Ideally you would like to have high airflow (cubic feet per minute, CFM) and high efficiency (particle removal) to give high effectiveness over a long period of time. 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 etc. If you Google those keywords and bring up manufacturers websites, K&N, Uni, No-Toil etc, but even there the provided 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. All filters require proper servicing, no filter is idiot proof. This means that none will allow you to run the saw with a dirty filter, even at a lowed performance level, without ingesting harmful dirt into the engine. Max-Flow chainsaw filters allow the longest period of effective filtering at high performance levels. Max-Flow filters are washable and re-useable and are easy to service with the correct oil at suggested intervals. I'll try to summarize why Max-Flow filters are the best filter you can install on your Stihl professional chainsaw.
Oiled foam does a tremendous job of filtering! The thousands of small pores offer tacky spots throughout the depth of the filter to trap even the smallest 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, the Max-Flow filter uses a larger area foam filter element and a larger cover so the CFM of our foam filter is equal or better that of a new gauze or polyethylene filter. The benefit of a foam filter in a chainsaw application is that it will filter fine particles immediately like a polyethylene element, and does not require the loading up of dirt like a gauze filter requires. The other advantage is that the foam filter will provide a nearly constant airflow as it traps dirt. Since it is the oil in the foam, rather than the filter foam itself is not doing the filtering, at some point the oil in the foam will be used up. At this time the foam filter must be cleaned or the dirt will pass through the foam and into the engine. Unlike a pleated filter whose microscopic pores are difficult to reopen once clogged, a foam filter can easily cleaned and the pores re-opened and re-oiled. Re-oiling a foam filter must be done carefully. While 2 stroke mix oil can be used, proper foam oil is much tackier and will stay on the foam longer. Simply put, the better the oil, the better the filtering. Bar oil and motor oil should never be used since it is not designed to be burned in a combustion chamber. There are a few disadvantages to oiled foam elements, the most obvious being the somewhat messy job of cleaning and re-oiling the foam. This is probably the biggest reason that they tend to be aftermarket, rather than OEM filters, some users simply don't have the skills to do even simple maintenance. The other issue is that at its dirtiest, foam has little restriction value. 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. As long as the user is aware of these limitations and by checking the inside of a Max-Flow 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 MS441, MS500i or MS661 saws, the interval is usually just part of their weekend maintenance. The good news is that since there is no restriction, there is virtually no chance of dirty air bypassing the element or entering through a crank seal. As long as the filter 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 get a spotless supply of air. Field testing has shown that Max-Flow foam filters offer cleaner air for a longer period of time than any other filter medium. They do an excellent job of cleaning the air your saw needs, while still allowing your saw to run at a consistently high power level. By looking inside the cage and observing if any dirt has passed, they are very easy to check to determine when they need to be cleaned.
Question: For a given airflow, if two filters are tasked to filter the same amount of dirt, is the only way one could show better "dirty performance" is by increased surface area or through less effective filtration? Do two filters of the same surface area that flow differently obviously have very different filtration capabilities? Can filters that fit the same application have radically different filtration areas? Is the only way to get better flow while being dirty to allow particulates to pass through to the engine? Is there a way to ensure the proper filtration and consistent power levels for prolonged running intervals? This question states the requirements of a filter perfectly. I'll first talk about filter media. There are quite a few styles of filter media for chainsaws. Most Stihl professional saws use K&N style oiled gauze, oiled foam and more recently pleated polyethylene. In some snow areas where there is no dust, screens are used. Flocked filters and pleated paper are for small consumer saws that see low airflow and infrequent use. Pleated paper is a style that is many engine applications, from riding lawnmowers to automobiles and heavy equipment. In vehicle applications, filters can be oversize and this allows for long life. Pleated paper is completely oil-less, and high CFM air flow and efficient cleaning. It is relatively inexpensive and easy to replace. Its design is effective by making the air pass through tiny holes that filter out most dirt particles. The downside of the paper filter (and more recently pleated polyethylene) is that the airflow holes must be microscopic, some 70% smaller than flocked or polyamide gauze, and there are only so many of them. Since the small pores of a paper filter makes