Everything you’ve ever wanted to know about air filtration but were too afraid to ask. A decent air filter is the first step to unleashing more performance from an engine. We take a closer look at the options available and how they work… Words: Jamie King.
A performance air filter has always been seen as the first step in engine tuning. It’s a quick, easy, and relatively cheap way to increase both power and torque outputs, and forms the foundation for any further engine upgrades. But with so many options available it can be difficult to know which is best suited to you and your car. You’ve got simple direct replacement panel filters through to fancy carbon fibre complete intake systems, and then you’ve got a selection of materials to choose from, too.
Each of these have their own set of advantages and drawbacks and, as such, there is no ‘one-size-fits-all’ answer to determine which filter is best suited to your car. For example, a rally car operating in huge clouds of dust and dirt will require a very different air filter to that of a Formula One car sprinting around on a clean race tracks. Likewise, a road car with 10,000-mile service intervals will need a different filter to that of race car that undergoes a full rebuild after each outing.
So, to help you understand exactly what an air filter does, how it does it, and how this can affect an engine’s performance, lets take a closer look at the subject…
WHAT IS AN AIR FILTER?
An air filter is quite simply what its name suggests – a filter for the incoming air. You may not realise it but the air your engine breathes in as you drive down the road is far from clean. It contains all manner of contaminants, everything from dried up old leaves through to dirt, dust, and other abrasives suspended in the air. If these foreign bodies enter the engine through the intake system, as they would without an air filter present, they can cause potentially catastrophic engine failure. Imagine pouring sand through the spark plug hole, it almost amounts to the same thing!
So, you can see, filtering out these contaminants is essential. But by filtering the air we are by its very nature restricting the airflow. And restricting airflow is not good for the performance of an engine. So, we need a compromise, and different types of air filter will offer different levels of compromise. Taking the extremes, at one end of the scale we have no filter at all; it doesn’t restrict the airflow at all but it also doesn’t stop any foreign bodies entering the engine either. At the other end of the scale, imagine blocking off the intake completely; no foreign bodies at all would get through but this would also stop all of the incoming air, too, so the engine wouldn’t even run.
In between, you have a slider scale of airflow versus filtration. The more filtration you have, the less airflow you have. The more airflow you have, the less the filtration.
Then you can add size to the mix. A bigger filter will flow the same amount of air but with a higher level of filtration. And we also have intake temperatures to consider; cold air is denser and therefore makes more power than hot air.
That’s why air filter manufacturers have developed clever ways of allowing as much air to pass through as freely as possible, while still filtering out as much dirt and debris as possible. They achieve this by using various different designs and a selection of different materials…
Most OEM air filters are made from paper as it is very good at filtering out the dirt and debris and is relatively cheap and easy to mass-produce. However, its performance is constantly diminishing. Imagine a paper filter as a piece of paper with lots of little holes in it (that’s essentially what it is!). When a bigger particle tries to pass through the small holes it can’t fit and gets trapped – great for filtration but that does mean that hole is now blocked and no longer available for the incoming air to pass through. Imagine a paper filter has 100 of these tiny holes every square centimetre; after 5000 miles, ten of those 100 holes may well be blocked, resulting in a 10 per cent loss of airflow; after 10,000 miles 20 holes may be blocked; after 20,000 miles 40 are blocked, and so on. And this will continue until the filter eventually becomes so clogged that it cannot flow enough air for the engine to operate correctly.
To increase the surface area, and therefore amount of time it will take before a paper filter is blocked, many paper elements feature fin-like pleats to squeeze as much material as possible into a given area.
Finally, while paper is very good at filtration, it’s not actually that great at allowing air to flow through it. You could fit a larger paper filter to allow greater airflow, but typically you are governed by the physical space available in which to fit an air filter. In these cases, switching to an alternative material that allows greater airflow from the same size filter is a better option.
Cotton Gauze Filters
One material that can flow significantly more air than paper is cotton gauze, and these types of filters are among the most popular performance items. They use multi-layer cotton gauze (typically four-ply) with a specially designed aluminium screen. This aluminium screen has a dual purpose: it offers support and rigidity to the cotton gauze but it also allows the filter to be corrugated in design, to increase the surface area available within a given space. This enlarged surface area helps with airflow but its main benefit is to increase the service life of the filter.
Cotton gauze filters use a specific oil to help trap the dirt passing through the filter. This is great for filtration as it means the filter can catch more of the contaminants in the air (some air filter companies claim to filter to a size less than five microns!) while still allowing the air to pass through and into the engine.
Another benefit of cotton gauze is that, unlike paper elements, they can be cleaned, re-oiled, and reused. Simply cleaning the filter to remove any dirt will restore its performance back to that of a brand-new filter.
Another type of filter available is a mesh filter. These use layers of fine stainless steel mesh to trap dirt and debris in the same way as cotton gauze items do. However, the general consensus considers cotton and foam to be far better at filtration than mesh filters. The upside to this is that mesh filters can generally flow vast amounts of air and therefore do allow for good power increases. The down side is this power comes at the expense of filtration. As a result mesh filters are general only seen on cars where maximum performance is more of a priority than longevity and reliability.
