Topic: K&N vs. stock air filter

[longshanks]

So I'm getting the stock airbox put back on and I'm wondering what the difference was between the stock foam element vs. K&N (performance, mileage, etc.)  Is it worth the $50 vs. $10?

[TwoTired]

The stock element was a paper type and not cleanable/reusable when the service interval expired.

The K&N IS cleanable reusable (see their filter service kit).  So, it lasts indefinitely baring severe abuse.

I don't know why you think the stock one was foam, but that is incorrect.  If you have a foam one, it should also be cleanable and reusable (until the foam begins to breakdown in 15 years or so).
Each filter type presents it's own characteristic pressure drop across the filter membrane type.  The filter's pressure drop gets reflected into the carb throat where the fuel jets exits are.  If you change the carb throat pressure, you also change the flow volume from the fuel metering jets, e.g. mixture ratio.

Foam filters often have more open pathways similar to the K&N, which CAN allow larger particles past/through it.  They are usually oiled so that the oil can catch and hold the particles when the air changes direction within the membrane.  Paper type filters use a smaller aperture size to absolutely block and trap particles within it without aid of an oil attractant/entrapment.  The paper filter weave makes it very difficult to clean thoroughly, which is why they must be replaced after a certain amount of use, in addition to the deformation of paper fibers from humidity and water effects.

Both the K&N and Foam type filters are therefore more of a statistical type (some objectionable particles get through it) rather than the paper's absolute type (none shall pass if larger than tolerated).  The trade off is that the paper type can't be flushed effectively, using a cleaning fluid, without collapsing the fibers and changing the pressure drop across the membrane.  So, you have to replace the whole thing, pretty much on a yearly basis.

In theory, a paper filter will help the engine last much longer than the replacement K&N or Foam filters when operated in dusty environments.  (Which is probably why it was chosen as the stock type.)
However, if you operate the engine in an environment where no particulates are in the air, then you don't need ANY air filter.

At any rate, the carb jetting was selected using the pressure drop characteristics of the stock paper type.  If the replacement filter, K&N or otherwise, isn't engineered to mimic the stock paper's pressure drop, carb mixtures are altered.  Which could be beneficial or not depending on your particular bike.  (Doubtful ANY cheap filter has much engineering invested in it.)

I've been using the K&N on my CB750s with no complaints (they have accelerator pump carbs).  It does run leaner than with a stock paper one and oxygenated fuels make this very border line and in need of the stock muffler.
I've been using the UNI foam type on all my CB550's except the 78.  They all run slightly leaner with this filter over the stock paper type.  The 78 is too lean with oxygenated fuels, so I must use the stock paper type for that bike, and replace it anually.

Cheers,

[longshanks]

The stock element was a paper type and not cleanable/reusable when the service interval expired.

The K&N IS cleanable reusable (see their filter service kit).  So, it lasts indefinitely baring severe abuse.

I don't know why you think the stock one was foam, but that is incorrect.  If you have a foam one, it should also be cleanable and reusable (until the foam begins to breakdown in 15 years or so).
Each filter type presents it's own characteristic pressure drop across the filter membrane type.  The filter's pressure drop gets reflected into the carb throat where the fuel jets exits are.  If you change the carb throat pressure, you also change the flow volume from the fuel metering jets, e.g. mixture ratio.

Foam filters often have more open pathways similar to the K&N, which CAN allow larger particles past/through it.  They are usually oiled so that the oil can catch and hold the particles when the air changes direction within the membrane.  Paper type filters use a smaller aperture size to absolutely block and trap particles within it without aid of an oil attractant/entrapment.  The paper filter weave makes it very difficult to clean thoroughly, which is why they must be replaced after a certain amount of use, in addition to the deformation of paper fibers from humidity and water effects.

Both the K&N and Foam type filters are therefore more of a statistical type (some objectionable particles get through it) rather than the paper's absolute type (none shall pass if larger than tolerated).  The trade off is that the paper type can't be flushed effectively, using a cleaning fluid, without collapsing the fibers and changing the pressure drop across the membrane.  So, you have to replace the whole thing, pretty much on a yearly basis.

In theory, a paper filter will help the engine last much longer than the replacement K&N or Foam filters when operated in dusty environments.  (Which is probably why it was chosen as the stock type.)
However, if you operate the engine in an environment where no particulates are in the air, then you don't need ANY air filter.

