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Is A K&N filter worth it

I am thinking about dropping a K&N filter into my Impreza 2.5 RS and I want to know if it's worth it and if not any recommedations on a good air filter?
Thanks,
Shawn
 

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Holy cow,

Shawn from Hulmeville is actually posting!

A K&N air filter is a great first mod to do to a car. I do it to every car I (or my family) buy. I did my rex at 8,000 miles.

You get better throttle reponse, filtration, fuel economy, and power. Obviously more air out (exhaust) would help with more air in (intake), but keep this in mind: A dirty K&N filter will out flow a clean OEM filter.

They are also permanent, you just clean and reoil them every so often.

Some may say otherwise, but I still think they are the best air filter money can buy.

How do you like you Silver 2.5 so far?

-Jim
 

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i agree, K&N filters are the best thing next to a CAI you can buy for your car. however, there has been alot of controversy regarding whether it actually increases performance. Vishnu racing dynoed it and it actually resulted in a decrease in hp. but, now K&N made a filter specifically for the wrx. before ppl would use one that was fit for a nissan ( i think) and it just happend to fit the wrx's airbox. now that there is a model-specific filter for the wrx, results might be different.
 

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Regarding the new wrx-specific filter...it's made of the same element, so should be the same 'performance'...

Stock, you won't see much benefit from parts like a filter and cat-back at all. Even CAI don't do much to improve 1/4 mi over stock.

I have both an ITG filter and Greddy cat-back. Also an MBC in there... 13.8 1/4. I do not attribute my faster runs to the cat-back as that's as fast as I've gotten prior to installing it. So no gain from that. The filter could be helping the top end, but I've never run the 1/4 with the stock filter. I believe the MBC is the reason for the quick times I've pulled.

However I do see benefits of filter and exhaust down the road. When I've gotten rid of all the cats in favor of one hi-flo (emissions testing in my area) and other more major mods. The Greddy sounds awesome and is 3" to 3.3" (4.5" tip). So it's ready for a 3" downpipe/midpipe.
 

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Si2WRX said:
Regarding the new wrx-specific filter...it's made of the same element, so should be the same 'performance'...

actually with the old filter, it didn't form the proper seal around the airbox, which is why it may have resulted in a hp decrease.
 

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Yeah, the Nissan filter is a really tight fit. It might have been cramping some of the pleats as well. I'm not going to be ditching my $45 filter just because there is a WRX specific one. I followed the directions to put mine in, which included using the foam strips to hold the filter in place.

shawn won't have this problem any way since he has a 2.5RS. Also, I think the 2.5RS has some kind of "cold air intake" already. Although, I've never had the opportunity to look under the hood of one. Shawn, can we do something about that?

Also, the K&N might not make a difference new vs. new, but wait until the two get dirty and see which one is still flowin' like crazy.

-Jim
 

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Okay.. maybe I'm wrong here but I read a report online about these types of filters and they found that the flow was better in the K&N over a stock filter but the Hp increase was fairly negledgable.. The other major thing to note was the amount of particles able to enter your engine. Because of the higher flow do to less area of fabric in the filter it can not hold as much "crap". This means that it lets through a higher amount of impurities, and reaches its saturation point faster, in which it lets through an even higher amount yet. This report went so far as to suggest in the long run the negligable power increase over a clean standard filter actually was at the expense of engine life to do impurities entering the engine. This could be magnified in areas where driving conditions are rough (heavy smog, dessert areas, and winter areas, lots salt particles in the air on the roads.)

Well just thought I would give you the jist of what I read... If I can find a link to this tonight I will post it.
 

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I'm interested in reading more into that. The ITG is said to out-flo and out-filter the K&N.

However, my Jeep has a K&N and my understanding was that as the filter gets dirtier, it actually filters BETTER. Because any holes where particles could possibly get through, would clog and therefore not allow particles through.

Please post a link if you can find the data.
 

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I would have to agree w/a filter having better filtration as it gets dirtier. Less area for particles to get through. This is, however, at the expense of air flow.