them very difficult to clean, in most applications they are thrown away. The Stihl HD2 with pleated polyethylene makes the filter more abuse tolerant, but it is expensive and it is not considered as disposable as a paper. It is also oil-less, in fact any oil will ruin the airflow. It has many of the advantages of paper, including high CFM and efficiency. Like paper media it depends on microscopic holes to filter particles. While this works well on small saws, this is problematic on high power chainsaws that need to move a lot of air. A chainsaw filter is relatively small, but the filtering needs are extreme, and the pleats can quickly pack with dust, especially if there is any oil contamination. Some users are adding a pre-filter or nylon to reduce this packing, but this significantly reduces CFM. Even with compressed air and repeated tapping, it is difficult to remove the dirt from deep in the pleats. Many users report the need to clean in as little as two hours. Aggressive tapping or compressed air cleaning leads to pleat bond failures and this new media has expensive replacement costs. Once the pleats and airflow holes are plugged with dirt, restriction occurs and performance quickly suffers. Besides lowering horsepower output, running with a restricted filter greatly shortens engine life because the air and dirt will find a way to bypass the filter completely. On high output saws, the constant cleaning and short replacement interval offsets their potential and with these unsatisfactory results, professional users are divided about the overall benefits over the K&N style it replaced. Most professional saws equipped with a pleated style filter will see a tremendous lifetime benefit when equipping them with a Max-Flow oiled foam filter.
Lets talk about the surface area, or effective filtering area, which is the usable portion of the filter media. Gauze and paper have 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 offset 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 higher. A factory filter has around 15 sq/in of surface or 3.5x service interval. A Max-Flow has around 30 sq/in or 7.4X service interval, nearly double the gauze or paper filter. With the Max-Flow, 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. An additional valid point about surface area is the texture of it. Most gauze or paper filters use pleating as a way to add rigidity to their media. It is also the only way to produce the necessary surface area. The downside is that large particles of sawdust can wedge into the pleats, reducing the airflow through large portions of it even though the filter media itself is still clean. To combat this, some filters include a felt sleeve that fits over the top of the gauze. While this sleeve does add some smooth surface area which helps shed unwanted sawdust, it also dramatically reduces the CFM of the element. In other cases a foam band is installed, which makes you wonder why you would not just use a foam filter in the first place. The sawdust choked chainsaw environment requires specialized filtering that negates some of the advantages of the paper or gauze mediums.
Regarding the gauze or polyethylene vs. foam efficiency questions, ("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 gauze filters don't initially filter very well. This may be one of the reasons that Stihl has discontinued them in favor of the pleated polyethylene. With oiled gauze, 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. This means that the filtration efficiency of the K&N actually increases as the filter traps 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 at the point the filter is efficiently removing particles, the CFM is much lower. We always test a gauze filter at their more efficient (somewhat dirty) stage. Constantly cleaning a gauze filter to achieve high CFM is not going to give your saw the cleanest air, it needs some trapped particles to gain efficiency. But at some point 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 bottom end 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 previously used this type of filter. It is important to note that the Stihl HD2 filter has changed the way the manufacturer dealt with air filtering, since their pleated polyethylene has pores 70% smaller than fleece or polyamide. This new filter element is better at trapping smaller particles, but at the expense of longer running intervals. Better filtering, but more time spent cleaning instead of cutting. Installing a Max-Flow filter will provide cleaning performance and longer cutting intervals.
CLEANING AND OILING YOUR MAX-FLOW FILTER
Whether you have many spare elements or field clean your filter element whenever it gets dirty, it is important to thoroughly wash the old oil and trapped dirt particles from the foam and then re-oil it. We do not advise motor oil, vegetable oil or gauze filter oil such as K&N. None of them will efficiently trap the smaller dirt particles. Remember the quality of oil determines the effectiveness. Suggested oils are Bel-Ray, UNI or No-Toil. DO NOT use bar lube, it is not designed to be burned in an engine and will cause your rings to stick! This is an easy job and with care it doesn't even need to be messy.
Suitable for the following Stihl Chainsaws: 044, MS440, 044, MS 460, MS461, MS500i, 064, 066, MS660, MS661, 084, MS880, MS881
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