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2018 2.5i Forester
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Discussion Starter · #1 ·
To preface this I hope this is the right location for this post, I looked in the ventilation section but I saw nothing about intakes, so I figured here would be a safe bet.
Now unto the guts, so I want to get as much power as possible out of my 2.5I, and I am looking and drop in filters or even AEM's CAI, but from what I heard a cold air intake is no good without a tune and no-one makes a tune for the 2.5I, plus I'm in CA so it would cause me to fail smog, so considering that I think I might be better off with a slightly high flow drop in filter, now I have heard bad things about the K&N filter damaging the MAF sensors due to it being oiled, so does anyone have any recommendations on a safer filter to boost power, even if its gains are tiny?
 

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2008 LL Bean (4EAT)
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4,996 Posts
... I have heard bad things about the K&N filter damaging the MAF sensors due to it being oiled...
I am using a K&N air filter for the past 10,000 miles in my BMW Z3. They do not affect the MAF unless over-oiled by an owner who cleans and re-oils them. They come from the factory pre-oiled with a very dry coating and no cleaning or re-oiling is required for 50,000 miles.
I plan to discard the filter before then, because cleaning and re-oiling is a pain, and unless completely drained and dried, oil can be sucked off and pass through the MAF.
In the 1980s I used a K&N filter on the aftermarket Weber carb of my BMW 2002. The filter came white and un-oiled, with spray can of red oil which I applied to the pleats. I drained it for a day, blotted it, and it still seeped oil on the filter housing. Liquid oil in the filter is what would ruin a MAF.
 

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2010 X Limited, 2.5L NA, 4AT. Purchased as the second owner in 2020 with ~126K miles.
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121 Posts
I also use K & N filter and service it myself. I can't really say you get much performance increase, if any. You get better performance doing regular maintenance with quality components and supplies. Keep everything clean, fresh, and tight.
 

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2011 Subaru Forester
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1,373 Posts
I wonder if the results would be similar using different models of air filters. The Fram Extra guard shown here is nothing like the one sold for the Forester. While I cannot judge the filtration material by eye, the Fram looks to be made as well as a Denso one which itself was pretty identical to one bought from a Subaru dealer. I tend to replace them every year.
 

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2017 Forester XT Touring EyeSight
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722 Posts
I know it's fun to experiment, but remember that less restricted intake flow may help power only at high rpm. The fancy variable-flow intake manifolds of recent decades' naturally-aspirated Japanese cars (Honda especially is in my mind here) had deliberately-restricted long-path intake flow at lower rpms to boost torque there. If you look at various YouTube videos showing plots of dynamometer results (hp and torque vs. rpm) you can get a sense of how many variables can be in play. On the other hand, maybe if you made your car weaker at low rpm, the feeling of it "waking up" more dramatically at high rpm, could be fun.
 

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cold air intake for your car will do absolutely nothing for power... with a stock car/truck 90% its engine bling. It can help but most of the cheap CAI are junk filters so of course it lets more air in but it also doesn't usually filter as well as a result. Basically any brand name air filter will be safe as long as you fully close/seat the airbox seals and replace it regularly. I gave up on K&N marketing bs years ago. They were and still are infamous for dusting turbos and I even switched back to paper on my side by side after some sand dune trips. the oil filters require more maintenance and theres a balance between not enough and to much oil before it can mess with the maf/map sensors in the intake tube. As much as people knock Fram I have 300k plus miles on a first gen 80s 4runner that used fram its entire life for air and oil and fuel filters... ZERO engine rebuild just timing chain replacement twice, valve guide seals and minor o rings, valve adjustments etc. I also have fram on my little commuter car and the 09 forester. zero difference in start up, running, oil test when changed. Fram isn't even that cheap anymore. Now to be clear im not saying fram is the ultimate in anyway but for a widely available and affordable option its been problem free on many diesels, gas, and equipment for 10yrs plus in my personal life.
 

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2019 Crosstrek 2018 Forester XT
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14,791 Posts
K&N does not filter as well as other filters. Aftermarket CAI will not improve your performance. Really nothing does. You got what you got. The FB was designed for efficiency not performance. I was after performance so I have an XT. My Crosstrek is made for efficiency. Both engines are 2.0 0ne gets 150HP, one 250HP. I love both.
 

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2018 2.5i Forester
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Discussion Starter · #9 ·
I know it's fun to experiment, but remember that less restricted intake flow may help power only at high rpm. The fancy variable-flow intake manifolds of recent decades' naturally-aspirated Japanese cars (Honda especially is in my mind here) had deliberately-restricted long-path intake flow at lower rpms to boost torque there. If you look at various YouTube videos showing plots of dynamometer results (hp and torque vs. rpm) you can get a sense of how many variables can be in play. On the other hand, maybe if you made your car weaker at low rpm, the feeling of it "waking up" more dramatically at high rpm, could be fun.
Yeah I have head about that, but I think I may stay stock for off-road
 

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2007 Forester Sports XT 4EAT
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@Delowi I've had a K&N drop in filter in my '07 FXT since I bought in '09 as a retirement gift to myself. Zero issues with the oiled K&N air filter messing up the MAF sensor. The COBB Surgeline dyno protune was done with the K&N air filter installed.

I'm NOT recommending a K&N air filter, it's just what I use. If you go off roading, where it's very dusty, you might opt for a plain old paper filter. ;)

Bobby...

