The air filter: which is better?

In the world of racing and competition, the air filter is just another option, as it is often replaced by a trumpet or simply not installed at all.

On street motorcycles, the situation is quite different, as one of the owner's goals is for the engine to last a long time, and to achieve this, we need the air entering the engine to be free of external particles that could damage it.
Most of these engine-damaging particles are between 10 and 20 microns in size (a human hair is 50 microns in diameter), and a good air filter can trap up to 97% of these particles.

The problem is that the more restrictive an air filter is, the less efficient it is, so different solutions must be adopted, as we will see below.

The filter also emits sound
More specifically, the intake. When a vacuum is created in the cylinders due to the pistons moving down, air is forced in, emitting a characteristic sound.
The fact is that motorcyclists typically appreciate the sound of the exhaust more than the intake, so manufacturers focus their efforts on creating the quietest intake possible to achieve these "stolen" decibels, all within the maximum established by regulations. In other words, a silenced air filter is required so that the exhaust can produce a louder "standard" sound.
Therefore, original equipment filters have extra layers of wave-damping fabric, and the airboxes often have smaller openings than recommended, or have Harley-Davidson-style Active Intake flaps. All of this allows the vehicle to comply with regulations, but it reduces its power.
We therefore conclude that replacing the stock filter with an air filter designed to be more efficient will allow more air into the cylinders, increasing power, torque, and performance.

Filter types
A good air filter is one that combines good airflow with maximum particle retention.
Both the ability to filter particles (SAE J726 test) and the airflow delivered by a filter (flowbench test) can be measured, thereby determining its performance and efficiency.

In motorcycles there are 3 main types of air filters in relation to the material with which they are built:
- Paper : This is the most economical filter and is usually standard equipment for most manufacturers. It is available in a pleated format to provide a larger filtering surface in less space, and it does its job quite well.

This type of filter gradually clogs as it traps particles, so it must be replaced regularly.
- Oil-impregnated foam : This is one of the most efficient designs at trapping dirt, and it barely loses efficiency even when dirty. The downside is that they require a fairly large size to ensure good airflow, so they tend to be bulky. They are primarily used in off-road applications.

- Oil-impregnated cotton fiber : This type of filter element is the one we usually find in high-performance filters, such as K&N , and combines the advantages of foam and paper filters.

It takes up little space, and the cotton fibers are capable of trapping more particles than paper. Most importantly, these particles accumulate only on the surface of the filter, so they do not block the air passage holes, achieving optimal performance and efficiency.

These filters must be "regenerated" at each service (cleaned and lightly oiled with a specific kit ) and can be reused about 25 times, i.e., more than 200,000 km.

The best filter
Now that we know their functions and the different types of air filters, all that's left to do is choose which filter box we like best. There are many options here, and in this case, the decision is usually an aesthetic one, as most high-performance filters are capable of providing more clean air than the engine may need at any given time.

As always, it only remains to remember that if we change the amount of air entering the engine, we must recalibrate (in the case of carburettor) or reprogram the control unit (in the case of injection) so that the entire system works correctly and balanced.

Frank Burguera

#harleymechanics