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 here) is meaningless, the K&Ns had zero adverse effect after thousands of miles. More to the point, considering the reputation and warranty of Underground Racing’s 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.

  • ToolGuy Price dropped $500 overnight. (Wait 10 more days and you might get it for free?)
  • Slavuta Must be all planned. Increase price of cars, urbanize, 15 minutes cities. Be poor, eat bugs
  • Sid SB Not seen a Core without the performance pack yet. Prefer the more understated look of the Core vs the Circuit, but both are great fun to drive.
  • El scotto Tesla has one team making EV's because that is all Tesla does. Farley -rolls eyes- decided to split Ford into two huge warring factions: ICE vs EV. Hey Jimbo, it says "FORD" on the buildings.Lord only knows what GM did internally because it's GM. I'm betting it's like Ford pitting ICE vs EVs. With GM being GM every existing division will be divided.Stellantis will keep building Challengers and Rams. Someday they may figure out that Jeep is the fugu fish of the automotive sushi world and unload to some Chinese. EV's? no, not really.If this site was The Truth About HVAC (TTAH) some on here would tell us that central heating and air causes unknown illnesses, will be bad, and cause a degradation of our nation's moral fiber. By golly they shoveled coal and carry ash buckets and that shouldn't change.
  • El scotto -channeling my inner Kenneth Mars- It is a coupe because Merzedes say's it est une kupe! Quiet schweinhunds! Dis is zee best kupe in zee vorld! Merzedes says so! Zee best or nothing Mein Herrs!