Facts On Air Filters

The air filter is an important component on a pro saw - it is also one of the most neglected. We believe pro saws could give a much longer service life if their operators paid more attention to the air filter. Hopefully deeper coverage of this topic will convince you that keeping the air filter on your pro saw in good condition is worth the time and expense.

Pro Saw Air Filter

The air filter on a pro saw is one of the hardest working components. Because of a saw's compact nature, the engine's air intake is only inches away from where chips are expelled. A saw never gets far away from all the sawdust it creates. Falling trees and other harvesting equipment working add even more air borne dust and debris.

On a pro saw, the air filter keeps foreign particles from getting into the saw's engine by accumulating debris on the surface of the air filter. In time, this debris plugs up pores in the filter and decreases its ability to flow air. If the filter isn't cleaned or replaced, performance will suffer.

How To Clean Filters

To clean an air filter, it must be removed and washed. Simply tapping the filter to remove surface debris is not enough to liberate the debris "in" the filter. It is best to wash filters in a non-flammable cleaning solution such as soapy water. Using a light brush to clean the filter surfaces is a good idea. Be sure washed filters are allowed to dry before reusing.  

It is also a good practice to have several filters and rotate them. One of the benefits of the Stihl HD filter system is the ability of its elements to fit pro models from the MS441 to the MS880. This allows a few spare filters to be used on almost any size Stihl pro saw. Stihl recommends air filter inspection whenever the unit is refueled. They also recommend replacement with a clean filter at least once a day. Without a spare filter or two, it is impossible to properly maintain the filter because it is difficult to do more than knock off surface debris in the field.

Air Filter

It is always a good idea to inspect a filter, any damage or leakage is a sure sign it needs to be replaced. But, it is also a good idea to replace filters after a given amount of use.  Sure, you can usually tell when a filter is bad, but some filters look fine on the surface, but debris inside the element may be blocking enough pores to significantly reduce air flow. Routine replacement is the only way to insure good filtration and top engine performance.

When a plugged filter isn't cleaned or changed, many saws run poorly. They develop rich running conditions as a result of low air flow and the choking effect caused by low pressure in the throat of the carburetor. When air flow is reduced by the filter, most carburetors do the opposite of what they should; they increase the amount of fuel that enters the air stream. 

Stihl's Intellicarb Feature

CarburetorToday's Stihl pro saws have a feature called "Intellicarb". With Intellicarb, the air for combustion and the air for the carburetor's fuel pump comes through the air filter. This leans down the carburetor to offset the effects of low pressure on a saw with a partially blocked air filter. As the air filter becomes more restricted, the engine gets less combustion air and less fuel.

 While no device can compensate for lost air flow, the Intellicarb feature does help keep the air/fuel ratio in range on the air that does get through the restricted air filter. 

Extending Cleaning Intervals

Air InjectionHusqvarna introduced Air Injection a few years ago. This feature is actually a type of mechanical air filter and is effective at extending the running time between filter cleaning. Pro customers like Air Injection and Husqvarna has introduced it on all their new pro models over the last few years. Stihl has adopted a similar feature. It is found on pro saws such as the MS441.

The idea behind Air Injection is simple; debris that enters the fan housing is swept up by the fan wheel. The fan blades accelerate the debris.  Centrifugal force causes the debris to go to the outside of the housing, which creates a band of air just above the fan blades that is clean. An  aerodynamic tube collects intake air from this area.

When Filters Should Be Replaced

Air Injection does a good job of extending the time between filter cleaning, but the whole system needs regular cleaning. Husqvarna also recommends that the filter be replaced after one month's use.

Stihl recommends replacing air filters at almost the same interval - after 100 to 150 hours of use or 30 - 35 days. From what we see in our shop, few pro saw users replace their filters this often. How do we know? First, we sell about 1.5 filters for every pro saw we sell each year. This means that neither new saws nor the saws we sold in previous years are getting new filters often enough. More telling is the results of poor filter maintenance we see in our shop in the form of worn pistons, coked bearing (heavy deposits in bearings), and so on.

Why Don't Pro Users Replace Their Saw's Air Filter More Often?

When we put this question to customers, we found that some didn't think ingesting sawdust didn't hurt a pro saw engine. Most told us, they replaced the filter as soon as they saw evidence of debris getting past it. The fault of this logic is: Sawdust does hurt the engine and the filter really needed to be changed before debris got into it. Once there is evidence of leakage, plenty of it has already gone into the engine.

Seized Piston

Perhaps the lack of concern about ingesting saw dust is rooted in a common misunderstanding of the way air/fuel mixture flows through a two-cycle pro saw engine. Many people think the air/fuel mixture flows from the carburetor, through the intake port, and directly into the combustion chamber. Once it's in the combustion chamber, it goes bang, and then exits out the exhaust port. They think sawdust just burns up with the rest of the air/fuel mixture.

Those who know more about two-cycle saw engines understand the flaw in this thinking. The air/fuel mixture does flow through the intake port, but is valved by the skirt of the piston. This is the first part affected by debris in the air flow. Sawdust and debris scratch and wear away at the piston skirt. In time, the skirt doesn't valve as well. When this valve leaks, performance suffers. Notice the pic to the left. The outside edges of the skirt are shiny compared to the material in the middle. The debris has actually sanded it shiny.

BearingsOnce the air/fuel mixture makes its way past the piston skirt, it flows into the crankcase. That is right, it does not flow directly into the combustion chamber. When the air/fuel mixture is in the crankcase, it cools and lubricates the bearings. (This is the reason there is no oil in the crankcase like your lawnmower.) So if there is debris in the mixture, guess where some of it stays. That's right, some of it remains on the bearings and other components in the crankcase. Notice the pic below left. One bearing still has shiny balls after serving it's life. The other is not shiny but coated with black lumpy stuff. (Dirt that has been ingested and stuck to parts in the bottom end.)

The point of all this is; debris in the air/fuel mixture damages the piston skirt and sometimes leads to other problems in the bottom end such as bearing coking. While damage caused by light debris may not be realized for sometime, its gradual toll eventually leads to expensive repairs. It also slowly degrades performance.

Why New Filters Are Necessary

As the air filter stops dirt particles, the porosity of the filter reduces. When you stop and clean the filter, you liberate much of the debris, but not all of it. After each cleaning cycle, fewer and fewer pores give up their debris. At the same time, some pores in the filter get larger after a cleaning cycle. This is especially true if any kind of compressed air is used to clean it. The good news is, these larger pores often mask the effect of other blocked pores by helping restore air flow. The bad news is, the larger pores reduce the effectiveness of the filter allowing larger particles to pass through it.

The bottom line is a new filter offers two things: it will stop fine dirt particles, and at the same time provide good air flow. Air volume is required for top performance and clean air flow is required for long life. It is that simple.

How Much Air Does A Saw Really Need?

Next time you are packing your gear uphill and you're out of breath, think of your saw's air filter system. A Stihl MS441 running in a log takes in the same amount of air an average person breathes in six hours every minute. In a day (six hours of running time), the saw takes in as much air as a person breathes in a hundred days. So if you think an old partially plugged filter really doesn't hurt performance, think again. Filters must be in good condition to handle this volume. Any impediment to air flow reduces performance.

Our Advice

Follow this advice. It will save you money. Poor air filter maintenance causes trips to saw shops. And yes, saw shops make good money changing pistons and main bearings. On top of this, saws that aren't fixed usually get replaced with new ones. That means more new sales. So why are we telling you this? Our job is to help you get the most out of your tools... that's why we're the Pro's Choice.

Got questions or comments about air filters? Call or stop in.