Originally Posted By: Phishin
Originally Posted By: nickolas84
I am refering to stock type "paper" filters. It is common knowledge that the dirtier it becomes, the better it filters.
Should we ever change our stock air filter unless there is a reduction in power?
My career has me in a clean room/clean lab. I get to monitor air quality every day and perform maintenance on extensive air filtration in the lab I work in...to ensure that I can analyze samples down to the parts per billion and trillion.
Here is the bottom line: Yes, the dirtier the filter gets, the filter can trap smaller and smaller particles....thus, becoming a "better" air filter.
As the filter gets dirtier and dirtier, the flow rate across the air filter will change.
One of three things will happen as the filter begins to "clog" up:
1.) the media remains fully intact and the filter begins to collapse, usually causing major leaks to occur where the filter seals along its outside edges; which causing massive amounts of unfiltered air past the filtration apparatus.
2.) air will begin to "channel" through the filtering media, finding a pathway of least resistence. Perhaps it starts at a small imperfection in the media. Once the air begins to channel through the media, the "hole" will expand. The air that passes through this channel isn't really filtered at all.
3.) As the filter begins to clog, the dirt and grit begins to break down the media (cutting it microscopically) and cause leaks to occur in the media...thus, allowing unfiltered air to pass through.
So, my conclusion is: a "seasoned" air filter is better than a new one. HOWEVER, if you run the filter too long, you might as well not have one installed....BECAUSE once the media is compromised, the media collapses, or the air is channeling through it, you have a HUGE source of grit and dirt (trapped onto the filter media) right at the point of failure that WILL get sucked right past the filter assembly.
Excellent info on the pitfalls of a willy-nilly, uninformed approach to air filter change intervals.
If you intend extend the OE interval, you shouldn't do so without some forethought. The number one answer is a restriction gauge. You change the filter according to a specified or reasonable restriction reading (15 in. of water gasser and 20 diesel are the low-end generic numbers), you have both headed off any of the bad things mentioned above AND you have gotten your money's worth out of the filter.
The gauge must be mounted downstream the filter but as close to it as possible, ideally in the plenum chamber just behind it. It shouldn't be placed at a bend, near a vortex or close to the throttle butterfly. According the engineer at Filter Minder, they aren't accurate on carbureted engines because of the instable airflow (choke, secondaries, etc.).
Over time and with careful observation, you could probably extend the FCI without a gauge. If, at the OE interval, there is little apparent dirt on the filter, you could extend the interval a ways and check again until the filter is obviously dirty. This can be tricky to gauge because looks are the least accurate way of determining restriction. Once you were obviously dirty, that mileage would then be the modified FCI for your normal driving routine. The danger would be if that routine changed... you took a long trip to a more dusty area, for example. You'd have to be on your toes watching for changes. The gauge is simpler and automatically accounts for all changes.
If there is a long time element involved, that should be factored in as well. In some cases, the sealing material is subject to heat aging and you may get a sealing issue. It really depends on the filter. Look at the cleanable filters for guidance, as their sealing media are designed for decades of service. The sealing material is the best and failure is unlikely. If the replaceable filter used a similar design and material, it's likely good for a long haul. Generally speaking, the foam type seals are the shortest-lived and the type that looks like an o-ring or a seal lip is the longest lived. The OE made this choice for you, so you have to work within the realm of the filter type. Generally speaking, the material in a premium filter is better no matter what the material. In polling various experts, a 5-7 year interval can generally be considered safe for any quality, name-brand air filter. My opinion is, often, the life in years can be a lot longer than that but it's something I'd hash out with a lot of application-specific research for myself.
Avoid molesting the filter seal as much as possible. Every time you disturb it, you risk a sealing issue. For the most part, and this is especially true for the foam seal, they loose pliability over time. They may not fit and seal well on the second (or third or fourth, etc.) fitting. The use of filter seal grease is recommended at the first fitting and during subsequent removal. Not only does it lube the seal so that it fits better, it helps to seal any small gaps and helps keep the seal material pliable. As to the grease, I have not done any research as to whether something other than "filter grease" is viable. Given the K&N filter grease is easily available, I have not taken the time to look past it.
Remember, the engine intake is the primary source for external contamination. A good portion of what finds its way past the air filter makes it into the oil, increasing wear and decreasing oil service life. Much of it is to small to be captured by the oil filter, especially after being ground into polishing paste by the rings. Enough dirt on the cylinder walls causes ring sealing issues and increases blowby, stressing the oil even more fighting the formation of acids from the combustion process as well as fuel dilution.
Finally, installation of a new filter is an important moment. Clean the sealing areas on the housing. Test fit the new filter to make sure it fits the housing well with no gaps. As mentioned above, use filter grease. Then, leave the thing alone as long as practical (note factors above) or the restriction gauge indicates it's time.