At any rate, the carb jetting was selected using the pressure drop characteristics of the stock paper type.  If the replacement filter, K&N or otherwise, isn't engineered to mimic the stock paper's pressure drop, carb mixtures are altered.  Which could be beneficial or not depending on your particular bike.  (Doubtful ANY cheap filter has much engineering invested in it.)

I've been using the K&N on my CB750s with no complaints (they have accelerator pump carbs).  It does run leaner than with a stock paper one and oxygenated fuels make this very border line and in need of the stock muffler.
I've been using the UNI foam type on all my CB550's except the 78.  They all run slightly leaner with this filter over the stock paper type.  The 78 is too lean with oxygenated fuels, so I must use the stock paper type for that bike, and replace it anually.

Cheers,

Cool, thanks for the thorough explanation. I am familiar with how K&N uses foam elements with oil but I didn't know CB750's used paper-type as the factory filter since I don't have the filter from the stock airbox. My question more more related to whether it enhanced performance at all or affected the air/fuel mixture.(which it sounds like it does)

Will some re-jetting be required? I figure it won't be too big of a deal to re-jet since I'm going from pods to stock airbox.

[Deltarider]

I was told by a filterexpert that when airfilter A filters out 99,8 % of the particles and airfilter B 99%, enginewear with B is already fourfold. It's your bike.
I always stayed with the OEM filter elements. No messing with oil (to little? to much?) or foam that can tear. Mine is changed within 5 seconds and as I can do easily 18.000 kms with them, they're outright cheap (check Dave Silver). Oh... and renewal doesn't need to be annually. Humidity in the air is no real danger to these type of filters. Quite some marine engines use 'paper' filter elements. The French even wash them out, but I don't think that is wise. Will try it with an old one someday. Did you know that towards the end of its lifetime a paper filter is actually at it's best?

[longshanks]

I was told by a filterexpert that when airfilter A filters out 99,8 % of the particles and airfilter B 99%, enginewear with B is already fourfold. It's your bike.
I always stayed with the OEM filter elements. No messing with oil (to little? to much?) or foam that can tear. Mine is changed within 5 seconds and as I can do easily 18.000 kms with them, they're outright cheap (check Dave Silver). Oh... and renewal doesn't need to be annually. Humidity in the air is no real danger to these type of filters. Quite some marine engines use 'paper' filter elements. The French even wash them out, but I don't think that is wise. Will try it with an old one someday. Did you know that towards the end of its lifetime a paper filter is actually at it's best?

No I didn't. Strange, I wouldn't think that. I also forgot to ask how viable it is to run stacks. I live in Los Angeles and I see a lot of guys run them but I always cringe thinking what might find its way into the engine. Is LA an ok place to run them?

[TwoTired]

Did you know that towards the end of its lifetime a paper filter is actually at it's best?

Only from the standpoint that when a paper filter collects dirt, the air passageways get smaller, which means even smaller particles can be trapped and prevented from passing on to the engine. Using this mentality, a completely clogged filter or a solid wall barrier is a better "filter" as it allows no contaminants at all to pass on to the engine, including the very oxygen the engine need to run. 

As the filter's passageways are blocked with dirt, the pressure differential across the membrane increases and the air flow capability through it decreases.  Yes, the filter's efficiency rating actually improves at the cost of blocking airflow, as well.
I find this a rather perverted way to evaluate an engine air filter.
It is similar to saying that having no money is a better condition because you are relieved of the burden of carrying it, and totally discounting the primary purpose of money.

[TwoTired]

Did you know that towards the end of its lifetime a paper filter is actually at it's best?

No I didn't. Strange, I wouldn't think that.

...And you would be correct from a practical/functional standpoint.

I also forgot to ask how viable it is to run stacks. I live in Los Angeles and I see a lot of guys run them but I always cringe thinking what might find its way into the engine. Is LA an ok place to run them?

If you don't care how soon the engine must be rebuilt and are confident the air quality will never allow ingested particles to "sand blast" the cylinder walls or rub between piston and cylinder sleeve...
... sure.

[longshanks]

If you don't care how soon the engine must be rebuilt and are confident the air quality will never allow ingested particles to "sand blast" the cylinder walls or rub between piston and cylinder sleeve...
... sure.

My thoughts exactly. You either have to be somewhat ignorant or prepared to rebuild when the time comes.

