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2003 Forester XS
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9 Posts
Discussion Starter · #1 ·
I've got a 2003 with the 2.5XS. It has 114700 original miles on it. Just checked the MPG after a week of city driving. Due to construction on the highway I drive about 10 miles on stop and go streets going to work then about 8 miles home each day with about 4 miles of that on the freeway and 4 miles through residential neighborhoods. I used the A/C each afternoon as well. MPG worked out to 23.64 mpg.

Today I swapped out the stock Subaru air filter for a K&N air filter. I'm curious if others have done any mpg comparisons with the K&N filter? Last time I tried this was on a 1991 4Runner on a 600 mile road trip. I was able to get an extra 22 miles out of a full tank of gas with the K&N. Not a big increase but an increase nonetheless.

I'll post back after I'm able to do a week of similar driving in about three weeks.

Two tips to offer: 1) If you do a K&N swap, keep the box to store your just removed paper filter for those times you need to clean the K&N and don't have time to let it dry before reinstalling. Just pop in the old paper filter for a day of two then oil the K&N and reinstall. 2) Remove the roof racks on top of your cars if not needed for immediate use. They just increase drag which contributes to decreased MPG. Probably not a big gas saver but at $5+ dollar a gallon gas prices, every little bit helps.
 

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2022 Forest Green Premium
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240 Posts
I have also heard there are tradeoffs with k&n products. Less effective as a filter, but can increase hp and mpg, especially if also done with cold air intake. Some decide the tradeoff is worth it. Personally, I would not use on a newer vehicle, but would try it in tinkering with an older model.
 

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2022 Forester Sport
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758 Posts
I think they are OK as a filter but don't much else.

My '89 IROC Camaro came from the factory with K&N filters. At the first filter change I went back to paper filters. I noticed no difference in HP or MPG.
 

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2014 2.5i Limited CVT
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5,986 Posts
Depending on the factory "tune" and the way that the ECU is programmed, K&N and similar filters can aid in MPG, but it's probably more of a long-term effect and not really something (in today's vehicles) that will be a huge gain in MPG over a single tank.

The bigger benefit of a K&N or similar "reusable" filter is the long term cost. While a K&N and the cleaner is a bigger expense up front, given that you can (as claimed) go 40 to 50K miles before needing to pull it out and perform the maintenance. Regular disposable filters (paper or whatever) are supposed to be replaced every 15K miles - so that means you replace it 3 times in that same time frame.

Those reusable filters can be bought starting at 30 bucks or so and go up in price. Paper filters start at around 14 bucks and go up. Depending on brand, the reusable can be a bargain based on price alone.

Truly - do what you want with your filter. Some will decry that the reusable filter will kill your engine (jury is still out) and this can vary by brand and all.

Best thing to do is to be sure that whatever filter you use meets or exceeds the filtration that the OEM filter provided. And remember that many of those claims of huge power and/or MPG increases are "old news" and may not really be true for your particular make and model, based upon how much the ECU controls the air/fuel ratio.

As a side note, the new audio head unit I installed in my Forester includes "gauges" for the air/fuel mixture and other readings that come from the intake and so maybe - just maybe - you can get a bit of a bump from one filter vs the other, but it's still your choice.
 

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2003 Forester XS
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9 Posts
Discussion Starter · #9 ·
Unlikely. Also filter passes more dirt
I appreciate the response. I used K&N air filters for years on big V-8 racing engines on dirt tracks and never noticed any hint of dirt in the carb or intakes which were ported and polished, so easy to see. If you have any reliable sources of data, beyond conjecture, for K&N air filters being less efficient at filtering dirt I'd appreciate you sharing some links.
 

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2022 Crystal White Pearl Sport
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182 Posts
I have used them for going on 30 years now in numerous vehicles including my own Baja race cars and trucks and have not had any problems with excess dirt or sand. They have noticeably improved my MPG as I have already stated and I will continue to put them in every vehicle I own. I do keep a spare paper filter for each rig in case I don’t have time to maintain the K&N (such as our frequent wildfires here). My old man has been using them since the 70s and never had an issue in any cars either and he frequented Glamis etc for half a century now. Just my $0.02

But ultimately what you put in your econobox is up to you. It is America after all!
 

