K&N air filter

Hi there,
I’ve got a question regarding K&N filters. They say that they are the best filters in the market and will give you more horsepower, better engine response etc etc.
Sounds too good to be true really.

Has anyone seen any difference fitting one of those? I am only asking because I have the option to either clean the standard one on my bike and re-fit it or pay around £38 (quite expensive) for a K&N one. Does it really worth that much?


BMC race filters give better results when combined with a tuned engine, i.e. the fueliing is remapped. I don’t know about K&N personally, but it’s the same principle. Just the fact that theyr’e servicable makes them a winner in my book.

Well I mean you can always take the standard one in the shower with you and clean it right?

No but seriously, in terms of performance gains did you notice anything?

Yep got one on mine ,i have noticed an improvement in the the low to midrange power,not a lot just a little more grunt,these filters are less restrictive and improved air flow will result in better performance ,just dont expect miracles.

As Jay says, the fact that they can be cleaned means it will be the last time that you buy a filter for your bike.

To get a performance gain out of them you really need to have race cans/system fitted and some remapping - although you will notice a small difference even without remapping.

A brief explanation: In the motor fuel & air is mixed and combusted in a specific ratio i.e. your a/f (air/fuel) ratio. The K&N will flow more air into the motor, but on its own this only puts the a/f ratio out so you need to add more fuel to get the ratio back to around 13:1 (if memory serves…) - this is done by remapping/re-jetting the bike. The problem is however that this increases the volume of combusted gases that need to go out through the exhaust, which is why you then fit a bigger bore/better design exhaust to extract the burnt gases quicker. The net result is an optimum a/f ratio but at an increased volume which results in more power.

Both filters work well, but i have gone for BMC. The engine will be able to breathe more freely now but its best to compliment the filter with a freer flowing end can to get better results and as Jay say a fuel remap or dynojet carb kit. Its worth the extra cost of a fuelling set up, you`ll have a sweet running bike and more economical.

I found that the k&n only worked better on some bikes. Never worked well on my zxr400 or my Gixxer6. Never made any more power back to back with the standard one! Both bikes were dynojetted etc.

thats interesting, so the leaning out is on ram air bikes or is it in general? how is the air actually introduced into the combustion chamber? i thought it was sucked in?

It’s on all bikes, but I guess the effect on ram air bikes would be slightly greater (but only at higher speeds e.g. over 80mph).

When the exhaust valve opens and the burnt gases are sucked out, there is a short period when the inlet valve is also open. The negative pressure from the exiting gases helps suck the fresh mixture into the combustion chamber. The negative pressure is then greatly increased by the piston going down in the cylinder and creating a vaccum effect like a big syringe (the exhaust valve is closed at this point). The air filter provides resistance though (compare breathing freely, to breathing through a dishcloth), so if you have a more freely flowing filter, more air can be sucked in by the same negative pressure, for the same given time that the inlet valve is open. What ram air does is to add some positive pressure to the air in the airbox, which then increases the pressure differential between the airbox and combustion chamber, and therefore increases the rate at which that pressure equalises i.e. air flows into the cylinder. Because the time that the inlet valve is open hasn’t been altered (different/reprofiled cams are used to do this), you get a greater volume of air in the cylinder. The air also flows into the motor at a slightly higher velocity (varies from engine design to engine design) which also helps the air and fuel mix better, and therefore burn more efficiently which gives more power - this is one of the reasons for multivalve engines, because two small valves can flow the same amount of air as one big one, but at a higher velocity (which incidentally is part of the reason why I4’s make more HP than V2’s - more valves). I sound a little sad, don’t I

haha, debateable

makes sense to me. the thing i’m curious/surprised at is that on cars (as far as i’m aware) the ECU monitors the fuel/air mix and if it is getting cleaner/colder/greater air flow it can adjust to this in terms of fueling - if you slap on a performance air filter (and often in the process put the filter in such a position that you’re getting ‘better’ air) you get respectable gains, and no real need to make adjustments at that point. the issue of leaning out wasn’t apparent when i’ve done it on the cars i’ve had altho the relative change might have been insufficient enough to notice given the bhp?

are bikes not capable of similar adjustments or does the increased ability to breathe outperform its self adjustment range?

Can’t add to the technical debate, but big thanks to Fridayman - great explanation. You should call yourself Dr. Fridayman

********! I had an answer all written out and it dissappeared when I hit send.

Basically FI bikes are the same, but have cheaper (less clever) ECU’s, so they do make adjustments, but not as effectively as cars do.

So in other words…

The bonus points are…

Better air flow, million mile warrenty, never have to replace it

You will get better performance gains but remember its not like your sticking a turbo on, you wont notice a 3bhp increase…

But Its a start…

Not necessarily…

The only person I know of that’s done a back to back (ie. run on dyno then whip out filter, replace & re-run straight away) comparison on a dyno between a std & a K&N filter ended up seeing a 4BHP loss with the K&N, that was on a ZZR1100

There’s a guy on another bike list I’m on who works in engine development including F1 engines & to be honest he thinks they’re utter cack, wouldn’t put one on his lawn mower let alone a bike!

What is he comparing them to though? His F1 filters, other race filters, std filters, pool filters… And why does he think they are cack?

I had a ZZR1100 too and fitted a K&N (with std pipes) - butt dyno couldn’t tell any difference.

BTW - how fast is his mower? (I take it, it’s on of those ride on race jobbies - if you have to mow the lawn, may as well!)

Acording to many people on the different R6 forums I frequent and racers I’ve spoken to, K&N air filters are ****e.

They make it difficult to jet or set up fuel maps becuase they have inconsistent air flow properties and don’t flow as much air as the stock filters.

Many people suggest (on the R6 at least) that you stick with the OEM filter. Infact I’ve heard that the AMA Yamaha race teams use the stock filter…Not sure if this is true though as many people ditch an air filter altogether when they race tuned bikes.

Heck…I don’t think it makes a big deal…I bought and am using a K&N before I heard the negative press.

I get 100 BHP at the wheel. To me that’s good enough, but I won’t be buying again on the next bike.

Hope that helps.