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2021 Forester Limited
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'21 Limited 17000 miles or so.
On the left is the engine filter and the right is the cabin filter. Change them because we live in a condo development that that is still being build. Dust/dirt all over the place. I'm really surprised that the engine filter is not as dirty as the cabin one.

Any thoughts on way that would be?
Rectangle Wood Automotive radiator part Gas Composite material
 

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2014 2.5i Limited CVT
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The engine air filter is pulling air from a different location (under the hood, at the top of the radiator) and is more protected from dust and such. The cabin air filter is pulling air from both inside the cabin (when in recirc mode) and also from the top of the cowl - less protected space - so it will have more contaminants.

At least in theory.
 

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2017 Forester 2.0XT
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61 Posts
The amount of air that is filtered by the cabin filter is also probably 2-3 times the amount that goes thru the engine filter.
I'm not so sure about that. ICE's need a lot of air, see this Engineering Explained for more details -

I think what we're seeing here is that the cabin air filter is much higher density HEPA type media and probably electrostatically charged; while the air filter is not.
 

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2001 Forester S auto
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The amount of air that is filtered by the cabin filter is also probably 2-3 times the amount that goes thru the engine filter.
How did you come up with that? Even the highest HVAC fan speed pales in comparison to the amount of air volume needed for proper internal combustion stoichiometry in most vehicles.
 

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"Cloe'" '21 Premium - Crystal Black Silica
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How did you come up with that? Even the highest HVAC fan speed pales in comparison to the amount of air volume needed for proper internal combustion stoichiometry in most vehicles.
Engine CFM = DISPLACEMENT (ci) x RPM / 3456 = Approx 80 CFM for a 2.5L engine @1,800 rpm
From what I can find, car HVAC blowers are in the 200 to 300 CFM range?
I am open to correction :)
 

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2018 Forester XT Limited CVT
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Make sure your 200-300 cfm number is the installed number, not the open-air number. Fans have different stiffnesses as well. Two fans that have the same flow rate in open air can have different flow rates when resistance is added to the air path. Like the difference between a dust collector and a vacuum cleaner. A vacuum cleaner has a lower flow rate but when you add resistance, there is a lot of suction.

OP's cabin air filter could certainly have a higher airflow x time that has gone through it.
 

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2019 Forester Sport Plus
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The amount of air that is filtered by the cabin filter is also probably 2-3 times the amount that goes thru the engine filter.
This is correct:
An engine burns a ratio of 14.7g of air to 1g of petrol. In Americaise this means to burn one gallon of gas takes 1500 cubic feet of air. A cars HVAC / AC at 2-300cfm will pass as much air in 5-7 minutes of use. A car doesn't use a gallon of petrol every 5-7 minutes of driving.
 

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2019 Forester Limited
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I think what we're seeing here is that the cabin air filter is much higher density HEPA type media and probably electrostatically charged; while the air filter is not.
The OEM cabin air filter is actually much more porous than the engine air filter. It keeps out bugs and debris, but not much else. Aftermarket cabin filters are available with finer filtration, but at the expense of air flow and replacement frequency.
 

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... An engine burns a ratio of 14.7g of air to 1g of petrol. In Americaise this means to burn one gallon of gas takes 1500 cubic feet of air....
Checking the math, that's about right.
As Wiki says for stoichiometric, "for every one gram of fuel, 14.7 grams of air are required."
An online converter also says: "Using a conversion factor of 3785 mL per gallon, we can determine that the mass of gasoline in one gallon is 3785 mL x 0.8500 g/mL = 3217 grams."
And so the weight of air to burn a gallon of gas is 14.7 x 3,217 = 47,290 grams = 104.2566 lbs.
The weight of 1 cubic foot of air is 0.0807 lbs.
Dividing 104.2566 by 0.0807 = 1,291 cubic feet of air to burn a gallon of gas.

If you did 25 mpg and an average 25 mph in stop and go suburban driving, that gallon of gas would run the engine for an hour.
In that 60 minutes, the engine would burn 1,291 cubic feet of air, while at 200 CFM, the fan would blow 200x60 = 12,000 cubic feet of air, ten times as much as the engine burned.
 

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2021 Forester Limited
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Also the SK has a baffle that helps keep out larger debris. At 11,000 miles my engine filter still looks brand new.

