Engine intake air filters (AKA, engine filters or intake filters) are a basic maintenance item found on just about every car. They protect the engine by filtering out things that shouldn't be ingested with the air that goes into the engine.
Let's start off with a few blanket statements just so we're all on the same page.
- First, an engine intake air filter protects your engine by filtering out contaminants like dust, dirt, hair, leaves, bugs, and other types of debris.
- Second, engine intake air filters all strike a compromise between performance and protection. All things being equal, the more protection a filter offers, the less performance it will have.
- Third, filtration media is the portion of the filter that actually filters air. There are a few types of filtration media used for filters, and each has pros and cons.
- Fourth, all things being equal, the more surface area a filter has, the more performance it will have.
- Fifth, what I say in the article below is based on a mixture of my research, my actual experiences, and my opinions.
Nets and Pillows
The easiest way to understand how filters work is with a pair of analogies: nets and pillows.
Intake filters work quite literally like a net. The net captures the unwanted contaminants and prevents them from going further into the engine. The closer-knit the net is, the smaller the contaminants that it can capture. The looser-knit the net is, the harder it is for the net to capture small contaminants.
Pretty simple, right? If you want to protect the engine as best as possible, just make the "net" very tightly-knit. The problem is that the more tight the net, the less it can flow and the more restrictive it is. The more restrictive a filtration media, the less performance you can get from your engine. That brings us to our second analogy.
Grab a pillow off your bed, press it against your face, and breathe through it. Not too easy, is it? Now, take the pillow case off the pillow, put the pillow case against your face, and breathe through that. Much easier, right? The pillow offers the most amount of protection as the huge amount of filtration media is best able to filter the air you're breathing in. However, it's incredibly restrictive, which makes you work that much harder to ingest air. The pillow case improved your breathing performance, but wouldn't be able to filter as much because it has far less filtration media compared to the pillow. These concepts are the same for filtering the air that goes into your engine.
PLEATS Are Still Stylish
They may have gone out of fashion for pants, but pleats are still used for air filters. Paper- and cloth-based filtration media are flat. The more surface area a filter has, the more air can flow through it. However, air filter size is often restricted by the space where it is positioned, so a filter can't just be made larger to increase its surface area. To solve this, pleating is used to add surface area without making the filter itself physically larger.
Pleating is especially important for paper media as it's by far the most restrictive type. Paper is very easy to fold into pleats, so manufacturers are able to create very large surface areas for those filters. This is especially true of Aston Martin's OEM intake filters. Astons use a pair of heavily-pleated paper filters which result in surprisingly good air flow for factory paper filters.
But even though they have a lot of pleats, it doesn't mean there's good air flow between the pleats. Here's how dense the pleats are:
You can see how narrow the space is between the pleats on the inside of the filter. Although the filtration media has a lot of surface area, the design doesn't allow the best airflow between the pleats. It does give a paper filter enough flow to provide the engine with an ample amount of air, but still leaves room for performance improvements.
Rinse and Repeat
Paper filters (like the OEM ones) are not reusable, but aftermarket filters usually are. When the service interval for the intake filters comes up, you just remove the filter, wash it, let it dry, and then reinstall it. If it's oiled, you need to re-oil ("recharge") the filter after letting it dry and before reinstalling it.
Being able to reuse a filter makes them much less expensive to use over time, but keep in mind that they generally become a little less effective after each wash.
Personally, I would only reuse a filter once and then replace it after that. It's still cost-effective, as it basically means I get every-other filter for free, but ensures I don't use a filter for so long that it's no longer doing its job. That's just me, though.
There are four basic types of aftermarket air filters: foam, metal mesh, oiled cloth, and dry cloth. Foam filters use a foam media to block or capture contaminants, while metal mesh filters use a very fine metal mesh. Both are typically washable and reusable. I don't use foam or metal mesh, nor have I ever, and they aren't nearly as common as oiled or dry cloth, so I'm going to skip over those.