['07 FSXT Member Journal] ['03 X Member Journal]
 

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'14 Forester XT Touring
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757 Posts
Well....
If you want power should have got the XT
All you NA guys are stuck with the power you got. Nothing short of Frankenstein'ing a turbo/tune kit will you gain more power.
The best "power" thing you can do to your NA, is use 91 octane.

Same reason why I didn't get the NA forester, I got the XT model.
 

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2008 LL Bean (4EAT)
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...The best "power" thing you can do to your NA, is use 91 octane.
...
Does that mean the NA engine can provide more ignition advance on 91 octane than its knock sensor will allow on regular 87 octane?
 

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2015 XT Touring CVT
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570 Posts
more related to higher octane fuels usually have more detergents to keep the engine inside cleaner. its about losing less power than gaining more.
 

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2014 2.5i Limited CVT
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I have a 2014 2.5 Limited and use the K&N filter. I also live in CA and know that we (as well as most or any of the states that have adopted California rules) have stricter requirements on filters - which is why you often will see disclaimers about not being legal for use on a vehicle in California.

Does it help with performance? No. I put it in place just so I don't have to spend 20 bucks every 6 months or so on a new paper/pleated filter. I do go off-road, but not much more than a time or two per month, most of my driving is around town.

What I'll also do is pull the K&N out, put in a paper filter and then use the high-pressure air at the local car wash to "clean" off the dust or whatever on the K&N and then swap back when done.

One other thing - if you want to swap out the stock airbox and filter set up for another aftermarket filter product, you can do this. Just be sure to swap back to stock when going for your smog check. Then you can swap back after the smog check is done. As it's usually every other year, there is little issue. Also, I've had an aftermarket cone-type air filter on my old 2001 PT Cruiser and never have had an issue with failing smog with that filter in place. But then again, I kept the stock air box with a slight mod to fit the cone-type filter and have never had anybody question it.
 

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2015 Forrester Touring CVT
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I've used Wix air filters exclusively for decades. I'm concerned most about keeping clean air coming into my engine. I believe Wix is as good as any and better than most.
 

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2020 Subaru Forester Sport/2018 Crosstrek Prem
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266 Posts
I have used k&n for our Subarus for the last 15 yr in all our 4 Subarus. No harm has done to any of our cars. Have I feel performance improvement? I don’t feel any of our car accelerate any faster. Don’t feel more powerful either. However, we do get 1-2 mpg improvement for every car we put k&n air filter in. I am happy with the mpg improvement.
Use k&n at your own risk. Don’t use it if you drive in dusty area.
 

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2017 Forester Premium 2.5 CVT
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K&N and similar oiled fabric type filters increase performance by reducing restriction at the expense of reducing filtration. It's that's simple. They DO NOT stop fine particulates at an effective level. Anyone who thinks they do is in denial or works for the K&N marketing department. Over time that reduction in filtration results in increased engine wear. These filters are not appropriate in any daily driver vehicle unless you don't care about reducing the service life of the engine. "Cold Air" intakes with an exposed cone filter are even worse. The factory airbox system IS a cold air intake, with a duct running to the edge of the hood. An exposed cone filter is sucking in hot underhood air.

The OEM designers are not ignorant. In an era where vehicle manufacturers are so desperate to gain fuel economy they are not putting a restrictive intake on the car. The factory intake is the best compromise they could come up with for filtration, performance/economy, and intake noise reduction through computer modeling and testing. If any aftermarket mfg came up with a cheap and simple economy and performance improving device that actually worked beyond the marketing they would long ago have been bought out or had their technology licensed by automakers.
 

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2015 ; 2.5i ; 15,000 Miles (I drive a lot less than I used to !!!!)
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I know it's fun to experiment, but remember that less restricted intake flow may help power only at high rpm. The fancy variable-flow intake manifolds of recent decades' naturally-aspirated Japanese cars (Honda especially is in my mind here) had deliberately-restricted long-path intake flow at lower rpms to boost torque there. If you look at various YouTube videos showing plots of dynamometer results (hp and torque vs. rpm) you can get a sense of how many variables can be in play. On the other hand, maybe if you made your car weaker at low rpm, the feeling of it "waking up" more dramatically at high rpm, could be fun.
You folks trying to get more air thru the filter with less pressure drop would reap superior results if you concentrated on the other areas of flow ... like the plastic air ducting, and finding ways to get the air to flow with a lot less turning and highly-tortured pathing. Remember , air also has weight, and constantly turning it causes pressure deviations.
 

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To preface this I hope this is the right location for this post, I looked in the ventilation section but I saw nothing about intakes, so I figured here would be a safe bet.
Now unto the guts, so I want to get as much power as possible out of my 2.5I, and I am looking and drop in filters or even AEM's CAI, but from what I heard a cold air intake is no good without a tune and no-one makes a tune for the 2.5I, plus I'm in CA so it would cause me to fail smog, so considering that I think I might be better off with a slightly high flow drop in filter, now I have heard bad things about the K&N filter damaging the MAF sensors due to it being oiled, so does anyone have any recommendations on a safer filter to boost power, even if its gains are tiny?
hi
the setup you have now from the factory is best you will not gain anything and just wasting money and destroy perfectly working part remember engineers have done all the work for you so save yourself money and headache and leave it alone and get something with turbo
 
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