What do people think about them? I've heard everything from there's no difference to its the ultimate upgrade. I can't believe either is true. They wouldn't be so incredibly popular if they didn't do anything. Aside from making the engine sound different, what benefits have people experienced? I've heard that it will both increase gas mileage and performance. Has anyone put one in themselves?

Demand empirical evidence, and be skeptical. To everyone else that has replied to this question: BRAVO! Well said! Add my name to the list of those that say no to K&N filters.

Look at my signature line. The K&N filters are, or were, incredibly popular among the RX-7 fans, and I had thought of installing one in my hopped-up 1985 SE. But, I found some data that shows they don't trap all the dirt that the factory paper filter does, so . . . Never, not in my Mazda!! Or my Volvo or Toyota either.

Reasoning like "They wouldn't be so incredibly popular if they didn't do anything." will put you at great risk of being conned, shilled, flummoxed, bamboozled, and mislead. And more.

Summer reading assignment: "The Demon-Haunted World: Science As A Candle In The Dark," by Carl Sagan. A great book, full of examples of how we are mislead, how we allow it, and how to reason our way free of it so we can live better lives, making our choices by reason based on facts.
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Scott Cook - 1991 745T, 1985 RX-7 GSL-SE, 1986 Toyota Tercel (Don't laugh, it is reliable, faithful AND gets 41 mpg!)

Well, the whole popularity reasoning was an idea, but not something to be solely based on. Me being skeptical is why I posted. Looks like I won't be getting one, not worth the money (at least not for a Volvo). However, might I remind people that everyone I have talked to in person who has gotten a K&N filter has had good results, but none of them had a Volvo.
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Manual tran 16v Wagon... Booyah!

Before I go any further, please understand that I'm not bashing, but want to help keep you from falling prey to the "demons" of ignorance that lurk in the dark world out there (paraphrasing Sagan - Really good book). They exist to take your money and whatever else they can get from you, like political allegiance, obedience, etc. And they will give you nothing in return.

Can everyone you have talked to provide you with solid data proving that they have gotten good results? If none of them can, then they may be evaluating the differences the filter made subjectively. Human perception is well known to be pretty fickle in this area. The data they provide should also include data on how well the K&N filters crud, and engine longevity using the K&N vs. factory filters. Unless you are just going to sacrifice an engine for the sake of winning one race, which is done, but on the track only.

An engine is an engine, Volvo, Mazda (including rotary), Ferrari, whatever. **If everything else is the same**, they are all going to respond to changing to a K&N filter the same way. In other words, performance-wise not at all, but likely with increased cylinder/rotor wear. You have to take into consideration ALL of the other factors involved too. These other guys you have talked to, are their engines all stock? What mods are involved? Is the stock design limited by its intake runners and system other than the filter (in which case removing the filter completely would do no good), etc. Also, if more than one or more mods were made at the same time they installed the K&N, how do they know that the filter made the difference?

Do these guys understand all the subtlties of tuning an intake system, cross-sectional area vs various airflow parameters, effect of lower intake resistance on torque and hp curves, effects of changes in air flow velocity, etc?

Also, sorry to say this, but some people may be reluctant to admit that they dropped a load of cash on something that is not worth it, so they, perhaps unconsiously, make believe that it was worth it. IMHO BMW, Porsche, Lexus, the personal computer industry, K&N, and many other companies have made lots of money exploiting this characteristic of human personalities.

Before you risk damaging any engine, or believing any claim from anybody, get strict and rigorous proof.
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Scott Cook - 1991 745T, 1985 RX-7 GSL-SE, 1986 Toyota Tercel (Don't laugh, it is reliable, faithful AND gets 41 mpg!)

Many manufacturers will void an engine warranty if the K&N filter is installed because most car owners do not know how to maintain the filter. It is old technology. You want to increase performance and fuel mileage........ just put a chrome valve cover on!!
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1965 Mustang A block 4 speed, 1969 Cougar Eliminator, 1990 Super-Charged Thunderbird and a car (740) that my wife can drive!!!

I have, in the past, used K&N filters on a Volvo. Given that I have now done some significant research on them, I would not waste my $$ on one again.

The factory Volvo panel filters are very low restriction designs. Large area for the engine size (air flow requirements). The K&N filters do, in many cases, offer *slightly* less restriction accross the filter media. I have personally dynoed an old stock paper filter against a new factory oiled K&N. The new K&N (clean, direct from the box) actually produced LESS Hp (by 1) than my old dirty paper filter. Somewhere, I still have that dyno sheet.

K&N filters do not trap more dirt, in fact they typically trap less (independant testing). I do not belive the difference is very large.
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JohnG 1989 245 MT @ 225,000

Snake oil.
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See the 700/900 FAQ under 'Select Link' button on the top right.

Crack cocaine is also incredibly popular. The general consensus here is that the added air flow is is a poor trade off for the decreased filtration. It's only a tiny increase in air flow since the volvo head can only flow so much air! So the crossland or mann or bosch air fliter you are used to is more than adequate for the engine. Mods you say? You'd have to be pushing well over 350hp in order to need higher flow. For example; even if you got all the go-fast stuff ipd sells you still wouldn't need a K&N filter. However if you want to impress your friends at high school, the ones who drive their rice cans with the wings that even an F1 car wouldn't be caught dead wearing than go right ahead. You'd get more performance from a nice new set of spark wires for the same 40 or dollars. Do a search here for "K&N" - educate yourself and form your own opinion.

Please save this post. The question comes up regularly and I am tired of answering it as well as you did here.

They wouldn't be so incredibly popular if they didn't do anything.

They are very heavily advertised to the very easily impressed. It's all about hype, IMO.
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Bruce Young
'93 940-NA (current) — 240s (one V8) — 140s — 122s — since '63.

If they are truly high performance, why don't high performance cars come standard equipped with them, i.e. Porsche, Ferrari, Mercedes, etc. They all use paper elements to my understanding......the factory must know something....

mechanical equivalent of a politician in a box, right down to the slightly oily exterior.
That said, anybody wanna buy a slightly used K & N for an S 70? I bought one on a recommendation from a friend, and took it out because I was concerned about the lack of adequate filtration.
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We have met the enemy and they is us. [Pogo] S70 T5 cop car : Rough Rider suspension, Walmart fog lights, eBay speakers, ambiance by Pall Mall, trim by Le Duc d'Tape, 8-channel THD by OEM amps

You can make your car go faster by putting a fart can on the end of the exhaust pipe- flame stickers help lower the wind resistance, too.

I have found a comparable effect from the fuzzy dice, after I learned to hang them from the INSIDE rearview miror. Related phenomenon: big stereo doesn't make you go measurably faster, but it does make the aurally (?) assaulted wish you would speed up and get the hell out of the neighborhood.
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We have met the enemy and they is us. [Pogo] S70 T5 cop car : Rough Rider suspension, Walmart fog lights, eBay speakers, ambiance by Pall Mall, trim by Le Duc d'Tape, 8-channel THD by OEM amps

"big stereo doesn't make you go measurably faster, but it does make the aurally (yes, that is correct) assaulted wish you would speed up and get the hell out of the neighborhood."

bru, i'm gonna use that one again and again.

back to the topic at hand, i've been vehemently dissuaded from using anything other than paper on the basis that the turbo+k&n=dead turbo.