[bollingball]

Where are these $10 filters? Best I can find on-line $35 +SH. Local Honda shop $40 out the door. 750K8. That is why I went with a K&N. Have one paper one to use before I put it in I maybe change mine to often I don't like a dirty filter plus I'm in farm land. Wind after a plow =lots of sand. If I go through a lot of it I blow the paper ones out when I get home. From the inside out and tap it.

[CrankyOldGuy]

David Silver Spares has replacement filters for $13.25 US plus shipping.

Harry O

[longshanks]

Where are these $10 filters? Best I can find on-line $35 +SH. Local Honda shop $40 out the door. 750K8. That is why I went with a K&N. Have one paper one to use before I put it in I maybe change mine to often I don't like a dirty filter plus I'm in farm land. Wind after a plow =lots of sand. If I go through a lot of it I blow the paper ones out when I get home. From the inside out and tap it.

$9.14

http://www.z1enterprises.com/Air-Filter-Honda-CB750-1290400-1972.aspx

[Deltarider]

As the filter's passageways are blocked with dirt, the pressure differential across the membrane increases and the air flow capability through it decreases.  Yes, the filter's efficiency rating actually improves at the cost of blocking airflow, as well.

Do you really think I cannot distinguish a good working filter from one thats need to be replaced?

[Deltarider]

There seems to be some misunderstanding about performance and lifetime of the socalled paper filter element.
Below I pass on the information I gathered from filterspecialists. Years ago in a motorcycle magazine I read an interview with the director of a company that imports filters for motorcycles. The man concluded by the volumes he sold that the average rider took extreme care of his bike. He demonstrated this by calculating that he sold far more airfilters than needed and he doubted whether people knew that by renewing their filters all the time ‘just to be sure and have the best’ wasted not only money but perfect filters too. It was in that interview I learned for the first time that contrary to popular belief a paper airfilter actually filters better when it gets older and is at its best, just before it clogs up.
A paper filter filters in three ways. Firstly by inertia. The air flows around a fiber. The heaviest particles cannot follow this track. Because of their inertia they collide with the filterfiber. The second way is the blocking effect. The somewhat lighter particles manage to follow the track of the airflow but hit the fiber sideways and so get blocked. The third way is by diffusion. The smallest particles can follow the airstream but by all kinds of accidental movements (turbulence) deviate nevertheless. When they contact a fiber, they get stuck. Diffusion is quite accidental. Enough filtermaterial and many thin fibers offer the ‘accident’ a helping hand.
In a filter not ony the fibers filter but also the already trapped particles. That’s why a new filter filters not as good as an used one. Although the filter during its lifetime traps more and more dirt, pressureloss-over-the-filter hardly increases if at all. It’s not before the filter is almost full of dirt, that pressureloss increases all of a sudden. Have a look at the picture below. It’s a scheme for a typical paper air filter’s lifetime and demonstrates that there’s no loss of pressure over the filter until… the very end of the filters lifetime. Then pressure loss over the filter increases dramatically. t1 actually is the ideal moment to replace the filter by a new one. BTW, on the horizontal axis it says trapped particles and on the vertical axis it says pressureloss.
Leave the trapped particles where they are, as they help your filter filter.

Where I live, I can do easily 12000 miles with a paper airfilter and it is still good. At David Silver Spares the filter for my CB500 will cost me US$ 23,97 (€ 19,04)* for genuine and US$ 15,51 (€ 12,32)* for a good imitation. I can change my paper airfilter in 5- 10 seconds and that includes opening the seat, removing the tooltray, the cover etc. Why would I mess with a foam filter that can tear, is from new already inferior in it’s filtering performance and then have to mess with oil that makes me uncertain about how much or how little, where I can have the pleasure at acceptable cost to have the genuine thing that will give me 12000 miles of riding pleasure without ever having to think about it? I don’t know about prices in the US, but here are some current prices at David Silver Spares: CB 550K/F for a Genuine Honda US$ 24,68 or € 19,60*. That's € 3,35 cheaper than Louis asks for an imitation! All genuine filters are available though Honda BTW and you'll find imitations in mailordershops like Louis.
 