Of course a higher-flowing filter is going to allow more particles in though...Hence, "HIGHER FLOW". Wouldn't that imply that it lets more through? There is a price to pay for any and every mod you do to a car. All I can say is this:

"Anything that gives you power gives you trouble."

Simple as that.
 

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K&N filters trap smaller particle than non-oiled filter. The oiling allows it capture the particles. The stock WRX filter is red, but I don't know if it is oiled. Regular paper pleat filters rely on small holes to stop particles rather the oil.

The ITG panel filter uses the new foam technology and I'm honestly not familiar with them at all. I do know that the some of the foam looked way too open for my taste. The new to Teague's Unipod filter look like a good foam density to me and it said to out clean and out flow pleated paper.

-Jim
 

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I think it comes down to the fact that when the filter is saturated as more is introduced stuff must still flow thus the crap in the air is pushed through... nothing can block all particles or it would'nt allow air flow.. allowing more air in theory would generally allow more other through too.. I'm still at school though and will look for the link tonight.. I didn't expect to get such a responce so quickly.
 

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In case you have trouble with the link here is the jist of the article:

The following information was taken from a posting by Jim Conforti (AKA the Land Shark). Jim is a well known tuner in BMW circles. His web site is the Bonneville Motor Werks. The testing was done on a BMW air filter but the concepts should apply to all manufacturers.
Comparing Stock to Performance Air Filters

First, a "prologue". This was a scientific test, not one done by filter manufacturer X to show that their filters are better than manufacturer Y. The test results are pretty irrefutable as the test lab tests and designs filters where "screw ups" are absolutely NOT allowable (I can't say any more for security. Think "Glow in the Dark").

OK, with that in mind, onward.

A scientific test was done on TEST filters where air was loaded with ACCTD (some standardized "test dust" called AC Coarse Test Dust) and sucked through the TEST filter then through an analysis membrane.

From the Quantity of dust injected and the amount that gets thru the TEST filter and is then captured on the analysis membrane we can calculate the efficiency of the TEST filter in Question.

First, the filters:

BMW Stock Filter, Eff. Area of Media: 8.4 sq ft.
K&N Replacement, Eff. Area of Media: 1.6 sq ft.

The filters are the SAME size. They both fit in the STOCK BMW M3 airbox. The difference is that the STOCK filter has 65 pleats 1.5" deep and the K&N only 29 pleats each 0.75" deep.

Now, remember this ratio: " 5.25:1". It's the ratio of the AREA of STOCK to K&N. It's very important and will come into play later.

The STOCK filter efficiency started at 93.4% at 0 loading and increased to 99.2% efficiency as the loading increased to a max tested of 38.8 gm/sq ft of dust.

The K&N filter efficiency started at 85.2% at 0 loading and increased to 98.1% at the max tested loading of 41.38 gm/sq ft.

Now, I hear you. "Jim, that's only a FEW PERCENT". But is it?

Let's look. If we had 100 grams of dust on a new BMW filter we would let thru a total of 6.6 grams of dust in. If we used the new K&N filter we get 14.8 grams of dust. Thats 224% (TWO HUNDRED TWENTY FOUR PERCENT!!) more dust ingested initially, stock vs. "free flow" and this ratio is pretty much held. Somewhere between 200-300% more dirt gets "ingested" anywhere across loading equivalence.

The more INTERESTING thing is when you look at what happens to the DP or Differential Pressure at a constant airflow as you dirty both filters equally with time.

The test used a rate of 75gr of dust per 20 min. Here's where the AREA difference comes MAJORLY into play. See, even though the BMW filter flows a bit less at the SAME loading, it also LOADS UP 5.25 times SLOWER due to it's LARGER effective area. So what happens is that the K&N initially flows better, but as the dirt continues coming in, the K&N eventually flows WORSE while still letting MORE dirt in.

