Friday, March 11, 2011

Car Air Filters: What To Look For

Best Air Filters Review: Air Filter Facts
Air Filter Facts Part 2: What makes for the best air filter

What makes for a good automotive air filter? To a degree, it depends upon your usage. Rural drivers in a very dusty environment will look carefully at capacity, while Friday night racing aficionados will look more at air flow. We look at criteria for an average user, who does not race, and whose environment is only moderately dusty. For our average user, we present below the most significant criteria for air filters, by decreasing order of importance.

Filtration efficiency

We already discussed here the importance of filtration, which simply cannot be overemphasized, as dust is the primary cause of engine wear: both oil and air filtration are the most critical factors to making an engine last.  Lycoming, the manufacturer of aircraft engine, states in its maintenance manual: "Dust or some form of dirt is frequently the principal factor in premature piston ring, ring groove and cylinder wear. If a worn or poorly fit air filter allows as much as a tablespoon of abrasive dirt material into the cylinders, it will cause wear to the extent that an overhaul will  be required. Evidence of dust or other dirt material in the induction system beyond the air filter is indicative of inadequate filter care or a damaged filter." The difference between filtration efficiency of 90%, 99%, and 99.9% is very significant:

The average capacity of all the filters surveyed in our series is 234 grams. If we assume that the life cycle of the filter is approximately 40% of its capacity (you will change it before it gets clogged), we can assume that the filter will see roughly 100 grams of dust at the intake. If the filter has 90% cumulative efficiency, what will pass through it is the amount of the first jar. If it has 99% efficiency, it will pass through the content of the second jar. With 99.9% efficiency, it will pass through the content of the third jar. Given that whatever passes through the engine goes straight into the cylinder chambers, which one would you like it to be for your car?

It is worth noting that the typical way to measure filtration efficiency is by calculating how much dust is filtered over the theoretical lifetime of the filter (i.e. until it reaches capacity), which is really measuring cumulative filtration efficiency. Why is this important? Filters get more efficient as they start accumulating dust. This means that, if you always use clean filters, your efficiency will be lower than that measured in ISO 5011 filtration tests! Amazingly, is actually better, for filtration, to have some dust in your filter. What this means is that, unless you keep your filters until they clog, your true filtration efficiency will be lower than that measured for your air filter.

We should mention, in passing, that we have frequently seen manufacturers' specs for filtration efficiency be proved wrong by third party tests. Unfortunately, it appears that we cannot trust air filter manufacturers to give us their own filters' specs unless the actual tests are conducted by third parties.


This is a frequently neglected factor. The variety of shapes and sizes across all cars is very, very broad, and there are many cases when a filter brand which performs in general well does not have perfect fit for a given car's intake. Big air filter manufacturers, such as Wix or Purolator, will often design and manufacture filters themselves for vehicles with wide distribution, but may sometimes factor out filters for less common vehicles, making fit and quality problems more likely.

As a result, it is not possible to evaluate fit on the basis of the air filter brand, since it varies by vehicle. If you are not going to actually check out the fit yourself, you are better off requesting an OEM filter for your vehicle, which is more likely to have good fit.


The capacity of an air filter is the amount of dust that it takes to increase the resistance of the filter to air flow by 10" H2O, and is supposed to be a good measure of what it takes to clog a filter. The larger the capacity, the longer you can take before you change your air filter. Of course, larger capacity is also preferable when driving in a dusty environment, where a lot of dust may end up in your filter in a short amount of time. While we do not believe that 20-30% difference in capacity makes much of a difference to the average user, we found out that filter capacity varies by a factor of 1 to 10. We do believe that a difference of one order of magnitude makes a significant difference.

Build quality

There are many ways to build a cheaper air filter - but how much do then also cheapen build quality? What material is the frame made of? How well dimensioned is it? What is the filtration medium? How are the pleats built? A better-built filter is likely to be more expensive, although not always better performing. Given that air filters can last through tens of thousands of miles (while oil filters only last thousands of miles), some difference in price may not be significant to the overall life cycle cost of your vehicle, while filtration improvement definitely will make a difference.

Quality control

The best filter does not amount to much is quality varies enough between units that fit is not guaranteed, or that filtration is not maintained. As air filters have become commoditized, quality control and build quality have become progressively worse.

Air flow

It is the last criterion on our list, and the most controversial as well. It is the least important of all criteria, and we will discuss why. The next few sections will investigate the influence of air flow on mileage and on performance.

Next we discuss if different air filters can impact air flow... So come back soon!

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