Let’s take a moment to appreciate the humble air filter. Tucked away in the dark depths of your airbox, it rarely sees the light of day and yet it’s fastidious about the quality of the air your engine sucks in every time you open the throttle. It doesn’t require much attention and it’ll stay loyal to its cause for thousands of miles. You can even neglect it and it’ll still do a good job.
You see, without an air filter, dust and debris would quickly make its way into your engine, where it would cause havoc with your motorcycle’s finely crafted internals. Piston-rings would wear and before long your pistons would be acting like sandpaper against your cylinder walls.
Eventually, your engine would give up. So the humble air filter, although it never gets much credit, is a vital factor in your engine’s overall health and ability to produce power.
There are three main types of air filter used by motorcycles, Paper, Oiled Gauze and Oiled Foam. The vast majority of road bikes come with a paper filter as standard. Most off-road bikes used an oiled foam but why are different filters used and which type is the best? Find out below.
Your engine needs air to help it produce power from the fuel its igniting. A standard bike is fuelled to match the rate of air that can flow past the standard filter, so it doesn’t restrict the engine’s ability to produce power in the way the manufacturer intended. They also have emissions levels to worry about.
However, if you tune your bike, you’ll be ‘leaving the handbrake on’ if you don’t swap your paper filter for one with a higher flow rate, to help the engine produce more power.
Most stock road bikes are fitted with paper filters. The paper material is like the thick but porous paper used in filter coffee machines.
They're usually pleated in order to massively increase the surface area and therefore increase the amount of air that can be pulled through the filter and into the combustion chamber.
A paper filter is very good at blocking out particles, which is why most manufacturers will use them as OEM. They give enough air flow but their priority is to protect the engine by blocking the transmission of tiny particles, dust and road debris that make it into the airbox.
A paper filter should be replaced once it has reached the end of its serviceable life, however you can carefully clean them to extend their life but you need to ensure you don't puncture the paper membrane. It's often better to replace it with a new one. Most bikers will opt for an oil-based filter instead of a paper one, for the reasons listed below.
An oiled-gauze filter does away with the paper membrane. Instead, they use layers of fabric, coated in oil, often separated by thin wire mesh frames.
One of the most well-known filters of this type is K&N. The fabric layer (usually cotton or a made-made fibre) is more porous than paper and allows air to flow at a faster rate. The particles are removed by sticking to the oil on the fabric. These types of filters are the best choice for anyone looking to fit an exhaust or remap their bike with the intention of improving its peak torque and horsepower but you can't just fit it and expect gains (more on that below).
Aside from improving air-flow, one of the major benefits of an oil-based filter is that you can simply wash them and re-oil them and they're as good as new. They're arguably better for the environment than a paper filter, which will end most likely up in landfill.
Oiled foam filters are common on motocross bikes and other off-roaders. They're a far better choice than paper for bikes that operate in dusty environments as the foam can trap dust particles while still allowing good airflow. A paper air filter would become blocked after minimal use, restricting airflow, reducing power and even possibly preventing the engine from working.
These filters are much thicker than standard filters and the porous holes in the foam are visibly large, meaning maximum airflow, but the surface is covered in oil and as the air travels through the filter, particles are removed efficiently. You can wash them out and re-oil them (most motocross riders will do this after every event), to ensure they function effectively.
A direct replacement for your OEM air filter, one that can be washed and re-oiled for the lifetime of the bike, meaning you won’t have to buy anything else.
K&N air filters are known for their quality. They’re more expensive than a paper filter but they can be washed and re-oiled as often as you like and they come with K&N’s 1-million-mile guarantee meaning you’ll only ever have to buy one. They’re a direct replacement for your OWM filter. Owners rate them for an improved induction noise and better throttle response. If you’re fitting an aftermarket exhaust, an air-filter like the K&N is an essential addition to maximise your power gains.
When it comes to off-road riding, you’ll value a filter that’s easily removable, robust, and free-flowing. For most off-road riders there’s only one choice.
Dutch firm Twin Air have been making air filters since the early 70s. They started in motocross but now make a huge range that also covers Adventure bikes and some other road bikes. A laminated dual-foam filter that’s coated in a thin layer of oil. It’s free-flowing and helps the bike develop power. They’re washable and 100% reusable.
In a majority of cases, fitting a performance (or high-flow) air filter to your motorcycle will only see a small power gain. Something in the region of 1% of the stock bike’s power is a reasonable assumption. So for a 1000cc superbike, you’re looking at approximately a 2bhp gain.
