Despite the spirited debate on this very forum regarding these filters, I decided I needed to find out for myself. I bought two K&N filters (unfortunately, I had no choice), and installed them on my 1994 E420. Under hard acceleration, I seemed to notice just a little more noise than before. By the seat of my pants, I definitely noticed a more urgent pull towards redline, particularly once the car got over 4000 RPMs. As many already know, this car can be a little sluggish due to the tall gearing, but once it gets rolling, the acceleration can be quite fierce. With the K&Ns, it seems to have gotten just a little angrier!

Assuming you are going to keep your car for a long, long time, the K&Ns can be thought of as cost effective. Since I live in San Diego, where it is rarely dusty, I am not overly concerned about some claims that the K&N filters do not filter as well as the paper filters, particularly since the K&N literature suggests otherwise. If my car suddenly requires a major repair, however, I may be singing another tune.

For now, I am a happy convert.

If there are any claims that the K&N does not trap dust as well as a paper filter, I believe that this must be due to lack of filter maintenance.

My $0.02,

Actually K&N claims the filter actually filters better when it gets dirtier (I dont know, it beats me). I have a K&N filter on my Integra and I have noticed no difference besides for it is louder. The intervals they recommend for cleaning the things are crazy, the box says cleaning is recommended between 50k-100k miles for street driven applications. Well I cleaned mine at 30k and it was a pain in the arse to clean 30,000 miles worth of dirt. It took me about 2 hours to "service" the filter, also the kit that I used to clean and oil the filter cost me $20. If I had to do it over again I would have never bought that filter and just stuck with stock.

I know my 500E certainly seems like it likes the additional air...I endorse this product for the big air-gulpers like the V-8s.

I can see this shaping up to be one of those arguments we have many times on this board similar to the "what oil to use", "what antifreeze to use" and "what wax to use" ones :D

I think this is another gray area that is up to you. There are Pro's and Con's for both types of products. As long as you stay away from the 'Quaker States' of the world you'll probably be OK.

Incidently my K&N is on its way in the mail ;)

If it is true that installing a freer flowing air filter increases power, than it is equally true that the stock air filter system is "choking" the car of power.

If that is the case, I am surprised that MB engineers were not smart enough to simply install a larger surface area filter box. It would surprise me that they would leave "free" (i.e. it doesn't cost in emissions, $$, safety, etc.) hp on the table. And, likely, also free MPG. Given the unbelievable R&D that all Mfrs put into making their engines as efficient as possible, I'd be surprised that not one of the engineers at MB has ever thought to test their stock airbox/filter system to see if it is choking the engine, given that if that is the case, a solution is so simple.

It has been proven by the most reputable Porsche mechanics in the world (Bruce Anderson, Jerry Woods, and others), using dynos and multiple tests, that with VERY few exceptions, Porsche engineers have been smart enough for decades to install large enough air filter boxes on their cars such that the stock air filter is not a bottleneck. They have undisputedly proven that a K&N gives no increase on a Porsche. In fact, they have even dyno tested with NO air filter at all, and shown no increase on most Porsches. The simple fact is that Porsche was smart enough to equip their cars with air filters/boxes that are ample to supply the stock engine with far more air than it can ever use (if you are talking about modified engines, of course none of this counts).

Additionally, K&N's stats (I've e-mailed them directly to get the info) never claim to filter as effectively as the stock filter. The best they have claimed, that I have seen, is that a stock paper filter will filter 98% of particulates, while a K&N does 97% in the same test (tests run by or on behalf of K&N). That is a deceiving number. It does not mean that a K&N only filters 1% less, in fact, it means it is about 33% less effective. Of all the millions of particles in the air, the stock only lets in 2 out of every 100. The K&N lets in 3 out of every 100, an increase of at least 33% (or maybe 50%, I'm not very good at math).

So, in conclusion, I think it is remotely possible that such a thing may increase power marginally on a poorly engineered car. I'd be surprised if it does on a well engineered car. In the absense of PROOF (valid, third party testing from a reliable, disinterested third party), I am willing to give MB the presumption that they were smart enough to not choke their engines with a too-restrictive air filter. That presumption can be overcome, but only by real, reliable evidence. Thus far, I've never seen it.

