Air Filters

Words Dean Mellor

Standard paper air filters are consumables. Sure, you can extend their life by blowing compressed air at them, but once they’re chock full of dirt and dust they’re ready for the rubbish bin.

An air filter’s purpose is to prevent foreign objects from entering your bike’s engine where said objects could otherwise cause extensive and expensive damage. The foreign objects we’re talking about are often so tiny they’re measured in microns, and include things like dust or plant material, or slightly larger things like insects that occupy the airspace as you ride through it. In the case of paper filters, as they become clogged with these particles the airflow to the engine is gradually restricted. If the filter becomes too clogged, engine performance will decrease, at which point the filter should be replaced.

While paper filters are very effective at preventing foreign objects getting in to your engine, their throwaway nature means they’re not exactly economical. And aside from the obvious expense of having to buy a replacement filter, throwing the old one into landfill is hardly best environmental practice. If you ride a roadbike mostly on the road, you may only have to change your air filter after several thousands of kilometres of riding, but if you ride a dual-purpose machine or a dirtbike off the road, chance are your filter will need regular attention.

As all dirtbike riders know, there is an alternative to disposable paper air filters in the form of washable and reusable aftermarket air filters, of which there are several types. But these filters are certainly not for everyone – while the economic and environmental benefits are obvious, you’ll need to be prepared to put the effort into regularly maintaining a washable and reusable filter, or you’ll risk the chance of the filter not performing as intended. So whether you opt to fit one of these filters will depend on several factors, including what bike you ride, where you ride it, how regularly you service it and how willing you are to get your hands dirty.

Foam Filters

Foam air filters are most often used in dusty conditions where a paper filter would soon choke up and need replacement. There are several brands of aftermarket foam air filters on the market so you’ll have no problem finding one to suit most dual-purpose bikes and dirtbikes.

The filtration area is the foam section, which is either housed in a model-specific filter cage or bonded to a model-specific base, usually made out of rubber or urethane. This base sits against the inner surface of the air box and an airtight seal is achieved with the help of a layer of grease.

When dry the foam does not capture contaminants, so it needs to be soaked in a special filter oil to trap small particles and, to a lesser degree, minimise the chance of water ingress. In order to trap particles, foam filter oils are very tacky; some are available in a spray while others come in a bottle. Most will incorporate a colour agent so you can easily see when the entire filter has been covered during application.

Like a paper filter, a foam air filter will eventually clog with particles, but instead of throwing it away it can be cleaned and re-oiled for reuse. Cleaning the filter can be a messy affair (always wear rubber gloves), and requires the application of a special foam filter cleaning agent. The filter is then squeezed out into a container before being soaked in a bucket of warm, soapy water. The filter then needs to be rinsed in clean water before being squeezed out again and left aside to dry. Speeding up the drying process with a hair dryer is not recommended as the foam can deteriorate or separate from its rubber/urethane base. You’ll need to set aside a couple of hours as the filter needs to be completely dry before applying clean filter oil.

If the filter oil is in a spray can, make sure you give the filter a liberal coating. If it’s in a bottle, place the filter in a plastic bag, pour the oil over it and squeeze through the filter. You can now apply some grease to the mating surface and refit the filter to the air box.

There are several designs of foam air filters. The Aussie-made FunnelWeb filter, for instance, has pyramid shapes on its outer surface design to increase surface area, while the Australian Unifilter employs a two-stage filtration process with a corrugated outer-foam. Some filters, such as those from Motorex, No-Toil, Twin Air and Hiflofiltro use a laminated dual-layer foam that’s bonded together.

Cleaning a foam filter out on the track is obviously not something you want to be doing, so many riders will carry a spare foam filter in their kit for extended rides. Some manufacturers, such as Twin Air, also offer dust covers to fit over the foam air filter for use in extremely dusty conditions.

Cotton Filters

A common style of washable and reusable aftermarket air filter for roadbikes is the pleated cotton filter, of which there are several manufacturers including K&N, DNA High Performance Filters and BMC.

The filtration element is manufactured from a cotton-gauze that’s soaked in a specially formulated oil. The cotton gauze allows for good airflow to the engine while the oil-soaked cotton strands trap foreign particles. Manufacturers of cotton air filters claim improved airflow compared to OE paper filters, even as the cotton filter becomes dirty.

The pleated cotton media used in K&N air filters is claimed to provide a large filtration area for low restriction and extended service intervals. The cotton gauze is housed in a wire mesh and surrounded by a model-specific frame that allows the entire filter assembly to be fitted to the factory air box without the need for modification.

The design of DNA filters is similar to the K&N system, and like K&N, DNA claims increased airflow compared with OE paper filters, which can result in improved throttle response and greater power and torque outputs.

The cotton gauze in BMC air filters is also contained within an external metallic support frame (like a flyscreen), with a soft rubber mounting system for model-specific fitment.

Regardless of brand, cotton filters need to be regularly cleaned and re-oiled. The cleaning process involves removing the filter, liberally spraying brand-specific filter cleaner on to both sides and then allowing it to soak for 10 minutes or so, but not long enough for the cleaning agent to dry. The next step is to rinse the filter with a low-pressure stream of water (over a bucket or other container) from the clean side only to flush the dirt out of the filter. The filter then needs to be set aside to dry naturally, which could take a couple of hours.

Once the filter has dried, clean oil needs to be applied (either poured or sprayed) along the crown of each pleat in the cotton gauze. The oil will have a colouring agent so you can easily see once full coverage has been achieved. Allow the oil to wick into the filter for 10 minutes or so before reinstalling the filter assembly into the air box.

Most manufacturers of cotton filters claim a significant increase in airflow compared with OE paper filters, but to get the most out of this, and to ensure the engine doesn’t run too lean, the bike should be retuned to suit the new high-flow filter.

A Polyester Alternative

Sprint Filters are similar in design to cotton filters but no oil is required, so the manufacturer claims the deep-fold P08 polyester filter element can be cleaned by simply blowing it out with compressed air. Sprint Filters also claims similar airflow and filtration properties to cotton filters but, of course, much less maintenance time, not to mention much less mess.

When cleaning air filters, ensure old filter oil and muck is disposed of in an environmentally sensible manner, preferably by sending to an accredited recycling facility.