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How Air Filters Work

There’s more than meets the eye in High-Performance Air Filters. Airaid explains what goes into designing a High-Flow Air Intake System.

Stephen Kim Oct 22, 2013
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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.

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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.”

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Airaid Carb Cover 4/14

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.”

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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.”

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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.”

“Our fifth-gen Camaro intake kit is one of our most popular systems, and is good for an extra 16 rwhp.” —Trent McGee

Build it Yourself

Swapping late-model EFI motors like Gen III/IV small-blocks into older muscle cars is very popular, but it often requires building a custom air intake system. Depending on your fabrication chops, this is a task that’s either stupid easily or extremely intimidating. Fortunately, Airaid’s U-Build-It (UBI) intake kit makes the process simple for hot rodders of all experience levels. “Our UBI kits include a long piece of molded plastic tubing that features multiple 30-, 45-, 60-, and 90-degree bends in addition to several straight sections of tubing. All you have to do is cut the tubing to arrange the bends and straight sections into a shape that fits inside your car’s engine compartment,” McGee explains. “The kit includes couplers, clamps, brackets, hardware, and a conical air filter to help build a custom intake into any shape you desire. There may be a little head-scratching involved when first laying out your design, but the UBI kit is very user-friendly and the only limitation is your imagination. The exciting news for F-body and A-body owners is that we’ve recently come out with cold air panels that snap into the stock radiator core support. These panels isolate the filter from engine heat to further enhance performance.”

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Filter Cleaning

In addition to increased airflow, one of the great perks of an aftermarket cotton gauze filter is that they can be cleaned and reused. Airaid recommends cleaning SynthaMax filters every 20,000, and SynthaFlow filters every 30,000 miles. While servicing a filter is easy, there are some very specific steps that must be followed for optimal results. “After removing the filter, gently shake or brush off excess dirt and debris. Next, spray Airaid cleaning solution onto the filter and let it soak for 10 minutes,” McGee says. “Rinse the filter from the inside out with a hose set at low pressure until the water turns clean, then let the filter air dry. Once the filter is completely dry, apply a light coat of Airaid performance oil until the filter is uniformly pink. After it’s dry, the filter is ready for reuse. Airaid offers filter tune-up kits that include the cleaning solution and performance oil.”

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Cleaning an air filter is straightforward enough, but McGee warns against over oiling the filter. “If too much oil is applied to the filter, it can enter the air stream and land on the MAF sensor. The way MAF sensors meter air is by sending voltage to a heated filament, then determining airflow based on how much the temperature across the filament changes,” McGee explains. “When oil gets on the MAF sensor, the heating and cooling characteristics of the filament changes, which causes it to read incorrectly. This can lead to an overly lean or rich air/fuel ratio. Some MAF sensors can be salvaged with TB cleaner, but sometimes they have to be replaced. Consequently, you should always take the necessary care to avoid over-oiling the filter.”

“In situations where oil has gotten onto the MAF sensor, it’s almost always due to a filter that’s been over-oiled during servicing.” —Trent McGee

Dry Filters

Airaid’s standard SynthaFlow filters are built from several layers of cotton gauze in addition to a synthetic layer that prevents the filter oil from entering the airstream. Although servicing the filter is a straightforward affair, Airaid has further refined the design to make it even more user-friendly. “In 2009, we introduced the SynthaMax filter that features a 100 percent synthetic filter material. It has the same airflow benefits as our SynthaFlow filter, but does not use any oil,” McGee says. “This prevents the potential of over-oiling the filter during servicing. Furthermore, SynthaMax filters can be washed out with a regular household detergent. They don’t require the use of a special cleaning kit. While SynthaFlow filters have a proven track record, SynthaMax filters are a great option for those who don’t want to oil their filters.”

Heat Isolation

Engines prefer ingesting cool air over hot air, so reducing the inlet air temperature is always a top priority when designing an aftermarket air intake system. “Colder air is denser air, and denser air packs more oxygen molecules, which makes more horsepower. If we can relocate the air filter to a cooler spot under the hood, we will incorporate that into our design,” McGee explains. Taking this concept one step further, some of Airaid’s air intake kits include a cold air dam that isolates the filter from underhood heat. “Our cold air dams replace the stock air box entirely or snap into the bottom of the airbox. Sometimes you want to retain the bottom portion of the stock airbox because it has inlets routed to cooler ambient air. Another nice feature of our cold air dams is that they have weatherstripping on the edge of the panels. The weatherstripping seals off the filter by turning the hood into a lid that blocks out heat.”

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Product Lineup

Airaid’s diverse range of products encompasses applications for fifth-gen Camaros, C6 Corvettes, S10s, and just about every Gen III– and Gen IV–powered truck Chevy has ever built. For older vehicles with TBI induction systems, Airaid’s Classic system features a filter adapter that bolts in place of the factory round air cleaner. Attached to it is a powdercoated intake tube that utilizes a conical air filter. “This arrangement allows us to move the filter from on top of the engine to a cooler location under the hood. The next step up is our CAD intake systems that feature a cold air dam, a molded intake tube, and a conical air filter,” McGee explains. “Airaid’s MXP Series intakes are our top-of-the-line models that include a conical filter, a roto-molded high-flow airbox, and a high-density polyethylene intake tube designed to maintain proper MAF sensor readings. Some MXP systems also include a velocity stack that further improves airflow and velocity.”

Budget Options

Since different enthusiasts have different budgets, Airaid offers a variety of entry-level air intake options that can be upgraded later on down the road. “Our junior intake systems replace the most restrictive parts of a factory intake system: the paper air filter and intake tube. These kits include a drop-in stock replacement panel filter, and an Airaid modular intake tube for an economical yet effective solution to boosting airflow,” McGee says. “The modular intake tubes are also available separately, and can be attached to a stock airbox. That way you can install a modular intake tube, and upgrade to a conical air filter at a future time. Some cars have very well-designed intake tubes from the factory. If that’s the case, we won’t bother designing our own tube because if there’s no gain to be had, we don’t want to sell people parts they don’t need.”

Carb Filters

Although Airaid’s air intake kits are geared primarily for the EFI market, the company hasn’t neglected the jet-and-booster crowd. “Airaid offers a full line of replacement filters that drop right into a carbureted air cleaner. We also have carb hats that can be used with our UBI kits,” McGee says. This arrangement is great for cars that have limited hood clearance, and it allows relocating the filter to a cooler location under the hood. If you prefer a more traditional look, Airaid’s Classic II air cleaner assembly is an exciting new alternative. It features a tapered 14-inch diameter filter available in 3-, 4-, and 5-inch heights. “The housing is built from hand-poured urethane for durability, and the assembly is topped off with an aluminum lid for good looks. Other features include a drop based for maximum hood clearance, and a contoured lid that enhances airflow and provides additional clearance for the carb vents.”

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Phoenix, AZ 85050



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