Gauze Vs. Foam filters in chainsaw applications
For a given airflow if two filters are tasked to filter the same amount of dirt... the only way one would show better "dirty performance" would be by increased surface area or with less effective filtration. Two filters of the same surface area that flow differently obviously have very different filtration capabilities. Do the filters in question have a radically different filtration area? If not then the only way one will allow better flow while being dirty would be to allow particulates to pass through the engine. I personally would much rather ensure the proper filtration than ensure consistent power levels well after I should have been cleaning the filter.
Your question states the requirements of a filter perfectly. Ideally you would like to have high airflow (CFM) and high efficiency (particle removal) to give high effectiveness. To compare two filters the answer is not as simple as you might think, especially where the filters use different filter mediums, oiled gauze vs. oiled foam. You can google those keywords and bring up manufacturers websites, K& N, Uni, No-toil etc, but information is surprisingly spotty. This is partly because each filter medium has its strengths and weaknesses and the websites tend to emphasize the parts where they do best. Even if you work through the information, most of it is directed at motorcycle or car applications and chainsaws are somewhat different. It would take several pages to try to present this properly, but I'll try to summarize.
Since you asked, we will cover surface area. Gauze has a high CFM airflow when new, foam will have a lower CFM for a given surface area, so typically they are made slightly larger to off set this. There is a formula to show how much air an engine needs and how much surface area that requires. An 084 has 7.4 C.I. and turns at 12,000 rpm = 88800/ 20839 = 4.2 square inches minimum. An 046 needs 2.9 sq/in. Typical off road filter application should have 2 times the surface area, but chainsaws live in a much harsher environment and need a higher value. A factory filter has around 15 sq/in of surface or 3.5x service interval. A Maxflow has around 30 sq/in or 7.4X service interval. With the Maxflow, an 084 has 7.4x interval, a 046 has 10.3x interval so it is clear that an 046 will run much longer between cleanings.
I'll speak to gauze vs. foam efficiency ("efficiency" being the ability to capture dirt). Since gauze has the advantage of high airflow when new, CFM tests are usually taken on new elements. But these tests don't give the real picture because new gauze filters don't actually filter very efficiently. It takes some time for the filter to trap enough dirt, which then acts as filter medium, before the filter gains high cleaning efficiency. From K&N "As the filter begins to collect debris, an additional form of filter action begins to take place because air must first pass through the dirt particles trapped on the surface that have become part of the filtering media. That means the filtration efficiency of a K&N element actually increase as the filter collects dirt". Unfortunately, while the efficiency in increasing, the CFM is already dropping because as the filter collects dirt, the restriction value increases. Using only CFM testing on new elements is not an accurate to measure gauze filters, because where the filter is efficiently cleaning dirt the CFM is much lower. We always test gauze filter at their efficient (somewhat dirty) stage. So constantly cleaning a gauze filter to achieve high CFM is not going to give your saw the cleanest air. Once the dirt barrier restriction becomes too high the mixture will richen and power drops. If cutting is continued, the dirt is sucked through the filter, OR it is sucked around it OR it is sucked through the crank seals. Fire cutting is the most extreme example of the limitations of gauze filters. When they are clean the fines pass right into the engine. Then the filter begins to load up with fines and the filter is effective, but as the restriction value increases the airflow drops. Either the filter must be changed, starting the cycle again, or the dirt will pass through the crank seals, quickly destroying the bearings. In these brutal conditions saws can seize up in less than a day. Somewhere between totally clean, and totally restricted is a sweet spot where they work perfectly. How long that sweet spot is depends on how large the saw is and how harsh the filtering needs are. For may users that sweet spot is adequate and this may be one reason that the manufacture uses this type of filter.
Oiled foam does a tremendous job of filtering. The thousands of small pores offer tacky spots throughout the depth of the filter to trap particles. As the airflow twists through the pores it is cleaned and this has the advantage of having immediate high filter efficiency, even on a freshly installed filter. The disadvantage of this twisted airflow path is that there is some additional restriction, however, if there is room for a larger foam filter, this can be easily overcome and the CFM of a foam filter can equal or better that of a new gauze filter. The benefit of a foam filter in a chainsaw application is that it will filter small particles immediately, and does not require the loading up of dirt that a gauze filter requires. The other advantage is that the foam filter will provide a nearly constant airflow as it traps dirt. To do this a foam filter must be carefully oiled to filter properly. While motor oil can be used, proper foam oil is much tackier and will stay on the foam longer. The better the oil, the better the filtering. Bar oil should never be used since it is not designed to be burned in a combustion chamber. Because the filter foam itself is not doing the filtering, at some point the oil in the foam will be used up. It is at this point that the foam filter must be cleaned or the dirt will pass through the foam and into the engine. Since there is little restriction value, (just dirty air) there is virtually no chance of air bypassing around the foam or through a crank seal. As long as the filter element is cleaned and replenished with oil, the chainsaw will experience CFM values far in excess of what it needs to run at maximum power AND it will ALWAYS get a spotless supply of air. Since the engine RPM will not drop, even when the oil is used up, it is very important to check the inside of the filter to determine that no dirt is inside the cage. If there is ANY dirt, then the saw has been run too long and the dirt trapping oil is used up. By checking the inside of a Maxflow cage at regular intervals, this overrunning can be avoided. Once you know approximately how long your saw will run, you can anticipate when a cleaning and re-oiling is necessary. For most users with 046 0r 066 saws, the interval is usually just part of their weekend maintenance.
Our testing has shown that Maxflow filters offer cleaner air, for a longer period of time than a gauze filter. While cleaning the air they will allow your saw to run at a consistently high power level. They are very easy to check to determine when they do need cleaning.
I hope this helps. Joseph