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2004 & 2005 FXT
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49 Posts
The only problem with a K&N is that if it is oiled improperly, the excess can lightly coat the MAF sensor. To see if this is happening simply check and clean your MAF sensor. I clean it every oil change, anyway. As far as the K&N letting in more dust, etc., for all practical purposes, that's a myth. A new K&N will filter about the same as a stock OEM filter.

But a used OEM filter will filter more (but not better) than a used K&N. This is because the material in the OEM expands and clogs and even puffs out much more than does the K&N. In general, the OEM filter begins to block air flow more than it filters air flow. If you have a machine to increase humidity in the house, simply hold both to the output. It doesn't take long to actually feel the difference.

This is in a stock Forester.

But is there anyone out there who still has an XT Forester that also still has a stock intake system? At the very least, most people have removed the stock inlet system, and left only the filter box and the MAF sensor tube. This, of course allows a lot more air into the engine compartment and into the filter box. Along with this added air comes added dirt and dust and, if it is raining hard enough, a lot more moisture. If there is any kind of tear or hole in the fender well cover, then this can be a problem, especially if you are using an OEM filter, or something similar.

We installed a new filter in the air box on an 05 FXT that we had just done a complete engine/exhaust build on. We were going to do a couple of tuning logs to begin the tune. We did one pass, and a really nasty thunderstorm hit. High wind, serious rain, etc. We cruised home, put the car in the garage to sit out the thunderstorm.

Later, when we did another tuning run (log) . . . the tune fell apart. We could not figure it out. Checked everything. Tried three different time to do another logging run. Finally, three of us were just standing there, looking at the engine, and I said, "It's got a new filter, right?"

"Check it, anyway," my friend said.

Long story short, there was a long split that no one had noticed in the wheel well cover that had opened wide with air and rain pressure, and enough rain had gotten in to soak the filter. And the filter had literally sucked, in a perfect cone, all the way into the MAF sensor. And when it dried, it retained that perfect conical plug. Bizarre looking, indeed, when I pulled it from the filter box.

Naturally, we replaced the wheel well liner. And I replaced the filter with a K&N. There's no way a K&N would come apart like that.

I've used K&N filters for years, in cars that have been constantly modified (as opposed to repaired). And every time the intake manifold has been pulled, and the inlet tube unclamped from the turbo, there has never, not once, been a problem with dust or oil in the inlet tube (AVO one-piece). As of this writing, the 20G Blouch in my car has about 77,000 miles on it. Montana driving, in all kinds of weather and road conditions. Virtually no oil consumption between changes.
 

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2022 Crystal White Pearl Sport
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182 Posts
Agreed, a K&N is not for lazy people as it requires maintenance. If you are not willing to do that, stick to paper filters, otherwise the K&N is a nice upgrade that is easy to install and pays for itself quickly.
 

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2019 Forester Touring
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2,304 Posts
Changed my wife’s Jeep to one and went from 25.3mpg to 27.6mpg.
The obvious question here is… If these filters do such a great job increasing mpg, why aren’t the vehicle manufacturers equipping all their new vehicles with them?
 

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2022 Crystal White Pearl Sport
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182 Posts
K&N $60+ paper filter to OEM company probably ~$1

why do they also sell with the cheapest tires, rubber instead of urethane mounts, etc. they literally do the cheapest crap they can to get it out the door. They have to meet a price point so they cut corners and reduce expenses where they can. These are economy cars and if they did everything with state of the art or performance parts they would cost twice as much and nobody would buy them.
 

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2019 Forester Touring
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2,304 Posts
K&N $60+ paper filter to OEM company probably ~$1

why do they also sell with the cheapest tires, rubber instead of urethane mounts, etc. they literally do the cheapest crap they can to get it out the door. They have to meet a price point so they cut corners and reduce expenses where they can. These are economy cars and if they did everything with state of the art or performance parts they would cost twice as much and nobody would buy them.
I disagree… the supposedly huge increase in MPG on a vehicle lineup created by such a simple change in filters would be worth pure gold to automakers. I can only imagine how fast they would jump at paying an extra few $ per unit to remain in compliance with EPA mileage standards.

And, like @bman400 … I’ve owned several “not inexpensive“ BMW’s that came with paper filters, and they weren’t economy cars by any stretch of the imagination.

As Carl Sagan once said… “Extraordinary claims require extraordinary evidence”.

Now, if one is really interested in extending mpg, this might just be your thing…


NOT!!!
 
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