I see in another thread Andy referred to my dislike of K&N. Sorry for the rant Andy, but you've touched on something I'm quite passionate about, so I'll put it down here for reference. If this post makes one person reconsider buying a K&N or equivalent oiled gauze filter, it was worth it. There's even a topic on the good tech page called "Air filter rant" from 15 years ago where I go over this. It's an interesting read when the K&N fans start trying to explain why they're so good. There is also lots of good tech in there on Donaldson precleaners and other relevant stuff.
K&N is the equivalent of "miracle anti aging cream" for blokes. They're marketing nonsense. Basic physics will tell you if a filter doesn't restrict air it won't restrict dirt.
To be clear, it's not an "opinion" it's because there is plenty of science that K&N filters are terrible filters. I was reading test results from filtration engineers many years ago, this isn't new news. I work in an unrelated (non-automotive) field with a high reliance on code compliance and data quality. I have a bit of a special interest that when someone puts a claim on the side of a shiny box in SuperCheap auto, it actually means something.
You won't find any auditable test data on the side of a K&N box. They use terms like "the best combination of fuel efficiency, filtration and power" which is meaningless. They also discredit ISO 5011 and then claim 96-99% filtration efficiency. 96% is terrible. Really dreadful- like
six times worse than a good filter. The tables in the article below demonstrate how poorly a K&N performs in like for like testing. Note that in the test below the filter under ISO testing achieves just over 96% efficiency while a the best boring old paper filter nears 100%.
You can read K&N's test procedures here:
https://www.knfilters.com/efficiency_te ... FHYXoeYgKYThey're almost maliciously misleading... it's almost like they're hiding something... Here's a clue though, if you're discrediting an international standard, and not because you think the standard is inadequate, you're not on strong ground.
So here's some ISO compliant test data (bless the authors of this article - this was much easier to find than the article I was looking for initially):
https://www.project200.com.au/dm-iso5011/It's damming. the TL,DR version is K&n filters poorly, it passes about 6 times more than a good paper filter. It can't hold much dust, and in a dusty environment very quickly starts to pass dust into the engine. The good news is when clean it offers very little restriction. Who would have thought a filter that doesn't slow the air down won't stop the dirt.
Their K&N marketing machine rolls on though and people will aggressively defend them because they believed what was written on the side of the box. and it makes more power. Well so does running no filter at all ::shrugs::
Most of what I've written below is the practical result of the poor test data. You might consider it opinion or take issue with it, but it's much more difficult to take issue with the test data. If there are practical ways to make a K&N perform better as a filter and I've overlooked them I'm all for it. Running one in a centrifugal housing, with a pre filter on it (I think they call them a filter charger) fed by a Donaldson pre cleaner might be a solution. It will still have a much shorter service interval than a paper filter in the same housing though.
A big issue with K&N iis that a manufacturer designs an intake for an application, generally including service interval, noise, power, a filtration efficiency required. this design is based on the function of a paper filter. Some kid* throws that away and then fits a cone type filter in its place with no effort to determine if it's enough filter. Maybe they've shortened the service interval to 5000km, 1000km.... maybe less - who knows? (and even then, it's still passing more dust when serviced than a paper filter)
* (or worse, a boomer with $150K of their super tied up in a 300 series and a caravan and they think they're protecting their investment because they read the side of the box)
..and then when they service that filter there's no way of knowing if it's really been done correctly. K&N actually specify the correct amount of filter oil to be applied for each filter. Remember this is done by aerosol. I'm not that good - how do I account for the weight of propellant, overspray, evenness of coating? I've gone as far as weighing the aerosol can before and after application to try and work out how many grams of oil I've applied to know if it's properly serviced because not enough or too much and the filter doesn't work properly. What nonsense.
Speaking of which, If you fit a K&N to a factory airbox, something K&N use as a defence of why ISO 5011 is problematic as it doesn't test in the stock airbox, the airbox stops working properly. I'll explain why.
There are two styles of airbox, panel and centrifugal. You'll notice with panel filters than the air is always drawn UP through the filter. This is so than when the engine is turned off (no vacuum across the filter) the dirt falls off the filter into the bottom of the box wherein collects at the lowest point (which is why they have a sloped bottom) A K&N is sticky so when the engine is turned off, the dirt stays stuck to the filter. This massively reduces service interval. The problem isn't quite as bad with centrifugal filter housings but it's still an issue. Problem is, how do you know what the "new" service interval should be? Note also that as 4WD's become more road biased there are more panel filters fitted to them now. Panel filters are not well suited to a 4WD application as they can't deal with dust very well and then that panel is replaced with a sticky filter that loads up and then starts passing dirt into the engine.
So, how do you make more power through an air filter while retaining good filtration? Fit a larger paper element. Fit a well designed centrifugal filter housing. A larger element (more surface area) means the air moves though the filter slower so, literally, the dirt doesn't hit it as hard, making the filter work better. I have a 1990's Hiace filter in my Suzuki, they use a large cylindrical element which is available anywhere, the housing seals very well and can be easily modified as it's steel. It has more surface area and can run bigger inlet piping than a Suzuki airbox I rarely bother to open my airbox. Every couple of years I'll just swap the filter over.
I've built Suzukis with a K&N cone filter. Note this was a large cone with a 660cc pulling though it, so the filter was huge for the airflow. It passed dirt. I have a K&N in my FG XR6T because it has a RHS intake mod. It passes dirt. I replaced the filter with a longer one and run a filter sock on it so at least the larger debris falls off the filter. Another of my road cars has a factory installed K&N (what were HSV thinking?? - even a HSV dealer told me I'd swapped the filter over "and they wouldn't service it" even though it has a HSV part number on it) but as I said I don't drive that and I need to keep it standard.
PS I've also owned oiled foam filters. They're also garbage.
PPS I'm aware there are now synthetic filter materials that work as well as paper. They are still dry, non-serviceable filters though and gain no respect at a car show.
