The humble air filter is seldom seen, but performs a very important job - it is the only thing keeping the dusty air out of the clean insides of your engine. Said engine needs oxygen to breathe and to burn fuel, which it sucks in via the intake manifold. Dirty air contains dust particles and other contaminants, which are very bad for your engine, greatly accelerating the rate at which it wears. That's why it is so important to properly filter the air before it enters the engine.

What Air Filters Actually do and Why They Need Replacing

There are many types of air filters for car models of every description, with some being circular, cylindrical or in a panel. Whether made of cotton, paper, or foam, they have the same purpose - the pleated material filters the air as it passes through, holding back any debris and dust. Air filter materials are folded many times, concertina-like, to give it a larger surface area. A simple sheet of paper would tear too easily, would resist the airflow too much, and would get saturated with dirt far too quickly. The expansive surface area of a modern air filter ensures it can do its job efficiently until its next scheduled replacement. It is placed in a special housing, or "air box", with its housing pressing down firmly on its rubber surround in order to ensure an air-tight seal and to prevent any air from bypassing it.

Since the air filter keeps on collecting dirt throughout its working life, it eventually gets clogged up and has to be cleaned and ultimately replaced. When the filter element becomes saturated, the flow of air to the engine is restricted. This can cause all manner of problems. What is most important is that you remember that your air filter is a service item with a finite life that has to be replaced according to your car's service schedule. It may have to be replaced more frequently than that under severe operating conditions. Reasons for changing your air filter include:

• Better performance with clean air to mix with fuel
• Decrease fuel consumption thanks to good combustion
• Avoid or decrease dirt exhaust emissions

How Many Air Filters are in a Car?

The engine is not the only component in modern cars making use of an air filter. Almost all cars in the USA now also have cabin filters to filter out dust, dirt particles, and pollen that might enter vehicles from the outside via the air-conditioning system. This helps to deliver pure, filtered air to the car's occupants, especially in dusty and smoky environments, and it helps to combat allergies. As such, the air-conditioning filter for the car needs to be maintained, too.

Some of these cabin filters, such as models made by Bosch, are of the HEPA (High Efficiency Particulate Air) filters and have a filtration efficiency of 99.97 percent at 0.3 microns. However, these only work properly when all the windows are closed and air is admitted to the car only via the filtered air-conditioning. Still, the engine and a/c filters are usually the only two types of air filters a car typically has, so maintaining them isn't overly difficult.

How to Choose an Air Filter From the Available Types

Most OEM car air filters are of the dry paper-like material, folded into a pleated filter. Manufacturers carefully select the materials they use for their air filters to represent a fine balance between filtration efficiency and performance. They tend to err on the side of caution and have high standards for filtration efficiency in order to ensure maximum engine life, which is why it's best to stick with the stock air filter. There are even more types of air filters for cars available after-market, though, and each claims many advantages in terms of performance and fuel economy. However, many of these claims are untested.

It would, therefore, be impossible to list the top 5 air filters for cars currently available in the US, because how is it measured? Many of the performance claims are not justified and when they are, there is no telling whether they still adequately perform their primary job of filtering the air properly. Among these are the sponge-type, cone-type, wet-element, oiled, reusable, and fiberglass filter; however, it is less common to find fiberglass A/C filters or a polyester filter in cars than in industrial applications. The market is huge and unregulated, so you have to take the claims of these suppliers with a pinch of salt. It is, therefore, very important to do lots of research and preferably obtain third-party test results confirming that a filter you plan to install is not detrimental to your engine's health.

When in doubt, the best air filters for your car are always the ones that the manufacturer prescribes. Avoid obscure brands that offer lower prices because you usually get what you pay for. There are tons of dodgy air filters for cars available out there, but the real deal is already cheap enough that you don't need to skimp. Cheap imitations might look the same, but their filtration performance could be far worse - and you wouldn't even be able to tell. After-market 'performance' filters are also a minefield, and you can't always tell what you're getting into. Many of their perceived advantages are overblown and do not make up for their worse filtration ability. Wet-element, foam, cotton, and reusable filters require special care, as do washable air filters, so if you don't feel like the trouble, these are probably not the best air filters for your car.

How to Tell When Your Air Filter Needs Replacing

The air filter is usually easy to access under the hood in all cars, whether it is a city hatchback such as the Mitsubishi Mirage or a cargo van such as the Ford Transit. There will be an obvious enclosure in the intake ducting that can be opened and that contains the filter. In classic cars with carburetors, the air-filter housing is usually right on top of the carburetor.

A physical inspection might just disturb the dirt. Your best option is to simply periodically change the air filter, but how often?

• Replace it as often as your vehicle manufacturer prescribes in your service and maintenance manual.
• It's never a bad thing to replace it more frequently, so if you're in doubt and you're not sure whether say a 24,000-mile prescribed interval is good for your usage pattern, replace it every 15,000 miles to be safe.
• There should usually be a stipulation alongside the replacement interval specifying what it should be if you operate your vehicle in very dusty conditions, like living on a farm and/or using gravel roads daily. If this is the case, it is always best to replace the air filter in half the normal time or prescribed mileage.
• You should definitely not try to clean and refit a disposable paper-element air filter. All it does is dislodge the dust, which will be ingested by the engine the moment you start it, no matter how well you think you cleaned it.
• Not replacing it in time can reduce engine efficiency and performance.

How Much do Air Filters Cost?

Air filters are among the cheapest service items to replace and they are easy to get to, so there's no excuse not to replace them timeously. Typically, car air filters cost between $20 and $50, which is small change compared to the peace of mind you get from knowing your engine is getting clean air and is running at peak efficiency. After-market air filters may be even cheaper, but in the case of the pride and joy that is your car, the price of the air filter should not be the determining factor. You stand to save little on such a cheap item and may lose a lot in the long run. So how do you know what is the cost of air filters for cars of your particular model? Phone the dealer or ask a professional mechanic who will not sell you substandard parts. On the other hand, some after-market 'performance' type air filters for your car model can be expensive and set you back more than $150. These have been said to increase power outputs from the engine, but some have not lived up to the hype. They are often accompanied by bold claims and while some air filters may improve car performance, it's not always guaranteed.

Conclusion

The air filter is an often overlooked, but very important service item, as it directly affects the running, economy, and longevity of your engine. Always replace it at the set intervals - or more frequently if you operate your vehicle in dusty conditions. Do lots of research when choosing after-market 'performance' filters and be sure to find evidence from independent, scientific, third-party experts that these can still do their job as well as the OEM product. Engine and cabin air filters give our car engines - and us - clean air to breathe and should never be neglected.