The main issue with a free-flowing air filter when you have a stock exhaust is that the amount of air the engine can pull in is governed by the amount of air (or exhaust) it can expel. A performance air filter on its own won’t make much of a difference. However, if you fit an aftermarket exhaust or a full system you’ll increase the potential volume of airflow through the system. Therefore fitting a free-flowing air filter will help the bike breathe and in turn, allow it to rev more freely and produce more power.
However, when fitting an exhaust and air filter you’ll also need to check the bike’s fuelling on a dyno. It is likely that you’ll need to flash the ECU or fit a Power Commander or similar to allow the bike’s engine management system to accommodate the increased air volume and rate of gas exchange.
When fitting a full system in conjunction with a performance air filter and a remap you can expect anything between 5 and 10% additional peak power and torque and a faster-throttle response.
The chart above demonstrates the power gains seen on a Honda CRF1000L Africa Twin with a Twin Air filter compared to stock. As you can see the gains are marginal with approximately 2bhp gained at peak power output, better pickup from zero-revs and small gains throughout.
If a Power Commander or an ECU flash was used, you’d expect somewhere in the region of 5bhp throughout. With the addition of free-flowing exhausts, you could expect up to 15bhp gain.
If you want to get a few more miles out of your standard air filter, you can clean it but you’ll struggle to get it back to as-new condition. That’s down to the fact you can’t wash a paper air filter, which would remove the tiny particles caught in the fibres. However, you can brush the filter to remove larger particles and extend the filter’s usable life.
The video above is a good guide for anyone looking to clean their paper air filter.
Oil-based filters are designed to be cleaned and re-oiled, meaning they will last far longer than a paper-based filter. They are more expensive to buy (roughly twice the price of a paper filter) but they’ll last for the lifetime of the bike.
You need to clean your oil-based filter and the gap between washing and re-oiling depends on the mileage you’ve covered and the type of conditions you’ve ridden in.
The video above is a good guide on how to refresh your filter.
There’s nothing wrong with an OEM air filter. The chances are your stock filter is a paper one and it’ll be set up to work with your bike’s stock exhaust and fuelling.
A motorcycle has to meet tough emissions regulations and so the manufacturer has to navigate the tricky triumvirate of Emissions Regulations, Outright Power and Drivability.
In stock form, a motorcycle’s engine isn’t producing the power it could if it had a free-flowing air filter, exhaust and the fuelling adjusted to suit.
If you need to replace the air filter but you’re keeping the standard exhaust and you don’t want to have to fiddle with the fuelling then stick with the manufacturer’s air filter.
There are lots of great motorcycle air filter manufacturers on the market. Below is a comprehensive list of motorcycle air filter brands that are available in the UK.
What does an air filter do on a motorcycle?
An air filter is primarily a barrier to prevent unwanted objects from entering the engine; anything from pieces of grit to tiny particles of dirt. They also smooth out the air flow to ensure its flows at constant pressure.
Does an air filter improve motorbike performance?
A free-flowing air filter, sometimes known as a performance air filter, can improve the engine’s power output but an air filter on its own will only show a marginal gain.
What happens if you ride without an air filter?
You’ll increase the number of unwanted particles that enter the engine, potentially causing wear a loss of power or even engine failure.
Does a K&N Filter make a difference on a motorcycle?
Yes, due to the fact K&N filters are free-flowing filters, which allow the engine to draw in more air, giving the motorcycle a quicker throttle response and they also usually give a very small power gain.
How often should you clean your motorcycle air filter?
This depends on where you use it and what conditions you use it in. In general, for road riders, your filter should only need cleaning every 20,000 miles. However, it is a very simple job to remove, clean, re-oil and replace your air filter and so we’d recommend you do this once every 10,000 miles or annually to ensure it’s working as efficiently as possible.
How much horsepower gain does an air filter provide?
An aftermarket air filter on its own will show only a small gain, around 1% of the bike’s stock power over an OEM filter. The real gains come when you fit a free-flowing exhaust to your motorcycle and map the fuelling to suit the aftermarket parts.
Can you use WD40 as air filter oil?
You can use WD-40 to help clean your air filter but WD-40 should not be used instead of air-filter oil.
Will an aftermarket air filter affect my motorcycle insurance policy?
Every insurer has a different view on ‘performance modifications’ but leading motorcycle insurer Bennetts stipulates that a performance air filter won’t invalidate your policy.
Thanks to the following websites which helped us research and write this article
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