How Air Filters Works


If an engine’s induction system is only as good as its weakest link, then a set of 400-cfm cylinder heads, giant intake valves, and mega-flow carburetors don’t mean squat if they’re getting choked upstream by an inadequate air filter. Even so, most hot rodders pay very little attention to the air filter and filter assembly. However, it’s foolish to assume that since they all look the same, they must perform the same, too. In essence, the air filter assembly represents ground zero for the entire induction system, but the challenge of designing a high-flow air intake is compounded by underhood packaging constraints, maintaining filtration efficiency, keeping inlet air temperature as cool as possible, and trying to get everything to play nice with modern EFI systems. To find out how manufacturers reach the best compromise between these often contradicting needs, we consulted with Trent McGee of Airaid. Here’s what we learned.

Cotton Gauze Advantage

While the OEMs overwhelmingly opt for paper air filters, cotton gauze filters are a staple of aftermarket induction systems. So what makes the cotton gauze medium so much more conducive to airflow? “Paper filters are very inexpensive to manufacture, but by nature, the paper media is very restrictive. Cotton gauze allows for much more airflow, but it also presents the risk of more contaminants passing through the filter itself,” McGee explains. To combat this, Airaid applies a layer of oil to the cotton during the manufacturing process so that any stray contaminants will get trapped by the oil. “The result is a substantial increase in airflow over a stock filter without compromising the filtration abilities. Airaid takes things one step further by adding a synthetic layer on top of our four cotton layers. The synthetic layer prevents oil from getting sucked into the motor and wreaking havoc on the mass airflow sensor.”

Surface Area

When designing a performance air filter, regardless of its shape, maximizing surface area is one of the best ways to improve airflow. Just like larger intake valves offer more airflow potential than smaller valves, the same concept applies to air filters. “Open-element cone filters are very popular in aftermarket air induction systems because they provide a drastic increase in surface area over a flat-panel filter. The greater the surface area of the filter, the more air an engine can ingest,” McGee says. “Even when we’re limited in space by the design of the airbox, we still try to increase the filter’s surface area as much as possible by increasing the number and the depth of the pleats. In addition to improving airflow, the larger filtration surface improves the longevity of the filter as well.”

Design Process

Aftermarket cold-air induction systems may look simple, but there’s far more to the R&D process than meets the eye. Interestingly, the design process starts before new vehicle models go on sale to the public. “Thanks to our partnership with SEMA, we can access the design schematics of a new car before they hit the showroom floor. With this data, we get a head start by beginning the design process first in the virtual world,” McGee says. Once Airaid get its hands on a new car, they put factory air intake system on the flow bench, pinpoint the weak spots in airflow, and design filters and induction tubes to improve upon the factory design. Some factory air intakes are designed very well, and others are a complete disaster. “In addition to measuring airflow, the flow bench allows us to monitor how changes to the air intake system design affects the MAF sensor readings. We always want maximum horsepower, but the EFI systems in today’s late-model vehicles are so sensitive to airflow changes that increases in airflow can really throw off the engine calibration and air/fuel ratio. Consequently, we may have to relocate the MAF sensor because the last thing we want is for customers to have to tune their computers after installing one of our air intake systems. Once we’ve compiled computer modeling data from our lab testing, we plug it into our rapid prototype machine and start making parts. Then it’s time to test them out in the real world, both on the street and on our in-house dyno. Airaid’s philosophy is to under promise and over deliver, so the horsepower numbers we quote are average power gains, not the best horsepower gain from a single dyno run.”

Sensor Compatibility

Since delicate electronics like the MAF sensor and inlet air temperature sensor are integrated into the factory induction tract, Airaid goes to painstaking lengths to make sure its induction systems are compatible with these factory sensors. “The name of the game is keeping the air/fuel ratio within the window the OEMs intended for. That’s why if you cobble together your own intake system out of PVC pipe, you run the risk of the MAF sensor not working correctly,” McGee says. “Once you redesign the air filter and induction tubing, you also change the path of the airflow. When this happens, sometimes the MAF sensor won’t read correctly if left in the stock location, which can lead to an overly lean or rich air/fuel ratio. With the help of the flow bench, we are able to find a location in the induction tract where the air path allows for more accurate MAF sensor readings. The same applies for the inlet air temperature sensor. To ensure accurate readings, you don’t want to position it in an artificially warm or cool location, since this will cause the ECU to alter the timing curve and negatively impact performance and reliability.”

Filter Shape

Air filters come in many shapes and forms. Some are round, others are cone-shaped, and many stock boxes house flat-panel filters. For performance air filter manufacturers like Airaid, determining the ideal filter design for each vehicle application is a balancing act between optimal flow and packaging constraints. “Flat filters create turbulence inside the induction system, so their use is typically limited to drop-in, stock replacement applications. While cone filters offer a significant boost in airflow, it’s not always practical to use them,” McGee explains. “In many applications, Airaid utilizes oval and rectangular-cone–style filters. Some of that has to do with underhood packaging limitations, and some of it has to do with airflow. With all the variations in induction system design from the factory, sometimes one shape will flow more than another, so it takes quite a bit of engineering and trial and error to come up with the ideal filter shape and design.”

OE vs. Performance Intakes

Installing an aftermarket cold-air induction system is one of the easiest ways to bolt a good chunk of horsepower onto a new late-model, so it’s not surprising that it’s one of the first modifications that enthusiasts make. Even so, those new to the hot rodding game might find it hard to believe that a simple cold-air induction system as much as 16 rear-wheel horsepower, as is the case with Airaid’s fifth-gen MXP kit. McGee says, it’s a difference in design priorities that accounts for such large horsepower gains. “Factory air intake systems are designed to be as quiet as possible. They often include various resonators in the induction tract to cancel out noises, but quiet operation can come at the expense of airflow,” he explains. “On the other hand, enthusiasts think it’s cool to hear the whooshing and sucking noises an aftermarket intake system makes. They want the performance gains of an aftermarket setup, but don’t mind the increase in noise, either.”

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