By Larry Carley
Technical Editor

Every engine is equipped with an air filter to keep out dirt. An engine sucks in a lot of air when it is running: about 10,000 to 12,000 gallons of air for every gallon of fuel it burns!

Consequently, the air filter has to have a large surface area and plenty of filtering capacity to trap and hold all of the contaminants that may be present in the air.

One of the most common ingredients in airborne dirt is silica, a hard abrasive mineral that's found in sand and clay. If silica gets inside the engine, it has the same effect as sandpaper. It scours the piston rings, cylinders, valves and bearings, and causes the engine to wear rapidly.

Air filters remove dirt by trapping contaminants as the air flows through the filter media. The percentage of particles removed is a measure of the filter's efficiency. A good quality air filter generally trap upwards of 98.5% of the incoming dirt. Filter manufacturers use SAE J726 test procedures to evaluate their air filters, and try to aim for the best balance between filtering efficiency, filter capacity and pressure drop (r...