The Science Behind Air Filters and Engine Performance

[Counterman]

Air filters are seemingly simple, but unfortunately often neglected until they are so dirty they’ve been unknowingly (to a vehicle owner) affecting performance. Selling them offers benefits to you and your customer. It’s never bad to ask the air filter question and, at minimum, replace them once a year. 

If a customer is skeptical, explain that even as little as 10% blockage of an engine air filter can begin to affect performance. Point out that the effects of a dirty filter were more noticeable on older vehicles, and modern fuel management systems are designed to compensate for reduced air flow. This is good for driveability and emissions, but bad for overall performance. Along with other normal maintenance items, filters are easy for a vehicle owner to overlook.

Be prepared for questions about high-performance filters, because you will get them. Many people ask about high-flow filters, cold air intakes or, on an older car, ditching the original air cleaner for a round, chrome replacement. Don’t be intimidated by performance questions; here’s what you need to know to answer them with confidence.

High-Flow Filters

High-flow performance filters are a very common upgrade. They are typically recognized and associated with a color, as opposed to the traditional white paper element. Often pinkish/red, the color is from a dyed oil that is soaked into the element. The elements are typically a cotton weave, held in place by a wire mesh. The weave is looser than a standard paper air filter, allowing a greater volume of air to pass through. Since the loose weave cannot filter out the smaller particles, it is soaked in oil, and the particles will, in turn, stick to the oil. Dye is used in the oil so there is an easily noticeable contrast between the oil and the element, allowing you to ensure that all areas are saturated. Even though red is the most common, there are multiple other colors available.

These types of filters absolutely flow better, and it is often noticeable in engine performance characteristics. But they do require regular cleaning and re-oiling to maintain their level of performance. You can be confident about selling them and touting the increased flow, but I do warn people that when they clean and oil them, be sure to heed the instructions, and don’t over-oil them. There have always been rumors of these types of filters damaging mass airflow sensors, but they are largely unfounded, and over-oiling them is the only thing that could possibly contribute to this.

Factory Air Cleaners

If you have a customer with an old car who wants to install a round chrome air cleaner, you may not be able to change their mind, but there are many facts about the original factory air cleaners that are often overlooked. Many people think they’re an eyesore compared to a shiny new chrome one, but there’s a lot more to them than meets the eye.

The inside of the air cleaner is designed to smoothly direct the air flow up to the top while reducing turbulence, so the air can be smoothly drawn down through the carburetor. They have a snorkel extending off of them to pick up cool air from the side of the engine compartment, and the snorkel also is designed to increase the velocity of the air as it enters the air cleaner.

In addition, the snorkel design incorporates a pickup for warm air to be drawn from around the exhaust manifold up into the air cleaner for improved cold-weather driveability. On the main body of the air cleaner, there is also a filtered pickup for fresh air to enter into the crankcase as part of the positive crankcase ventilation system, so they are important for emissions, too.

This is all theoretically great information, but if someone wants a chrome air cleaner, I get it. They look great, and they are a common characteristic of some old muscle cars. The completely exposed element offers the maximum amount of air flow necessary for the type of performance sought after in these cars.

The minor affect in driveability due to lack of a heat riser and less air velocity at low RPM is not missed on a car with straight line, high RPM performance in mind, and one that is most likely only driven in the summer. I do like to point out that there were a handful of top-dog muscle cars that came originally with chrome open-element air cleaners. On these cars, the crankcase ventilation was routed to the air filter base, and the aftermarket units generally have a stamped breakout in the base and come with a fitting so this can be reconnected. I always encourage them to reconnect these emission-related devices.

If your customer is replacing the air filter on an old vehicle with an OE air cleaner, it’s important to use a high-quality filter and always look it up for the application. You can’t see it when the lid is on, but the filter is designed to seal on the top and bottom so that all air flows through it. If a filter is installed that is even a little bit too short, unfiltered air will be drawn over the top of the filter into the engine, plus mice can fit through just about any small crack, and that carburetor is an awful inviting cubby to store the kibbles `n bits you keep in the garage.

Cold Air Intakes

Aftermarket “Cold Air Intakes” are the rage on newer cars and trucks, and these can offer performance benefits as well in terms of the flow, but there may be a sacrifice in driveability when ditching the original air box on a modern vehicle.

Modern air boxes also have provisions for warm air during cold starts. They have crankcase ventilation connectors, they reduce noise and they are designed to improve air velocity at low engine RPM, just as the original air cleaner on an old vehicle.

Another drawback to cold air intakes is they often drop very low in the engine compartment to pick up air. Most factory air boxes draw from above the radiator core support or from behind one of the front fenders, the point being they draw from high up. It doesn’t matter if a cold air intake picks up down low…that is, until there is a bad rainstorm and deep water. And, since a lowered suspension is popular on new cars, watch out!

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