not by the micron size of particles that pass through them. The standard rating system for automotive air filters requires a standard test dust comprised of particles from 0 to 80 microns to be fed at a concentration of 1 gram of dust per cubic metre through the filter media. When the filter starts to restrict the dust flow causing a significant pressure drop across the filter the test is stopped. The filter has reached its end of life. At this point the filter is weighed.
The weight of the filter determines how much of the test dust was collected in the filter before it became too restrictive. This weight gain determines the filter's capacity. An absolute filter downstream of the test filter (it collects all the leftover particles) is also weighed. The increase of weight in the test filter versus the weight increase of the absolute filter is the measure of the filter’s efficiency.
The point here is a good filter has to catch as much dirt as possible without letting any bits through all the while not restricting flow.
I have removed and installed a lot of air filters in my time. Do not underestimate the importance of how the air filter fits into its container (the air box). The filter must seal properly in the box so that there is no way the incoming air can bypass the filter media. That is many times what separates a good filter from a bad one. It is also the responsibility of the installer to install the filter properly. We find many air boxes that are distorted from poor design or poor installation and removal practices.
What happens when dirt get in your engine? Basically, the dirt will scratch the cylinder walls and wear them out prematurely. Your engine will die a premature death.
Air filters are more often sold on how easily air flows through them and any performance improvements that can come from using them. The most common versions are cotton filters that you spray oil on to catch the dirt. On modern fuel injected engines that use mass air flow sensors to measure incoming air, too much oil sprayed on the filter media can cause problems.
As the air flows through an over oiled filter it pulls an oil mist from the filter and deposits on the mass air flow sensor that can result in performance issues.
Air filter choice therefore requires high efficiency, high capacity and excellent fit. After the product is chosen a proper installation is mandatory.