 * Prices March 2009

[TwoTired]

Your initial analysis is good as far as it goes. But, you left out a couple of important details. One is the fourth (actually first) way filters do their job, that of aperture size.  When an ingress particle exceeds the size of a filter path way it is blocked from flow through.  Further, that trapped particle seals that pathway and reduces the overall area or the total number of pathways available for air to flow through.  This creates a cumulative "choking" effect and carburetor fuel enrichment over it's use lifetime. 

The second oversight relates to the graph you've introduced.  I've seen similar graphs before.  It is valid only at air flow rates far below the max airflow capacity of the filter selected.   Your graph shows the air filter operating well below the point where the pressure drop across the membrane becomes significant, such as at idle when the volume of air demand is minimal.  At such speeds, when the filter approaches an overall area restriction due ONLY to reduced available pathways, the filter finishes clogging rapidly.  But, your graph does NOT account for air demands above a minimal air flow requirement, such and high RPM operation.

Other filter performance graphs, chart the pressure drop across the filter membrane at air flow rates where it becomes significant.  The filter cross sectional area is also significant and the paper filter loses cross sectional area as it traps particles.

The first graph compares a filter manufacturer's product offerings, and demonstrates characteristic of the same filter media offered in three different sizes.  This shows how cross sectional area impacts the pressure drop and further shows how the increases in velocity (equivalent to engine volume demand increases with RPM) impact the pressure drop across the filter membrane.
You should be able to see that at any given velocity, the larger the filter, the less resistance or pressure drop occurs at the filter membrane.   Further, the chart shows that if you are operating any of the filters at very low velocity, there is very little difference in pressure drop between the three different sized filters.

The second chart typifies how dust loads impact pressure drop at the filter, with 4 filters having different efficiencies relating to dust particle size blockage. The finer the filter (using the same initial filter area), the more rapidly the onset of a pressure drop increase with dust volume capture.

Perhaps to get a better overall understanding of filter parameters/functions, and capabilities, you might study this link.  It's is NOT motorcycle specific,  But you can gain a wider understanding of the various aspects or parameters involved in filter "performance".
http://www.dieselbombers.com/chevrolet-diesel-tech-articles/16611-duramax-air-filter-testing.html

It's a well observed phenomena that fuel mileage on carbureted engine, worsens as the air filter becomes more restrictive.  The increased pressure drop from the reduced membrane area choking effect, causes more fuel to be drawn through the fuel jets, which leads to fuel enrichment, even when the exact same volume of air is drawn through the filter membrane.  This fuel enrichment is what diminishes fuel economy AND can rob power at high RPM due to less than ideal fuel fuel mixture.  I've demonstrated this many times on my own Hondas, with air filter replacement or cleaning alone showing a dramatic change in both spark plug deposits/carbon fouling AND fuel economy.

As for prices; a new, stock Honda filter can't be had here in California for under $50, and I've seen dealer prices near $75.  (If someone has a cheaper source, I'm sure willing to hear of it.)  I need to include shipping charges if it isn't a local pick up.  The Uni costs me $26 with tax at the local MC shop, and the cleaning recharge kit was about $25.  Both have lasted about 15 years.  It's pretty easy math to see a significant cost savings... for me, anyway.

Yes, I know the K&N and the Uni don't scrub the air quite as well as the stock paper one.  But, I minimize that risk by not riding through dust storms.  If your situation makes such practice unavoidable, a better filter, as well as frequent replacement, is more economical than engine rebuilds.

Cheers,

[detdrbuzzard]

i have a k&n filter ( and have had them in several of my bikes for years ) i won't say that k&n filters are better or worse than a stock air filter. some don't like cleaning them but around this time of year my bikes are in or going into hibernation which is a good time to clean the k&n filter

[HondaMan]

My first choice is the K&N, not just in the CB750, but in my cars as well.

In my CB750K2, I installed it at 10,000 miles (the first 5 months after I bought the bike). It has been there since, now at 140,000 miles. The engine has always stayed absolutely clean, no bore wear or ring wear issues. It (the filter) has been cleaned and LIGHTLY re-oiled exactly 4 times in these 40 years. I had 80,000 miles on the last set of rings, having installed .010" oversize pistons at 53k miles (not for wear reasons, but because I wanted to...) and then I took the top end apart in 2005 to see if there was any trouble after the bike sat 4+ years when I was sick: it was fine, so i installed new rings then. It still has the same air filter, though.

