Very interesting video on how K&N filters work

[DaveW]

This is in the RX7 Club 3rd Generation Specific (1993-2002) forum. IMO, if you haven't seen this, you'll find it very interesting. This is the post it's in. Warning: the video is ~23 minutes long. The thread containing that post is also interesting.

https://www.rx7club.com/3rd-generati.../#post12583418

I know they work well (excellent filtering, low restriction for their size) because I have them on my F2000, my 342K mile 1974 Chevy G30 Van tow vehicle, and my '93 RX7.

[stonebridge20]

Vizard knows his stuff.

Very interesting video.

He just threw everything I thought about K&N filters right out the window.

Thanks for posting this Dave.

[Steve Demeter]

I have them on pretty much everything I own. Not available for my chainsaw or lawn mowers>>>>>

[billtebbutt]

Ah yes, David Vizard.....

From my chair right here in the home office, I can see my 40+ year old copy of "How to Modify your Mini by David Vizard". I must have been through that book hundreds of times as a teenager, trying to make my Mini quicker on a shoestring.

My gosh, the man's ability to think laterally and sheer mechanical genius was and still is amazing to me.

best
bt

[E1pix]

This is in the RX7 Club 3rd Generation Specific (1993-2002) forum. IMO, if you haven't seen this, you'll find it very interesting. This is the post it's in. Warning: the video is ~23 minutes long. The thread containing that post is also interesting.

https://www.rx7club.com/3rd-generati.../#post12583418

I know they work well (excellent filtering, low restriction for their size) because I have them on my F2000, my 342K mile 1974 Chevy G30 Van tow vehicle, and my '93 RX7.

Thanks for posting, will watch now, used them since 1981, but…

YOU STILL HAVE THAT CHEVY VAN???

[DaveW]

Thanks for posting, will watch now, used them since 1981, but…

YOU STILL HAVE THAT CHEVY VAN???

Yup - it'll probably outlast my racing career.

[E1pix]

Van’s got 20 more years in it.

Meaning, No, it won’t.


Video confirms what I “thought,” but didn’t know. What we hear over and over from mechanics is “I can’t trust a filter I can see through,” ignoring that paper blocks air.

The idea that everyone using them, and especially in racing, isn’t an obviously good sign just blows me away.

PS — Cool to see Howard Coleman on that forum, he was the only SCCA guy in our town when I was a kid. He raced a Pinto in B Sedan all through the latter ‘70s.

[DaveW]

...PS — Cool to see Howard Coleman on that forum, he was the only SCCA guy in our town when I was a kid. He raced a Pinto in B Sedan all through the latter ‘70s.

Howard is one of the most informed contributors on the RX7 Club forum. He takes a very intelligent look at everything he does or sees.

[E1pix]

He’d retired I’d thought, then showed up at the ‘84 (I think) Runoffs with a Badass RX3 and almost podiumed.

I was shocked to see him. May have been the former Pro Mandeville car.

[DaveW]

Originally Posted by TeamRX8 in the RX7 forum:
...on an engine the filter flow isn’t constant in that every intake cycle results in frequency pulsing/resonance that cause the cotton fibers to dance around and this greatly increases their true filtering capability. That’s not going to be replicated by the fram guy testing method...
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My response:
Maybe "dancing" vs not doesn't precisely define the issue. I think it's a similar phenomenon, but not, IMO, "dancing."
Here's what I (nerdy engineer) think it is:

During high-speed airflow a K&N's small fibers take a set, bent over in the direction of airflow, like wheat in a field when the wind is blowing.

When the airflow periodically slows, the fibers can stand back up and catch the small particles. Since it takes time for them to bend (take a set) again with airflow, on-off airflow, which happens frequently during driving (either shifting or intake pulsations), allows many of them to remain close to "vertical" in a position to catch small particles.

This also means that if the filter is too small, air flow rate never decreases enough to let the small fibers return to vertical. So there's a double "whammy" with a too small K&N filter - more airflow restriction, and impaired filtering ability.

With a paper filter, this same issue doesn't apply, so high airflow rate won't reduce its filtering ability. This also corresponds with paper filters having more restriction.
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[cjsmith]

To add to Mister Vizards results, I did a test a number of years ago in the company’s air side lab. The test was to compare a new paper filter to a new K&N and a used K&N of the same size. I don’t remember the exact model, but it was a round motorcycle filter. The used filter was washed and dried. It was not oiled. We found the paper and both K&N filter had the same pressure drop as long as the mass flow was at normal cursing speed for the bike. We did the math to establish CFM based on engine RPM. As the RPMs increased looking for max RPM and increasing to max velocity, the paper filter saw increased in pressure drop compared the K&N. From the camara we put close to the filter, we could see the plys of the paper filter condense together at the higher velocity, which we attributed to the increase in pressure drop. For our test, we did see roughly a 5 to 8% increase in pressure drop between the new and used filter at max velocity if I recall correctly. To be clear, the used filter had been on the motorcycle for 1 riding season. We then looked at where the air was entering the 2 K&N filters. The new filter had considerably more flow through the base of the filter, closer to the carburetor or for our test, the test fixture. We used smoke sticks to observe the flow. The same test on the used filter, the flow was more equal across the majority of the filter media.
I did a chassis dyno test 2 years ago and compared air filters, during the testing process. In short, I found a small but repeatable drop in power when comparing a new K&N to a filter I had used for the previous race season. The test procedure was base line with new filter, then used, then both repeated again. The used filter was not washed, hence it didn’t give the appearance of being dirty or damaged. I did blow it out with compressed air, blowing from the inside. I’m not saying Mr Vizard is incorrect, just saying what I experienced.

[DaveW]

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