Aftermarket Airfilters

Testing of aftermarket airfilters on the RC51 has yielded poor results. Typically there are no real world benefits to aftermarket filters for the RC51 as small hp gains can be obtained on topend, but only with a trade off for mid-range hp.  Truth be told almost every sportbike in the last 20 years has had losses in power at some point in the rev range or even throughout it from the addition of K&N, EMGO, BMC filters etc... There are exceptions to that rule, but I will save delving into those anomalies for a different time and place.

Since this article was originally written we've also had multiple issues with drivability on the Honda 919 that resulted in small stumbles and hesitations at lower rpms. None of these were able to be ironed out with mapping changes, but simply swapping out the K&N filter & reinstalling the OEM air filter instantly fixed the problem. I don't think much else needs to be said on that...

Interesting info #1: Flow testing on the stock filters performed by Brian Sheridan of Sheridan Racing Design proved to us that the OEM filters are more than capable of flowing more air than the engine can use

Interesting info #2: Dyno Testing on the BMC filters showed a definite decrease in topend horsepower with only a very slight gain in the mid-range. dyno charts from testing on 02/23/02

Note: when testing new high flow filters it is imperative to allow them to be broke-in properly by getting some miles on the bike at normal riding speeds. The oil applied from the factory can be initially too thick & can cause flow problems which can mis-lead dyno results in either direction. You may initially get good dyno results, but when the oil dissipates & the flow increases your dyno results may worsen or vice versa. Simply put with the extra oil in the filter you might get a good or bad initial dyno run, but after the oil is thinned out the dyno results may change (& usually do) just depends on the bikes flow characteristics

Additional common dyno testing discrepancies are discussed here dyno tuning basics

Back in the old days all the bikes benefited from slapping some individual pod filters on & adding some larger fuel jets...  Things have changed greatly in the last couple decades whereas it's not uncommon for modern custom jet kits to use smaller main jets with slimmer needles etc... to gain more horsepower & a more linear power delivery instead of just following the old cliché of bigger is better. Fuel Injection has made that process even more convoluted as it allows the laws of physics that Constant Velocity carbs must abide by to be thrown right out the window in favor of any combination of Air/Fuel ratios at any point in the rev range or throttle position or any other sensor in the system for that matter an engineer wants to obtain.

Since originally posting this page I have received hundreds of further inquiries about testing methods involving ram-air & how do I know the aftermarket filters don't work etc... Well there are more ways than just dyno testing to get results. Utilizing an Air/Fuel meter while riding in real world conditions will reveal alot of what your bike is actually doing, but even then not everything may be as it seems, sometimes a rich condition will be reported when the reason is that too much air inside the airbox is causing turbulence & stalling the airflow etc...

Some airfilters do work on certain bikes & some don't affect performance at all, but most aren't worth a damn for the overall performance advantage.  The OEM filters are tuned to a specific resonance inside the airbox & once you go altering that you get various results.  You might get better top end, but poor low end or vice-versa or you might get better performance with a bad stumble that lasts only for only a 200 rpm range or you may simply get a dip in power at some point.  The combination of results is almost infinite.

Ultimately if the aftermarket air filter introduces turbulence into the airbox and causes a disruption in air flow or a harmonic problem that results in an issue with fuel atomization then all sorts of little problems can occur.  Something as small as a little hiccup at a specific rpm or even fuel that doesn't atomize and burn evenly in the cylinder that can cause piston slap and liner wear problems due to the uneven flame propagation. Strange things can happen when you start messing with induction variables. Many of these things can happen and the rider is 100% totally oblivious to it.

The main thing to remember is that on streetbikes you need drivability.  If you were to put a graduated scale on your throttle housing & make a mark on your throttle grip you would find that you rarely exceed 10% throttle while riding around, yup no crap, 10%! Even when aggressively leaving stoplights & such you really don't open up the throttle all that much.  So you need to have your bike as responsive as it can be at low throttle positions & in the mid-range of the bike where you ride the most.   Way too much emphasis is put on peak hp when in fact you rarely ride around at redline with the throttle pinned to the stop...  It's all about better mid-range performance & that is coincidently where the high flow filters usually hurt performance.


