A typical upgrade consists of replacing the standard air filter with an upgraded high performance filter (or sports filter) with a claimed increase in horsepower.
These are typically pod style filters (see left image) which attaches directly to the air flow meter or to a pipe attached to the air flow meter. To understand whether a filter upgrade is actually an upgrade, it is important to understand the function and restrictions of the air filter.
A typical air filter consists of a fine-meshed plastic/metal sieves or with fine-porous paper, felt or fleece. A performance filter will usually combine the two methods using coarser mesh and material and optionally fluid (oil) to allow for more flow while still provide filtering capabilities.
The basic function of an air filters is to prevent impurities and foreign matter from entering the inside of the engine together with the air, providing increased wear or damage to the sensitive engine internals.
The restriction in standard air filter is typically flow, temperature and noise (and size for heavily upgraded engines). The filter will usually be encapsulated to decrease noise, and located away from the engine using multiple bends on the intake piping. All of this decreases flow.
A (larger) pod style filter will allow for more free breathing due to removing the air filter box and minimizing the various intake piping bends. However it is crucial that care is taken to keep intake air temperature as low as possible.
The underbonnet air temperature can easily rise 30° C (54°F) above ambient temperature for a naturally aspirated engine and 50° C (90°F) with a turbo engine. A approximate power decrease of 1% can be expected with every 7°C (13°F) the intake air temperature rises.
For a 300 hp engine, a difference of 50°C is equivalent to 20hp. This illustrates why cold air is important for maximum power.
To achieve this, a number of measures can be taken to ensure cold air intake temperatures:
A good example of this is the image on the right with a cold air intake 'kit' fitted to a corvette engine. The location of the filters is far away from the engine and with appropriate heat shielding. Finally the piping has few bends and the filters are of large size. All this results in optimum flow with minimal resistance.
A high performance air filter upgrade is not a justifiable expense as a solemn upgrade. If nothing else has been done to the engine the intake flow requirements has not changed and a minimal benefit will be provided with a high performance filter.
Many turbo cars are seen with no air filters, typically just with a fine steel mesh on the turbo inlet side. Unless the car is a purely for car shows, this is not recommended. A properly dimensioned & installed filter will not reduce HP. All the sand, dirt and grit that do make it into the engine will soon reduce HP due to the additional leakage past piston rings and compressor blades or rotors. See Air Filter - Advanced for more information.