Does dirty air filter allow more dirt to pass through than new one?
 

I can't find any data or discussions on this topic online so I figured I would ask here.

So if we compare a brand new air filter (2012 Honda Fit) with the same filter that has been filled with dust, i.e. ran for 40000 miles.
Can you say that the dirty filter will allow more dirt to enter the engine because it's dirty?

NOTE, I am not talking about filtering efficiency or resistance, or total volume of air they pass. I am ONLY talking about the amount of dirty that is passing through both filters with only variable is that one is new and the other is old (dirty).

Thoughts would be appreciated.
https://cimg1.ibsrv.net/gimg/www.fit...ed8cfc3c65.jpg

They would both block the same amount of dirt (not allow dirt through). . Once a hole is clogged with dirt
more dirt will just back up on it.

Once your sink strainer is filled with junk it doesn’t allow more junk to pass through it just slows down the flow of water. Dirty air filter makes it harder for the air to get to the engine.

Good question. I was wondering the same thing and just changed both my filters. I was worried about dirt getting through the old filter, causing more wear. I guess not? So, with a dirty filter, there is not a performance issue? (hope you don't mind my jumping in on your post!)

Filter
 

as they get dirtier, the size of particles getting through is smaller. But, impedes airflow as they plug up.

My thinking is no if I am thinking of it correctly which could be wrong as I can be a moron :)

Engineering Explained tested this and his test results were inline with other tests saying that the car's computer will adjust fuel and timing parameters to accomodate for the dirty air filter. However he did see a reduction in HP not a reduction in MPG (if I remember correctly, I'll have to look for the video). So a dirty air filter will rob you of horsepower but the car (modern car) should maintain the same MPG.

Thank you for that information. So, actually, I can run the car and never change the air filter and that won't hurt the engine. (But I will keep a clean one in there) Thanks again.

Well eventually, like really eventually, it might tear .. Once it can't shed moisture well because air is not flowing through. But I agree, you CAN safely put it off, but they're so cheap. After market is like 8$ on Amazon.

Thanks Golnat and Jerry for responding.
I wish it was that simple. All the web sources state that the dirty air filter allows dirt to enter the engine, but they never explain why. They just want to see you a filter or labor.

However, I can hypothesize scenarios where this could be the case. For example, as the filter gets dirtier, the pressure on it increases. This will actually result in either engine not running at all or the air filter can get torn and sucked in (hence they have plastic/metal reinforcement).

Anyway, as the pressure increases, some dirt particles might be forced through the opening that they didn't pass through initially (imagine squeezing cottage cheese through the strainer, or the crowd pushing onto each other until people break through the fences)... hard to imagine exact scenarios but I imagine both dust particles and filter media (basically paper and/or fibers) can be spread apart wider with some force... unlike the kitchen sink strainer...

Would be great if someone could find articles online dealing with this question. Even bob is the oil guy filter analyses focus on other questions...

Thanks guys. I really have no knowledge about how the mechanics and engine works but the whole reason I was concerned is that I had heard that a dirty air filter lets "dirt" though, into the engine and will cause more wear, by friction, I suppose. And I suppose that is the reason for an air filter in the first place-----to supply clean, "grit" free air. At any rate, I now have a new one installed, mostly to follow Honda's recommendations.

Some good comments here. In terms of vehicle performance I heard one time about how a restrictor plate affects performance in a NASCAR engine.

Their analogy was “Picture yourself running a marathon breathing through a straw”.

That is how the dirty filter affects your engine. With all of the engine sensors it will adjust to keep the correct air fuel ratio but since the volume of air the engine can draw in your full throttle power will be reduced.

In theory since dirty filter restricts the airflow, so the engine will start sucking air from every gap it can find, sometimes pulling the filter gasket in and creating a gap around it, where unfiltered air can get in. Remember, this is just a theory:)