Paper for Protection
Best overall protection
Typically inexpensive to buy
Prevent water ingestion
Don't hold contaminant buildup
Most restrictive media
Worst for performance
The vast majority of OEM filters are made with paper filtration media. Paper is inexpensive and provides the most protection of any media type - both characteristics of high interest to vehicle manufacturers. Something to keep in mind with intake filters is that vehicle manufacturers don't make different filters for different regions. A car going to Dubai is going to have the same filters as a car going to Florida. That means a filter has to be able to handle everything from fine desert sand to chunky swamp bugs.
Vehicle manufacturers also have to deal with "what if" situations that most of us don't consider as normal occurances, like flooding. Paper filters offer the best protection from water ingestion, and you can see the difference right here:
NOTE: I made that video, which proved to be rather controversial, quite a while ago and have started using and selling cloth filters since then. The video was made only to show the difference in water protection (and air flow) for paper filters compared to cloth filters, but a lot of people took it as an attack on aftermarket filters and said I was trying to drive sales to the OEM filters I was selling. That wasn't the case by any means - I already had my cloth filters in development at that point. I wanted to show the risk of water ingestion with cloth filters so people know the actual drawbacks to aftermarket filters. It was an attempt to inform my customers so they could make the best decision for their own situations.
Oiled Cloth For Performance
Probably the most common type of aftermarket intake filter is the kind that uses oiled cloth filtration media.
Least restrictive media
Best for performance
Reusable (require "recharging")
Cost effective if reused
Worst for protection
Potential to damage MAF sensors
Don't prevent water ingestion
Hold onto contaminant buildup
Using our net analogy from before, oiled cloth filters are like a loosely-knit net. That loose knitting allows them to flow the best, but the loose knit allows contaminants to pass through. To mitigate this, oil is used to make the net sticky. A contaminant doesn't need to be caught by the net - it just needs to come in contact with it.
While having a 'sticky net' catching contaminants is a good thing, it's a double-edged sword. Since it's sticking to the filter, contaminants aren't falling away and eventually create a buildup that can slowly reduce the filter's air flow potential. It might be worth cleaning oiled filters every year instead of every-other year as their service interval would usually be. This would ensure the filters don't get buildup and are able to flow their best, but it will also accelerate the wear rate of the filters themselves.
Oiled cloth filters are usually reusable. You can wash them, re-apply oil (sold as intake filter oil, or "charge kits"), and then reinstall them without actually replacing the filters with new ones. After one additional use, they've basically paid for themselves. The challenge with oiled filters is that the re-application of oil has to be done just right. Using too much oil will saturate the filtration media, allowing oil to get sucked into the intake system and potentially damaging the MAF sensors. Using too little oil will result in reduced protection for your engine.
The debate over the risks of oiled filters is a long and tired one, and I'm not going to rehash it. As far as I'm concerned, the choice between dry or oiled filters is up to each person. I have seen MAF sensors damaged by the oil from oiled filters - I've even had it happen to my grey V8 Vantage (the previous owner had oiled filters on it, I returned to OEM filters when I replaced the ruined MAF sensors).
I consider it a risk-to-reward decision. If you want the most power from your engine and aren't worried about the risk, then oiled filters are the way to go.
Dry cloth for balance
I'm a big fan of dry cloth filters. They're less restrictive than paper filters and don't have the risk that oiled cloth filters have - they're something of a compromise between the two.
Best balance of performance and protection
Cost effective if reused
Reusable without requiring recharging (re-oiling)
Don't hold contaminant buildup
Not the best for performance
Not the best for protection
Don't prevent water ingestion
In my opinion, dry cloth filters offer the best balance of performance and protection. The 'net' is tightly woven so it can actually catch contaminants without needing oil, but is still loose enough for air to easily pass through.
One of the benefits of not having oil is the natural jolts and vibrations the filters get while the car is being driven will knock some of the contaminants off the filter surface, helping maintain the filter air flow. Paper filters do the same thing, but oiled filters don't.