Now, does any of this additional dirt cause problems? I dunno. I suppose we could have a few people do some independent oil analyses on different motors using both K&Ns and Stock filters. Get enough of them, and you'd have a good statistical basis. For me though, it's simple: More DIRT = BAD.

The additional short-term airflow might make sense on a track car. IMHO, it doesn't for the street.

-- Jim Conforti
 

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I guess the major thing that caught me in this article was that over time the stock filter will flow better and let in less crap...

Obviously if you maticulously cleaned the K&N this wouldn't be the case.. but then comes the question of how often are you really willing to do this?

Well anyway let me know what you all think.. and if you don't like it lets keep the flames down.
 

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About the article

MustGoFast,

Thanks for the info. You should've pasted the whole article, it's only text and the methodology looks important. I took the liberty of putting in the whole article.

My question is: why is the effective area so different from 8.4 square feet versus the K&N at only 1.6 feet^2. This automatically says to me that this isn't an apple v. apple test.

The K&N I put in my car has a much larger effective area that the stock Subaru filter. I will take measurements to verify this, but I remember a few more pleats and much more depth on the K&N filter.

BMW apparently puts very good filters in their cars. I would like to know if BMW uses an "oiled" filter.

One thing is for certain, K&N filters claim to filter better with time:
The pleated design provides five times more filtering surface over the element circumference. The cotton/screen wire filter media is then saturated with a formulated air filter oil. Acting like a fluid curtain, this oil attracts particles of dirt and debris which build up on the outside of the filter. This buildup of dirt particles suspended in oil creates more and more irregular passages that actually create an additional filtering barrier with use. In most cases, up to 1/8" of dust can cover a Filtercharger® before performance is significantly affected.

From: http://www.knfilters.com/filtercharger.htm
This test data refutes that claim.

-Jim
 

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The reason there is more surface area is do to the layering of the filter.. the stock filter contains many layers possibly less area on the single visible layer but if you had 10 of them as apposed to 5 of them each with very tiny film of air between them you have much more total area to collect the crap in the air.. and also this explains why the flow is lower initially as it has more layers of filtering to flow through...
 

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I should have added to that last post:

This is what I have read.. I did not verify anything personally.
 

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So as usual, everything has advantages and disadvantages. You didn't really think than Subaru and every other manufacturer never considered putting on a high flow filter like K&N. Yeh, those manufacturers are so stupid and don't even do these basic common sense things to give you more free hp.

No, they're trying to improve engine life, avoid scratching up the bores, avoid premature compression loss, burning oil and blowing smoke. Use your K&N, but understand, everything has a trade off.

Now about that larger sway bar that those stupid manufacturers never even considered putting on their car...
 

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k&n

Hey guys, My names aaron willen. I just got a wrx last week, after having a plymouth acclaim for like 3 years...quite a difference haha. I dont really know my way around a turbo engine yet but im learning. My question today is...would it be worth getting a k&n filter, or is the stock filter a high performance one? Thanks guys
 

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You will likely hear two stories.

As a primer, I have the K&N replacement panel in my WRX. I just tend to like to put one in all my cars right off, since they have a good name for increased performance, plus you never have to buy a filter again.

As for my experience, without doing a dyno run before and after, there is really no way to tell if it really provided an increase in power.

Now, the other side of the coin. Vishnu Performance, in CA, is a premier tuner for Subarus, expecially the WRX. Vishnu's Stage Zero kit for the WRX does not include a K&N replacement. Shiv at Vishnu has mentioned several times that the K&N actually responded with less power than the OEM filter. This information was gathered on the dyno. They would run the car with it and then without it, with no other changes. They have strongly suggested against the K&N, at least for low levels of tune and modifications. There are potential issues with the oil in the filter fouling the MAF sensor, since it is a hotwire-type sensor.

I should note that, to my knowledge, I have had no issues with MAF fouling after several thousand miles on the K&N.

The K&N is a good product. I happen to like them. Just wanted to let you know that there will be differing opinions, just like most everything here and on i-Club.com. You will see a lot less attitude on this site, however.

Have fun!
 
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