As mentioned in the air filter for fuel economy section, there is another benefit to aftermarket air filters: horsepower increase. The concept behind how aftermarket air filters increase horsepower and torque lies in their ability to increase the airflow into the engine. Typically, aftermarket air filters allow a higher cfm value or cubic feet per minute of air into the intake plenum. Compared to stock, restrictive air filter flow, the extra air combines with gasoline to provide the motor with more combustible fuel than it previously had to produce energy. This results in a more energetic explosion within the cylinder walls. As a result the pistons are forced down with even more force, improving your vehicle’s horsepower, torque, and acceleration.
Now, stock air filters are generally quite restrictive to air flow as their main purpose is to prevent dust and particulates from entering the motor. This is what they were designed to do from the factory and are very efficient at doing so. In fact many stock replacement paper air filters claim particulate trapping efficiencies upwards of 99.7%. While this is great for filtering, it is very restrictive in terms of ability to flow air into the motor.
This is where aftermarket filters and performance kits (i.e. intake tubing, cold air intakes) come into play. Typically aftermarket filters are made of a cotton gauze / screen mesh combination. Some are oiled, others are not. The cotton gauzing is usually more gas permeable than the paper filters. In other words they allow more air into the engine. The oil increases the particulate trapping abilities of the gauze in an effort to compensate for the increased flow through. In addition many aftermarket air filters are in the shapes of cones which increase the surface area through which air can flow into the motor. Stock filters are flat and rectangular, limiting the amount of air that can physically be pulled into the motor. Aftermarket filters can typically raise horsepower by anywhere between 2 to 25 horsepower depending on the size of the motor it is installed in.
The purpose of the aftermarket cold air tubing is to replace a vehicle’s stock restrictive intake tubing which is not optimized for performance. In fact vehicle intake tubing is designed from the factory to help quiet the motor down in many instances via the use of resonator tubes attached to the bottom of the intake tubing. Aftermarket tubing, made of aluminum, stainless steel, or other materials is usually of a larger diameter than stock to allow more air into the motor as well. These units are also designed to keep the incoming air as cool as possible, as colder air is denser which means more oxygen will be available to the fuel to create more power.