Air filter... again (K&N vs. OEM)

[Kito]

Dear GS'ser

Quick one: Which air filter do you recommend? K&N (washable  - regular fit) vs. OEM?

[mr72]

OEM, if you care about the ability to filter dirt from the air. It's a far more effective filter.

[Kito]

Tks Mr 72!!!! I will applaud you for give me a push on this not so thrilling topic!

What you are saying is that OEM mesh is more closed.. retaining more small abrasive particles... right?


Any thoughts in performance?... would K&N give something more in power?



how many engines wear OEM   and K&N are far from each other?

[qcbaker]

The K&N allows more air into the system, theoretically allowing for more fuel to be added via a rejet, which theoretically results in a gain in power. However, real-world gains are often minimal in a best case scenario, and messing around with getting the AFR right after the filter swap could leave you with more issues than you started with. I would personally stick with the OEM filter to minimize issues.

[Kito]

Nice!

I really believe in you that have no benefits in power increase... however the possibility of reusing the filter is interesting...
My gs is (since I returned a crappy filter to the shop) working pretty good without any filter...  so even with a KN i believe that my carbs will work ok!....

[qcbaker]

Nice!

I really believe in you that have no benefits in power increase... however the possibility of reusing the filter is interesting...
My gs is (since I returned a crappy filter to the shop) working pretty good without any filter...  so even with a KN i believe that my carbs will work ok!....

Filters are relatively inexpensive and they don't need to be changed often enough for me to care too much about re-usability, but thats just my personal opinion. As for running the bike with no filter, its definitely going to be running leaner than normal, so I wouldn't run it like that for prolonged periods of time. If you do end up using the K&N filter, I would strongly recommend rejetting the carbs so you don't burn an exhaust valve or crack a spark plug or something.

[mr72]

The K&N allows more air into the system, theoretically allowing for more fuel to be added via a rejet, which theoretically results in a gain in power.

I don't think this jives with CV carb operational theory, but it's a pretty common misconception.

The K&N filter has less resistance to airflow so the air velocity is slowed, vacuum reduced, needle position lower at a given throttle position/load/rpm, resulting in a need for bigger jets to get the same amount of fuel into the slower-flowing air as you would have had with the regular air filter. You are not actually effectively adding more fuel or more air. The quantity of air (and fuel) is a result of demand from the engine due to piston pressure and displacement. Which is why...

real-world gains are often minimal in a best case scenario, and messing around with getting the AFR right after the filter swap could leave you with more issues than you started with. I would personally stick with the OEM filter to minimize issues.

Bingo. Changing the air velocity throws a wrench into the pretty carefully balanced carburetor design on a GS500 and often has unanticipated negative consequences.

But it's not "more air + more fuel = more power". It's "slower air = lower slide = whole thing gets screwed up"

[Kito]

you guys are sharp on this friday !  making me remember Pai Mei (of carbs)

tks

[user11235813]

Even if you were going to get a K & N (who by the way are unscrupulous lying asshats) wouldn't you be better off, seeing as the OEM filter already has a nice metal casing, simply remove the paper and replace with with some foam you can buy yourself  and oil it up.

I recently bought a block of specialise foam that's made for this purpose and some proper filter oil. Not for the GS but for the S40, although I have not gotten around to installing it yet.

[qcbaker]

I don't think this jives with CV carb operational theory, but it's a pretty common misconception.

The K&N filter has less resistance to airflow so the air velocity is slowed, vacuum reduced, needle position lower at a given throttle position/load/rpm, resulting in a need for bigger jets to get the same amount of fuel into the slower-flowing air as you would have had with the regular air filter. You are not actually effectively adding more fuel or more air. The quantity of air (and fuel) is a result of demand from the engine due to piston pressure and displacement.

Thinking about it, you're right. But now I'm a little confused about where the "gains" are supposed to come from with filter swaps like this. I'm probably wrong, but it seems like lowering the resistance at the air intake, even with a rejet to accompany it, wouldn't give you additional benefit that a rejet alone wouldn't also give.

[mr72]

Thinking about it, you're right. But now I'm a little confused about where the "gains" are supposed to come from with filter swaps like this. I'm probably wrong, but it seems like lowering the resistance at the air intake, even with a rejet to accompany it, wouldn't give you additional benefit that a rejet alone wouldn't also give.

There would only be "gains" if the engine was being restrained by inadequate airflow to begin with. Those gains would happen at high rpms since that's when the maximum demand on airflow exists, and it's the head/valves that limits airflow at the top end for a GS. So, in theory, there are likely no gains except maybe noise, unless it's simply a result of richer jetting all around.

Stock (US) main jet on a Mk1 carb GS500 is 122.5, which is probably quite lean. If you swap the air filter and switch to, say, 135 jets (IDK the correct jetting for a K&N drop in), then you likely wind up making the bike more rich at the top end while at the same time affecting airflow. But you probably would have gotten exactly the same gain in power if you'd left the stock paper filter and rejet to 125 or 127.5.

[on a side note, the air filter in my Triumph is maybe 25% larger in surface area than a GS air filter, this is on a DOHC/8V twin with almost 400cc more displacement and nearly double the power, which anecdotally suggests the GS OEM air filter is enough]

[herennow]

What happens when you use a more or less restrictive air filter on a motorcycle?  You either increase or decrease the resistance to air flow. A more restrictive filter, through the greater resistance, creates a greater pressure differential between the front and the back of the filter. This pressure differential obviously has a relation to the volume of air flowing on each side as per Boyle's law. (P1V1=P2V2).

Using a more restrictive filter is, therefore, the same as going up in altitude. You end up with "thinner air" or less oxygen for a given volume. This leads to lower dynamic pressure in the cylinder and hence less power.  The free flow filters work the same way, less restriction equals less pressure differential and hence "more dense" air after the filter. The greater density leads to higher pressure in the chamber and greater power.
CV carbs are fiendishly complicated and they are designed to take pressure variations into account through the diaphragms operation. I'd love to buy a CV carb designer a beer or 6. Hell, I'd even take him to a Karaoke bar if it would make him open up!!!!

Here's the kicker, variations in altitude (or in filter resistance, a close proxy) have no effect on carb mixture, except at idle or at WOT. The reason for this is that at these extremes the carb does not rely on the diaphragm and its pressure differentials to control fuelling.  At idle the diaphragm is yet to lift and at WOT it is fully open.
In between these two extremes, the pressure differential between the carb mouth holes and the slide vacuum holes is always the same for a given set of engine operating conditions (RPM/Load) regardless of exterior atmospheric or airbox pressure and takes into account the density of the air. This gives good enough fuelling, not perfect, but very close to it in virtually all cases. However, at idle and at WOT you will have a leaner mixture with a free-flowing filter.

Some info re the altitude point:
https://www.motorcycleproject.com/text/carbs_altitude.html

Note the vacuum in the manifold has an important effect on air volume, a 1/4 open slide might have a 20 kpa vacuum in the manifold. At 20 kpa pressure lower than atmospheric, air has 1.2 times more volume than at atmospheric.

[Kito]

 

this thread is getting awesome!


I was googling to see the behavior of the pressure, flow and area... to see if it was linear... but I will stick with your considerations!



However,   I still not decided yet.. which will be the winner filter for me....