With my 2010 Impreza, at less than 8000 miles the engine filter would be covered in dead bugs and debris. It was much more open.

After hitting a deer, the air duct wouldn't fit in place anymore, so I taped off the hole, to keep from sucking in hot air, and used the hole that the resonance tube was attached to and pulled air from the inner fender.

After that, the filter stayed clean just like the Forester does.

Oddly enough, the cabin filter in the Impreza never got that dirty. I might have washed it off twice in the 7 years I owned it. I never bought a new one. Those were a tougher style than the SK. I don't recommend washing those!
 

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The amount of air that is filtered by the cabin filter is also probably 2-3 times the amount that goes thru the engine filter.
Not sure about that... stoichiometric requirements for an ICE is 15lbs of air for every pound of gasoline. Not positive, but I think air at sea level weighs in at 820L/Kg and if correct then you're looking at 5740 liters of air being continuously ingested through the intake. Where all that is compared to throttle opening is beyond me, but I think its safe to say that volume goes up as the throttle opens.
 

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2015 Forester 2.5i Premium 6-speed manual
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'21 Limited 17000 miles or so.
On the left is the engine filter and the right is the cabin filter. Change them because we live in a condo development that that is still being build. Dust/dirt all over the place. I'm really surprised that the engine filter is not as dirty as the cabin one.

Any thoughts on way that would be?
View attachment 571823
Based on the supplied data and decades of personal and professional automotive education, training, and service experience these air filters appear to have have been used in normal to severe duty environments and look according dirty. I thoroughly clean and vacuum out air filter housings when I change air filters for this reason. Especially the cabin filter housing which often has the shortest most direct air intake routing to the outside environment through an opening into the debris catching plenum below the windshield in most vehicle designs.

In short, they look normal to me for the enviroment the vehicle is operating in.

Many modern engines have intake snorkel and plenum designs that effectively ensure air entering the intake system is devoid of all but the smallest of aerosols prior to the intake air passing through the primary intake air filter. Over the years some makes of cars have incorporated a so called "snow filter" at or near the point air enters the intake system which keeps anything bigger than kosher salt from entering the intake system.
 

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Over the years some makes of cars have incorporated a so called "snow filter" at or near the point air enters the intake system which keeps anything bigger than kosher salt from entering the intake system.
No such thing on the duct to the air filter box on at least up to the 2021 Foresters. Just a baffle that air has to flow around to enter the duct. That alone may cause heavier particles to be "tossed" out of the air stream, as the air goes around the baffle, the debris will be too heavy to make the turn.

Simpler, and seemingly just as effective from my personal experience. I wouldn't want another "filter" to have to worry about being clogged with cottonwood fluff every spring.
 

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Wow! Way over complicated for most! (Myself included!) maintenance schedule calls for two cabin air filters for every engine air filter. The pictures looked very normal to me. Way to go keeping up on your maintenance! I recently helped somebody go over maintenance needs of their car. When I asked about the engine air filter, they said “what engine air filter?” Lol But they had been keeping up with the cabin air filter…
 

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2018 Forester XT Limited CVT
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No such thing on the duct to the air filter box on at least up to the 2021 Foresters.
This thing is called a "resonator" and probably does something to that effect but it looks like there might be a bit of a vortex there where bigger droplets etc might drop from and into the tube. It does look like the bottom of the tank has been wet.

 

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2001 Forester S auto
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Engine CFM = DISPLACEMENT (ci) x RPM / 3456 = Approx 80 CFM for a 2.5L engine @1,800 rpm
From what I can find, car HVAC blowers are in the 200 to 300 CFM range?
I am open to correction :)
Is everyone driving their Forester at 1800 rpm the whole time that it's running with the HVAC fan set to ONE constant speed? Highway cruising rpm in a 2001 4EAT averages 2500 rpm or greater (not sure about anything manufactured after 2010/2011). There are too many variables, and to make such a bold statement like HVAC fan air volume is 2-3 times greater than engine combustion air volume is an 🍎🍎🍎 to 🍊🍊🍊 comparison at best.

Either way the OP was clearly confused and seems to have triggered a debate that is not worth any appreciable attention other than straightforward replacement of the cabin air filter due to surrounding dusty conditions.
 
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