K&N Explanation please

[bone48]

I have just been reading a thread on the accelerator module and have noticed comments on K&N and other higher flow rate filters making the bike run lean.
Could someone with a lot of technical knowledge please explain why this is the case?

I would imagine that the map developed by manufacturers covers different air pressures, densities and flow rates to allow for sea level to high altitude, thus allowing for more air flow provided by certain air filters.
I would of thought that there was more chance of your bike running lean with a more restrictive air filter, i.e. less air = less fuel injected, surely new maps mean that the A/F ratio is maintained at all levels, so if you put more air in it puts more fuel in to maintain the correct ratio.
Also I believe the bike has knock sensors which when activated fatten the mixture up anyway.

Isn't the notion that a free flowing air filter leans the mixture out just a remnant from the days of fixed main jet carbs, where due to a fixed amount of fuel delivered to the engine a free flowing air filter will indeed lean the mixture out?

Facinated to find out why I'm wrong

 

[cookie]

wot i think:

unlike cars, BMW (and probably the others too) bikes don't measure the air flow into the engine. also, as i understand it, the lambda sensor only works at lower revs range, so that cannot correct at higher revs where the ecu resorts to a fixed map which expects a certain air flow based on the oem filter..

knock sensors detect pre ignition and retard the timing.

 

[bone48]

cheers for that cookie

surely there must be some measurement of air in or air burnt somewhere, if there is no high rpm measurement surely there is a chance that even a standard bike is going to run very lean or very rich depending on altitude, if the ECU is just making an assumption with a fixed map?

 

[MWH]

I've wondered about this to with modern technology. I would've assumed the engine used what it needed as its not using forced induction. Think lots of people, myself included, think the ECU etc is cleverer than it actually is! Isn't it only air intake temprature thats measured?

 

[trainman]

I've wondered about this to with modern technology. I would've assumed the engine used what it needed as its not using forced induction. Think lots of people, myself included, think the ECU etc is cleverer than it actually is! Isn't it only air intake temprature thats measured?

think you are right, map will assume certain air densities for certain pressures and will 'know' air flows as it knows rev's (piston pump) and road speed (any air forced into the inlet) - I assume the issue with a K and N is that these assumptions go wrong and hence you flow more air than what the beast thinks in its brain?

 

[Bendy toy]

I was told the engine ECU can compensate for ay two of the following three changes
free flow headers (downpipes)
free flow silencer can
free flow air filter.

If other things are added such as cleaned throttle bodies, head porting cams, etc there will be even more for the ECU to cope with.

The knock sensor is there to allow the engine to burn low octane fuel without pre-ignition. However late ignition leads to hotter exhaust gas as there is literally less time for the engine to convert the fuel's heat into power energy. if the engine is also running weak you risk burnt valve seats if not a dropped valve head. The exhaust valves are large and already run hot. They get less cooling than in a similar capacity multi cylinder engine.

I have all three of the above mods with a Power Commander 5 and the engine runs really well. if the PC-5 is ever taken off (without a remap), the standard air filter will go back in it's place. I also run only high octane fuel such as Shell Optimax or Tesco Momentum 99. It feels as flat as a f**t on plain 95-RON.

I terms of what the ECU "should" be able to accommodate, I personally think the BMW does seem to have limited adaptability compared to car ECUs. But, most car engines are relatively low stressed even when tuned so the ECU has more room to adapt as it has to be less precise (relatively). The R1200GS with the usual mods and PC or mapping will make close to 100bhp at the back wheel. A 16V Fiat 1250 with no mechanical tuning (porting etc) won't go beyond 80bhp at the wheels. Its about 65bhp as standard.

 

[Steptoe]

Why would anyone want to fit a K&N air filter in the first place. Don't be seduced by the advertising.

 

[Alan R]

Why would anyone want to fit a K&N air filter in the first place. Don't be seduced by the advertising.

Exactly! American snake oil product that has its roots in NASCAR etc. Only needs to filter out tumbleweed sized particles for one race before an engine rebuild. OEM filters are half the price and you chuck them away and use a new one from time to time. Much better for engine life.
Alan R

 

[Bendy toy]

Mine's got a Pipercross oiled foam element.

 

[seahorse]

Much is made by K & N of the fact that they never need replacing. They do however need to be cleaned and re-oiled, and I am not convinced of their effectiveness anyway. They cost 3 to 4 times as much as OE, the service interval is 24k for OE. Given the pitiful mileage most bikes do, its not even worth doing to save money as a K & N would take at least 70k to pay for itself, and changing an OE is a piece of piss to begin with. There seems little point to my mind.

 

[bone48]

Thanks for your replies everyone

Shell V power for me where I can.

To be fair I'm not bothered about whether or not a free flowing air filter is the right move, be it performance or financially driven, was just curious as to why it was deemed to make the engine run lean.

