Piston Slap: An Air Filtration Fallacy?

Sajeev Mehta
by Sajeev Mehta
piston slap an air filtration fallacy
Josh writes:

Dear Sajeev,

I can remember many times where I’d hear, in passing, someone say that K&Ns and other performance air filters were actually bad for my car… daily or otherwise. Their argument was always that the performance filters allowed that 1 percent of dirt to get through. This argument came up again as Engineering Explained did an air filter comparison. His tests showed a small increase in power at a very minor risk of extra dirt (which seems to be less than 1 percent in many cases).

My question is: If an engine is an air pump and you’re filtering the majority of the dirty out, who cares about the 1 percent that gets through and enters the combustion chamber? When was the last time you heard of an engine dying due to dirty air?

Sajeev answers:

Even in my years monitoring the performance of half-dead hoopties (that often run K&Ns, FWIW) as a 24 Hours of Lemons judge, I’ve never heard of engine failure via air filtration issues.

Much less seen verification of it!

While the extra 4 hp in a whipplecharged Ford GT (backstory twin turbo kits, odds are cotton gauze filters do an adequate job for extreme use… Lemons racing or otherwise.

But what about normal stuff? The K&N (purchased in 1998) for my first Lincoln Mark VIII (reused for my second, 2003 purchase) met my expectations because it:

  • Has 150,000+ collective miles with no signs of wear.
  • Filters almost everything: upon doing a resonator delete in 2010(?), the tube was almost as clean as this video.
  • Burns zero oil between 8,000-ish mile intervals, no signs of blow by, and has plenty of compression: running hard enough to hang with modern performance cars (until they destroy me with their 6-10 speed gearboxes).

As the tests in the video above suggest, I doubt paper air filters do a significantly better job. But what keeps me a cotton gauze loyalist is:

  • The unbelievably vague promise that meaningful upgrades (computer re-flash, forced induction, etc.) benefit further from that 4-ish HP boost.
  • The “throatier” intake sound, especially after removing intake resonators/silencers installed to meet (conformist) consumer expectations and/or possible EPA noise requirements. The latter might be why Active Sound Systems are a thing, but this mod won’t attract attention if you’re modest with throttle input around pedestrians/law enforcement.
  • Most importantly, they seemingly last forever: comforting for a vehicle that will remain for the life of the owner.

Bonus! A Piston Slap Nugget of Wisdom!

Let’s talk about a cotton gauze air filter’s oil causing problems with a MAF sensor, which is clearly a sore spot for the manufacturers considering this link. And if this link is true, the quote of the “2,500,000 filters K&N sells a year, they encounter less than 200 dealership problems annually” is a good indication that YOU haven’t seen fouled MAF sensors either.

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry…but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.

[Image: OP]

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2 of 86 comments
  • Dantes_inferno Dantes_inferno on Apr 29, 2019

    After using K&N filters for years, I made the switch to AEM Dryflow filters. No more oiling - all you do is wash (with air filter cleaner), rinse and dry. Comparing the two reveals that the AEM feels more solidly constructed (especially the seal) than the K&N.

  • Don1967 Don1967 on May 01, 2019

    I've got three-quarters of a $20 cleaning kit left over from the $90 filter I justed traded away in my previous motorcycle, for anyone who wants to "save money" like I did. Let's face it, the real purpose of a K&N filter is to give yourself an excuse to spend more time in the garage.

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