The first thing it did for me when I installed it was that it slightly leaned out the fuel mix, a real blessing on the CB750K0-K3 bikes. It changes the mix ratio about a 5 jet size (i.e., 5% on the 750, from 105 main to 110 main at sea level on post-1973 bikes). The earlier 750 came with too-rich jets (120 in K0, 115 in K1, 110 in most K2 and some K3), and installation of the K&N was the "quick fix" for the problem.

All my bikes have it, now. My cars get better MPG from them, too.

[Tews19]

Not sure if it is relevant, but back in my rice burner days I bought a AEM cold air intake with a K and N air filter.. Got better gas mileage, studies proved gained horsepower. Although this I could not concur as i have no clue. There was a distinct louder noice from the engine when I floored it. Is this the same for bikes? Also, when I bought my 550F there was no fitler. First thing I thought was to contact the PO. He stated he had no clue there was no air filter an drove it like that since he bought it. I was nervous as hell there was some engine damage. He state dhe put on a few thousand miles.. Of course who knows if the facts were true.. Either way the engine now has an air filter and is strong.

[dhall57]

I run K&N air filters in my 3 bikes also.

[ekpent]

I made a post earlier about always hoping for a K&N in there whenever I pick up a new project. Bottom line though is to always check before attempting that first start up because you may just find something like this,an old dusty,crumbled wreck of a foam Uni ,mouse nest or such. NICE----------

[chewbacca5000]

I am not a filter genius, but one day when I was strapped for cash I found that you can clean a stock Honda paper filter using an ultrasonic cleaner.  Place the filter in the cleaner blow out with compressed air and let dry in the sun.  Put it in my bike and it ran without any problems.  I have used K & N, but can't say if it's any better.  Maybe a little boost?  Not really sure.

[grcamna2]

HondaMan,
                   I know that K & N's filter better when they are a little dirty,as the dirt is trapped in the oil impregnated cotton mesh of the filter...how often do you recommend cleaning a good K&N filter ?

[HondaMan]

HondaMan,
                   I know that K & N's filter better when they are a little dirty,as the dirt is trapped in the oil impregnated cotton mesh of the filter...how often do you recommend cleaning a good K&N filter ?

In truth: I clean them when the dead bugs (spiders, mostly) are filling up some of the mesh, or if it has absorbed enough moisture to start turning the oil whitish on the mesh.
I have never seen any degradation of performance from the dirt, partly because the surface area of the K&N is so huge that it would feed the 750 perfectly well if even 80% of the surface were blocked off.

It is exceedingly easy to over-oil the filters since K&N introduced their spray oil cans, though: 5 seconds of sprayed oil onto a cleaned filter will be too much, letting it drain back into the airbox and make a mess. It also fouls plugs that way for quite a while, since the surface area is so large that the 750 is hard pressed to suck the extra oil out by itself. So, I wash it in soap and water, let it dry a day or two, then spray a thin line of oil along the bottom edge of the mesh, and turn it upside down to sit for several hours to wick the oil up toward the top. Then I turn it right-side-up again and instal it. The mesh is just barely pink when it is just right.

When new, the last 2 I bought from K&N were seriously over-oiled. I had to wash them out and start over, right out of the box.

[HondaMan]

I made a post earlier about always hoping for a K&N in there whenever I pick up a new project. Bottom line though is to always check before attempting that first start up because you may just find something like this,an old dusty,crumbled wreck of a foam Uni ,mouse nest or such. NICE----------

Wow, Ek: I'd bet most of that would have found its way INTO the engine!

[mrbreeze]

I have a stock air box filter (K&N) but only ran it for a short time before pulling the trigger on some pricey K&N pods. I have only cleaned and oiled them once since new 3 or 4 years ago. I really should pull one off to inspect but the bike runs just fine and hasn't really had too many miles put on since then. You will know if your filter plugs to the point where it really needs maintenance. You will have fouled plugs and black smoke. You will have a noticeble lose of power. I will hang on to my stock one in case I decide to keep one bike stock but I probably won't (I'm not a stock box kinda guy!!!) I want to hear more about the Dragon set up.

[Spanner 1]

Paper or K+N, paper please !..... changing from stock filter ( paper) will effect your bikes performance  almost as much as pods ( ugh ! )........ Get real guys , Honda SOHC's carburation was designed at the airbox intake and ended at the tip of the exhaust.  Change any one of these 'paramaters' hoping for better something' fill in the blank' is upstaging  '60's Honda engineers... good luck with all that