I have had doubts about oil based filters for some time - if you search my posts I have been hesitant to recommend oil based filters and do not use one even though I have started to mod the car somewhat.

I finally tracked down what appears to be good research confirming my initial impressions about the so called high flow filters. All is not as it appears - after reading this I am unlikely to change to an oil based high flow filter

An interesting read and some great facts and figures there. Much better than the average review that's for sure.

Whilst I love high air flow into my engine, when I use a high flow filter I know that it WILL impact on my engine. It has to eventually. Might take a fair while but it will. Anyone with experience in filtering knows it as its a reality of physics.

As long as there are people out there though that get ultra excited about high flow filters enough to convince themselves that there is no way more grit can reach your engine, then the myth about the filters magically letting lots of air flow but no additional particulates get through will persist.

You can get SLIGHTLY higher flow filters that happen to be better designed than the factory ones that let SLIGHTLY more air through without any additional particles. The same applies to using different technologies like oil/foam vs dry material.

However the moment you get near anything like a high flow filter, it comes at a cost, and that cost is filtration efficiency. Its not brain surgery - to let more air flow you need to offer less resistance. To offer less resistance there has to be less in the way to stop the air flow. If there is less in the way, there is also less to block the particles, because the particles are generally blocked physically, not by magic. The same thing that blocks the particles is what blocks the air flow. You simply cant have high air flow and high blocking of particles as there is always a trade off of one against the other. The higher one is, the lower the other is.

High air flow, lower filtration.

Low air flow, higher filtration.

Average air flow, average filtration.

Every petrol engine air filter is one of those three, or somewhere between them.

And for the sceptics, read that review mentioned in the intial thread as it confirms what I am saying.

Great find mate!

Thanks

High air flow, lower filtration.

Low air flow, higher filtration.

Average air flow, average filtration.

Mick,

We've covered this before but this is not always right. The way you get more air through a filter is to increase the surface area ie: more pleats. The greater the surface area the lower resistance (pa) across the filter, resulting in better air flow. Increased airflow does not have to mean a lower level of filtration.

High air flow, lower filtration.

Low air flow, higher filtration.

Average air flow, average filtration.

Mick,

We've covered this before but this is not always right. The way you get more air through a filter is to increase the surface area ie: more pleats. The greater the surface area the lower resistance (pa) across the filter, resulting in better air flow. Increased airflow does not have to mean a lower level of filtration.

I'll add one qualifier that can affect filtration performance, and that is velocity.

If the air travels thru the filter too quickly, filtration performance will be affected.

Filters are rated for efficiency at a rated airflow (l/s) and at a given velocity (m/s).

very good write up!! 10 points for the find.

High air flow, lower filtration.

Low air flow, higher filtration.

Average air flow, average filtration.

Mick,

We've covered this before but this is not always right. The way you get more air through a filter is to increase the surface area ie: more pleats. The greater the surface area the lower resistance (pa) across the filter, resulting in better air flow. Increased airflow does not have to mean a lower level of filtration.

Heres a few random points to consider which are important to filter design.

I get most of this information from my knowledge of compressors, as filters are hugely important.

Firstly, you aren't blowing into the filter, air is being sucked through it. This makes the shape of the filter less important and has less effect on the air flow as one might think. The reason is that the negative pressure will cause air to be sucked through the filter in a uniform fassion, not just through one spot.

Aftermarket car filters are folded by design giving something like 3 times more surface area.

This means that the larger the filter surface area, the lower the air velocity through any single surface of the filter.

I think it should also be mentioned that independant tests using K&N filters in the past have shown significant increases in filtration and airflow...

I guess the moral of the story is; don't believe every new article that comes out when it isn't specifically directed towards your situation.

Quality air filters that are available in Australia (mainly K&N, BMC) are perfectly safe and will provide cleaner air and more of it (through less restriction) to your engine. This has been proven far too many times to suddenly go "oh wait, no it doesn't because this article says so, AND it has fancy graphs".

That is the same as saying "People over the last 20 years, where high performance cars have used this type of filter and gained significant power increases over greater periods of time (ie, less power lost), have been all wrong."

I don't think they are wrong, the results on the track prove it.

I have a foam (oiled) unifilter rampod in my ute.

After racing (and still riding) motocross for several years, foam-oil filters are the only thing that you can use. Properly cleaned and oiled, ain't nothing getting through those things for a long time (until they cake up beyond belief!) and given the sometimes dusty conditions, they work an absolute treat - and flow well to boot.

I figure, if they work in those conditions, then why wouldn't they stop the piddly amount of nasties that my car my try to inhale?? Proof is in my pudding.

I think the evidence that "high flow" filters are of marginal benifit especially for normal day to day driving is now quite strong - I refer the readers to:

Autospeed

and This

How about someone on their next dyno run puts their stock filter in and performs a comparison showing the difference.

That will satisfy me.