Weighing against that is the fact the K&N has NEVER, to my knowledge, claimed that their filters actually filter dirt as well as a stock filter (I's suspect they won't make that claim, for liability and false advertising reasons, because it appears that their testing indicates that their filters DON'T filter as well as factory paper filters).

(They can easily get around the false advertising rap on their HP claims, because you have to carefully read their ads to see what they actually claim. They claim "up to X hp increase", but don't ever specify on what car, or what conditions. They also generally only claim their filters flow more air, but that is meaningless. If the stock filter flows X, but the engine can't use more than X (as is the case in most well engineered cars), than increasing the flow of the filter to X + 1,000,000 will do nothing).

Just my opinion.

Brian brings up an EXCELLENT point.

The gain you will see will be determined greatly by how well the original filter is sized to the engine. In the case of a properly sized filter there will be negligible or no gain.

In the case of my Vette, the filter area and mass air flow size are undersized for the car. The injection system and associated components were originally developed for a 305. Then before production, GM brought back the 350, so they simply stuck the 350 under the same system which was probably marginally sized even for the 305.

My C240 has less than half the engine displacement than the Vette and it has a mass air flow sensor that is at least twice the size of the one on the Vette. One of the air filters on the C240 is probably as big as the single air filter on the Vette. The MB has twice the filter and mass air flow area but only half the displacement. Adding the K&N on it, or even taking out the filter element all together would probably not increase the flow.

Brian, "you're the man!"

Have a great day,

Like I said in the absence of "evidence" from either side...its still up to you. Personally I am going with it for ease of maintenance but then your going to have some people who can't stand to clean the damn thing, so your back to square one with the opinion score sheet.
Bottom line is your probably not going to get any hp increase worth a damn from the change of an air filter on 90% of modern cars. It will probably be the equivalent of welding on some chrome tips to your muffler. But chrome tips look good to some people.
We keep regurgitating this K&N argument on this forum and many others and it won't be the last time. Someone will probably throw in some mathematical formulas next....

For what it is worth, their ad copy states that not only is flow increased, but the filter "straightens" the air flow so that it is not as turbulent. I believe that it is this straightening effect which makes the K&N "flow" better than no filter at all. Assuming there is some validity to this straightening effect, that may be where a few HP lie even when the air flow capacity of the intake exceeds the needs of the engine.

Yes, "crooked" air can lead to decreased hp! LOL!
If you believe that crock, I'd suggest taking a look at the "Tornado" product. You can get it from JC Whitney, I think.
Basically, its an impeller type thing (like a windmill) that you stick in your intake. As the incoming air rushes through, it spins the windmill, not only "straightening" the air, but also giving you a "free" turbocharging effect! All for only $19.99!
The bottom line is this: A normally aspirated engine is only going to use as much air as its head can intake and exhaust through the intake and exhaust valves, and as its pistons can pump in and out of the cylinders. Period. It is not rocket science to develop an intake system that can easily meet the engines air intake requirement (exhaust may be a different story, I am only addressing the intake side).
Sometimes, this is not done by the factory, like in Larry's Corvette example. That example does not surprise me, because nothing GM does surprises me. Such poor engineering on a modern MB, however, would surprise me.
On the other hand, I really don't have anything against K&N, and I actually think in many instances its a good value. Why? Because actual hp increase is not really important, what is important is OWNER PERCEPTION. Therefore, if for whatever reason (either the owner believes the marketing, or thinks he is getting more power because his intake sounds different, whatever) the OWNER THINKS he is getting more power, he will be happy and will be glad he spent the $$. Since most cars are not subject to a stopwatch, reality really doesn't matter, its only the individual owner's perception that matters.

Imagine if K&N came out with a filter that actually circulated a closed loop freon stream in it, then you would cool the air, therefore, making it denser, consequently delivering more oxygen in the same amount of space and thusly improving engine response and acceleration that can be easily proven...

Off to the patent office I go...