Another very popular performance air filter is the foam type filter. These are common in many forms of motorsport, having been used in everything from WRC to Touring Cars to Formula One.
Unlike a paper filter, which is classed as a ‘surface media’, foam filters are known as a ‘depth media’. In simple terms this means they offer filtration throughout the entire thickness of the filter. Let’s go back to the example we used when looking at paper filters; if there are 100 holes per square centimetre there are 100 different routes for the air to take to pass through the paper. That is until 50 of those holes become blocked with dirt and debris, and then there are only 50 routes for the air to pass through. Now imagine stacking 100 of those paper filters on top of one another; those 50 blockages won’t all happen on the same level, and even with 50 of the holes blocked there are still plenty of alternative routes for the air the take in order to pass through the filter. For this reason, many foam filter manufacturers believe this type of filter offers greater service life and maintains optimum performance for longer than surface media filters.
Another huge advantage of foam filters is that you can stack different grades of foam on top of one another to alter the filtration effects to suit your required application. For example, you can have a coarse outer layer that traps only the largest of particles on top of a medium layer to catch mid-size particles and finally a fine layer of foam to catch the smaller particles. Laminating these together means you can choose from various different grades of foam to produce a truly unique air filter for the task at hand. For example, WRC cars run a different filter at each event to ensure optimum performance for the conditions.
As with cotton gauze filters, foam items use a specific oil to help trap the contaminants as they pass through the filter. This also means that, like cotton gauze units, they can be cleaned, re-oiled, and reused without the need to purchase a new filter.
TYPES OF FILTER
The simplest form of performance air filter is a replacement panel filter. These fit within the OE air box and are a direct swap for the original paper filter. But being made from a performance media (typically cotton gauze or foam), they allow more air to enter the engine and therefore offer performance benefits. It is believed that even simple panel filter upgrades can allow as much as 40 per cent more airflow compared to the original paper items. However, as they operate within the standard air box you are still at the mercy of the air box design. If the air box flows pretty well then a performance air filter will offer notable gains but if the air box itself is restrictive, fitting a performance panel filter might not make much difference. In these cases you’d need to look at removing the air box and fitting something that allows more airflow into the engine, such as a cone filter or cold air induction system.
Open Cone/ Dome Filters
Moving on from a panel filter upgrade, one of the most popular performance air filter options is to fit a cone or dome-shaped air filter. These are often referred to as ‘induction kits’ as they remove any restrictions caused by the standard air box and allow the filter to draw air from all around.
Typically cone filters are well-suited to naturally aspirated engines, as many OE air boxes pose significant restrictions to airflow, and removing this restriction increases the engine’s power potential.
This is not so much of a problem (but still a restriction) on forced induction engines, where the turbo/supercharger physically forces air into the engine, but a cone filter will still allow more airflow into the turbo/supercharger compared to an air box. Forced induction engines generally require the use of a larger cone than naturally aspirated engines simply because they make more power and consume more air, so the filter needs to be larger to flow the increased volumes of air being consumed.
For naturally aspirated engines using individual throttle bodies (ITBs), a specific filter is required as none of the original induction system is used. For ITBs there are two main options: multiple small filters for each throttle, or a single larger common filter that feeds all the throttles. The style used will depend on the application and its specific requirements; one application may require the largest airflow possible whereas another application might be limited by available space within the engine bay.
The biggest down side to an open cone filter is the potential for heat soak. Because the air filter is able to draw air from all around this can result in hot air from the engine bay being consumed by the engine and, as we all know, hot air is not good for performance. Many induction kits will come with heat shielding and cold air feeds to combat heat soak (with varying degrees of success depending on the application) but the best way to get all the airflow benefits of a cone filter but without the heat soak issue is to consider a cold air induction system.
Cold Air Intake Systems
Many induction kits will come complete with some form of heat shielding to install in the engine bay to prevent the cone filter drawing in hot air from the engine bay, or cold air feeds designed to channel cool air from the front of the car to the air filter. However, the best way to combat the powersapping heat soak issue is to enclose the cone filter within an aluminium or carbon-fibre box. That way you can fully control the air entering the enclosure, the air at the filter, and ultimately the air entering the engine.
By placing the filter within an enclosure it is no longer subjected to the heat transfer from the surrounding engine components, such as hot exhaust systems and turbochargers. Instead, you can run a cold air feed from the front of the car (where it is receiving plenty of fresh, cool air) and channel this directly to the air filter. These types of system tend to be among the most expensive, simply because they are the most comprehensive, often feature complex designs, and involve more materials (often expensive materials such as carbon fibre). However, they do, generally, give the best results on both naturally aspirated and forced induction engines.
These complete intake systems also allow you to take advantage of clever designs to improve performance, too. Some tuning companies will actually make use of the ‘venturi effect’ in their intake design. This uses an inverted cone within the intake system, which not only allows the air to flow freely into the filter, it actually uses a funnel effect as air flows through the cone to increase the speed of the air entering the engine – effectively turning the whole intake system into one big velocity stack!
The only down side to complete intake systems is the price. If you want the ultimate performance air filter then this is what you need, but it does come at a price. For example, the Eventuri Kevlar intake system from the F80/F82/F83 M3/M4 costs a staggering £2520!