Honda's are definitely more applicable to this issue than the other manufacturers. It would appear that Honda spends more R&D time than any of the other manufacturers to insure that the intake system on their bikes is finely matched to the flow characteristics of the motor.  The airbox is perfectly tuned to accept x amount of air thru the filters & deliver it to the carbs or throttle bodies in a metered volume & every thing works perfectly in sync in the state of tune they must be in to pass EPA Emissions testing.   Typically an alteration of the filter leads to too much turbulent air entering the airbox & usually ends up either leaning out the motor or stalling the airflow altogether.

It some cases a larger aftermarket airbox can be more beneficial for creating additional horsepower or even assisting in better mid-range throttle response, but this has not worked in favor of the RC51 either at least not with a stock engine as can be
seen here. In some cases a larger airbox would make a larger amount of air "available" for the motor to use as opposed to a greater volume of air that the engine would be "forced" to use if you went with hi flow filters or filterless. What you don't want to end up doing is putting more air into the airbox than the engine can mix with the fuel. If that happens then you have screwed up your air/fuel ratio & you will lose power.

I'm not going to outright say that there is nothing to be gained from aftermarket filters, but I will say that most top-notch tuners will sway you a different direction especially if you are riding a streetbike as opposed to a racebike. Citing drivability issues as the number one concern.

Another point routinely thrown in my face is that the aftermarket filters are "re-usuable" Which simply put means that not only are you spending $80-$100 for a filter, but you also now have to purchase an additional cleaning kit so that you can wash, dry & oil your expensive filter, but in less time that it takes you to just wash the re-usable filter I can remove my old OEM filter, throw it away, install a new one & be off riding... If you are one of those people that are going to argue the cost savings of aftermarket vs OEM filters then you are into the wrong sport as nothing about a sportbike is cheap, not the cost of the bikes, insurance or maintenance period & you should be prepared to pay a few bucks every 4000-8000 miles for a filter. Another important issue is that these fabric media filters have been independently proven by many sources to allow way more dirt and debris into the engine that paper filter elements. On a racebike maybe that is worth the risk, on a streetbike it is not.

Let's take a different look at it. If K&N filters really were the instant bolt-on horsepower that they are claimed to be in those ridiculous tv commercials then why doesn't K&N simply put a picture of the dyno chart showing the gains over the the stock power curve right on the box for every air filter they make??? Could you think of a better marketing strategy proving to the public that the filters actually work? Unfortunately they don't always make horsepower and sometimes decrease performance so obviously this simple tactic won't work... Caveat Emptor

In the end one thing is for certain if you are going to use aftermarket filters & get any appreciable gains from them it's going to require a Powercommader, some dyno time & probably some creative airbox mods too, maybe even to the point of fabricating custom bellmouths to get it dialed in correctly & you must also consider that in some cases there are no gains to be had & in many a loss of performance or drivability can occur! Do not blindly believe the hype that people on the web forums making claims of how much horsepower they got with their air filter installation. If they did not do a COMPLETE dyno tune & test before the installation then claim they got x amount of horsepower after they installed an aftermarket air filter & then did a dyno tune they simply have no way of knowing what their gains or losses were from the air filter itself. Very few people go through the trouble of properly testing their parts to get apples to apples comparisons like I have over the years because everybody just wants to make themselves feel good about buying a $100 air filter so they will pretend the first dyno chart they see with a good number on it is golden to them when it truth it doesn't mean squat. It is very reminiscent of the 10 year long airbox debate where the airbox actually loses performance and barely gets the bike back to stock numbers after doing a full tune, but everybody that owns one claims it helped the performance... Link to airbox testing page


BTW because someone always brings up the fact that they use K&N filters in their car or truck & they do work under those conditions I must add that I too use K&N filters in my vehicles & they do show definite increases in power & throttle response, but I have also had problems with my Mass Airflow Sensors getting dirty and I can tell you from years of testing K&N and other high flow filters that they do indeed let a lot of debris past the filter media that an OEM type paper filter would catch. Additionally the oil from the K&N filters always gets shed off and mixed with that debris making a sticky mess. Back to the point though cars are different from current bikes in that cars have a closed loop Fi system that can compensate for the air flow variance whereas most bikes use an open loop Fi system that cannot automatically compensate. Adding supporting evidence to my theories of airbox turbulence you will be interested to know that K&N has developed many different types of turbulence diffuser inserts for many different makes of cars & trucks that lessen intake turbulence when using their filters to increase performance. So far nobody has developed a way to even test for turbulence on motorcycle induction systems let alone cure it.

 
 
 

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