I have finished servicing my bike tonight and when the 911 was plugged in I looked at the live information which included incoming air temp, and ambient air pressure, which may well be one bit that the ECU relies on for the map?
The amount of air the engine can consume is governed by the intake system, which as far as I am aware is large enough as standard to supply enough to the engine?
The volume of air required at all RPM is a known quantity due to intake size and cylinder volume, so the only thing that can affect the air is the density/pressure.
As potentially the volume doesn't affect the mixture as this isn't measured, I don't see how a free flow air filter will affect the mixture?




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[Bendy toy]

I got my oils foam Pipercross from the for sale section for the same cost as a standard are filter so I thought why not. Ive used oiled foam filters on Japanese bikes and all was well.

Faster air flow with the same fuel flow can lead to weak mixture. But as you say will it really make "that" much difference. I do however question the size of the intake ports compared to the size of the intake to the filter. Double the diameter and the gas flows goes up by 16x. Yet the intake tube is smaller than either of the throttle bodies.

On that basis, chopping off the intake stub is likely to be far more effective than any fancy filter stuff. If you can live with the noise.

 

[Greenman14]

Also, bear in mind that it's very easy to over-oil a K+N filter then it's much worse than the OEM unit. I know because I've done it and my Firestorm felt like it had lost a cylinder.

 

[Sgt Bilco]

Why would anyone want to fit a K&N air filter in the first place. Don't be seduced by the advertising.

Spot on......they are a pointless add on.

 

[karlp]

K & N 'filters' are a waste of money.

If an engine can run lean with a std filter then air flow is not the problem

 

[Pukmeister]

I had a K&N when living in a dusty environment with sand everywhere, I'd clean and re-oil it regularly. Thats what they were designed for.

Back here in UK with my more modern bike I stick to the OE paper filters (much less hassle).

I believe the ZFE unit has an inbuilt barometric sensor to measure altitude for fuelling corrections, but I could be wrong. As stated earlier, the swept volume of the cylinder X RPM (less flow losses) will be a known variable.

I don't think the knock sensors "fatten up" the fuel mixture as you put it, they simply retard ignition as necessary to prevent detonation (but I could be wrong).

 

[bone48]

Again, thank you everyone for your replies

I am not concerned about whether or not a free flowing air filter is the right move for financial or performance gains, I am curious as to why they are deemed to make the bike run lean. Telling me they are a waste of money and the OE is the best doesn't really satisfy that curiosity, although it is valid information and I thank you for it

Would just like to know the definitive reason why everyone says that free flowing air filters (on injected bikes) make your bike run leaner?


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[Deleted account 190920001]

In short it runs lean because the ecu doesn't know because it doesn't have the sensors to do the job properly.

There is a MAP (mass air pressure) sensor on the ecu for altitude adjustment. This is not in the airbox. Cars will have this but they will also have a MAF (mass airflow sensor) as well. These are large items that sit before the air filter and are usually hot wire. Because of the size they aren't typically used on bikes so they make assumptions in the software. You could use a second MAP sensor in the air box. BMW could use one of these and why they didn't I don't know. It's is minimal cost and the ecu has the input pins to cope with it.

On the way out you have the lambda sensor that only gets used in closed loop/cruse/steady state. Lambda or narrow band sensors are shit but cheap and can only operate in a narrow band of mixture so are only used for emission control. A wide band sensor would be far more useful but are expensive as are the electronics to control them. Obviously this is the primary reason for manufacturers not to use them.

Ultimately it's a cost against expected usage. If 95% of owners are expected not to need something then any manufacturer won't spend the money on it. It's just unfortunate that emission laws get in the way and make an engine lean out the crate. You could argue that BMW refusing to break with tradition and not making the engines water cooled years ago is equally to blame. An out the crate bike benefits from a PCV or remap but of course don't meet emission laws.

Off topic but I think it is only a couple of years away that bikes will be emission tested at the MOT. Once they are all euro 5 compatible but only bikes manufacture from that point and not the older bikes. Once this happens decating and remapping will basically be far more difficult especially the cat.

 

[trainman]

if we get emission's testing for bikes I would think a ban on modifying engines wont be far behind, I think its been talked about before but more around noisy exhausts etc

 

[Deleted account 190920001]

if we get emission's testing for bikes I would think a ban on modifying engines wont be far behind, I think its been talked about before but more around noisy exhausts etc

Will be no different from the car world is at present and has been for a number of years. You just need to be more careful with what your doing. Ultimately they can only measure the exhaust at idle. Proper emission measurement would require a rolling road and the costs/setup/control of that may prove too great.

The exhaust noise limits have come and gone a few times and abandoned. Accuracy was difficult so complaints were upheld all too often. I think it stands now at if it has not for road use stamped on it or is deemed to be excessively loud it's a fail. The stamp gets polished off and nobody wants to stick their neck out and fail on noise unless it really is offensive. The police are much more likely to pull you than an MOT fail.

There may of course come a day where there is a EU blanket ban on changing anything and manufacturers or OEM parts must be used at all times. Some nations are for this like Germany others are bitterly against it because of the devastation it would cause to the aftermarket business.

Anyway, all very off topic. Sorry!