After reading a story about a car that gained hp on dyno day by going from a 7" diameter K&N filter to a 14" diameter filter, I remembered that many large cc bikes have a panel filter on top of the air box rather than inside/up against the air box outlet, and thought it might be a plus to use the volume of my 250's air box in a better manner?

Soo I picked up a $4 paper filter made for another bike, cut some of the extra flange off of it on the right side, and modified the KLX air box lid I wasn't using anyway to accommodate the filter's case.

Looks almost factory! (squint)





This filter then will replace the K&N that was inside the air box. I've already duct taped over the holes I drilled my air box; I can use them again when I get the hole plugs with built in foam filters. I sealed the drain and emissions outlets, and this paper filter will now be the only filter and source of air.

Since the air filter is the restriction in the system, it should be a good thing to have a large volume of filtered air already inside the air box waiting for the next intake cycle. A volume much larger than 250cc btw so there will be clean air to spare!

I'll probably hold this filter in place with short screws in the lid/nuts on the top side; then be able to place the air filter frame on to the 'studs' via drilled mounting holes aprx where you see the blue dots around the filter's perimeter.



The filter's plastic casing is an interference fit in the lid and just needs to be held in place. I could always add a little caulk around it if I decide to leave it like this.

This was an easy $4 thing to try. The harder part will be testing to determine its benefit, but I'm expecting good things.

 

Don't forget - 4 cylinder litre displacement is simply 4 250cc cylinders drawing in alternating patterns.

It will filter fine once sealed up. Only question will be if you actually need extra air flow capacity. There is a point where there is no gain. if there wasn't you would see HUGE air filters on cars.

You will see huge filters on the off road trucks and cars, but that is to maintain flow as the filters start to clog with dirt/dust.

If it was possible to test, the key is just how much air the carb body itself can flow and then the actual head. The biggest air filter made won't flow any more air than the most restrictive portion of the intake tract. And if you actually do some reading on it, you will find there can be the case where too free of flow works both ways, the velocity drops on the flow and the loss of ram effect makes for less charge into the cylinder. It's all about balance and efficiency of cylinder filling.

This is highly evident in automotive intake/head/header design and has far more published about it than anything with bikes. The media is constantly testing out heads, manifolds, and header designs. Part of the reason I read them a lot. They actually DO something with the engine, not just test cars. My biggest complaint about the motorcycle industry. Imagine if they actually did a build on a KLX250 like Gordon Jennings did with the SR500 in 1979. He did carb, cam, pipe changes with dyno runs to verify what was happening to the power.

I will say for a road bike that kind of filter set up - in the cover - would be sooooo much easier to deal with than that stinking foam derby that is jammed in too small a space. Honestly that stinking derby has more surface area than the entire top of the air box minus lid. Fact is that filter is larger than the one in my KLX650.

 

"If I actually do some reading on it?" Really?

Sorry I just can't recall articles where the test had to back up a few steps because they'd provided too much air to the carb.


Mr Jennings hisself was known to put long hours and money into ideas that had no assurance of bearing fruit. If you lament the m/c mags don't experiment enough, then any KF member experimentation should be a good thing.


- I did accidentally just find and read a non-Kawasaki m/c forum story about putting 41mm FCRs w/ velocity stacks on some 900cc BMW adventure bike I think it was. Forum members there btw are all positive and happy to have someone putting in some effort towards something untried. No dire warnings or repetitious ramblings of theory, just anticipation. Lots of fabricating etc and many pages of incremental progress, but HP and torque were up afterwards.

Adding choke circuit(s) to the FCRs that come w/o choke will add cost to the mod. Even so, still there was just cyber applause from assorted forum members worldwide.

...

 

My apologies for commenting. I will no more.

 

That's an interesting idea. Kind of makes sense though. I look forward to hearing how it affects performance!

 

Did you increase the size of the tube that goes from the air box to the carb?

If not, I'm not quite following where any benefits would be had. Once you remove the airbox lid, the shape and location of the filer is inconsequential, as long as you are already flowing as much air as downstream will allow.

To put it another way, the size of the water tank has no impact on water flow once you've reached the maximum capacity of flow that the pipe from the tank will allow.

I'm hoping you did a dyno run first, it would be interesting to compare power curves before and after. My guess is there would be little to no change, but it would be interesting to see.

 

There is some merit to what you're saying...if...the size of the airbox boot is at its limit to achieve maximum flow that matches the airbox intake hole. That's not usually the case with most carb or FI setups out there. While the boot at the throttle body or carb is usually about the size of the carb or TB bore, the intake benefits from being able to draw a large volume of air because the engine is a pump drawing the air into/through the carb or TB. Your water tank example is relying on gravity flow, isn't it? You stick a pump inline somewhere in that setup, and you can just about blow paint off a wall. If I'm understanding your description. This is why you see huge fabric or other filter medium style filters in use in performance applications often in concert with a large airbox and equally large or multiple openings to gain more intake hole volume. Or again, maybe I'm missing an element in your description.

 

Everyone seems keen to increase flow to/from the head and I can understand this for max power. But what about throttle response and low/mid range power? The extra flow could easily make things worse lower down.

I guess it depends what you are after, and it's always interesting to read about people's experiments and results.

 

The example given would have the force of the weight of any water above, based on gravity, so there is actually pressure involved that will push the fluid. Funny part about fluids and flow is when turbulence comes into play and all containers and conduits have some surface turbulence that slows flow. The one thing that really freaked me out in the Fluid Dynamics class was the thought that you could have 100psi at the spigot hooked to a hose long enough that the internal turbulence could actually STOP any flow! The length would require more than 100 psi to overcome all of the surface turbulence restriction.

I actually saw that in play when working at Gradall. They had a heater flow problem within the cab defrosting system. It was running 2-1/2" plastic flex tubing from a heater under the operator seat to the dash board area along side the cab. The problem was farmed out to a flow specialist. Their results found that by the time the air had passed through the few feet of tubing within the cab at the pressure there was, the area of active (live) flow was about the size of a dime! They needed to go to smooth tubing and manifold to maintain the air flow from the heater under the operator seat. Incredible how fluids are affected. Of course this is similar to why smooth bores are so much better than the standard carbs and why any intake/exhaust tract should be as smooth and seamless as possible.

One thing that comes into play in air intake that shows up now in well tuned large volume air boxes (not on the KLX and other bikes with cramped space) is the shaping leading to inlets. The large air box presents a lot of air, clearly as much as the space allows to provide more than enough air for an engine. The area around the carbs/injectors will be contoured for the best flow characteristics.

None of this is present on most off roaders dues to space requirements for the shock and the required narrowness. But I think you guys know that. So the best thing that can be done with a bike like the KLX is to try to figure out how to smooth the inlet if possible. The trick is to end turbulence. If the air filter was relocated to the lid, this would allow the old filter mount "spider" frame to be cut out. The old filter mount area has a lot of room for improvement to smooth the flow into the intake. It could be shaped with epoxy and grinding to try to make it a bit more bell mouthed and smooth. Same effect as the bell mouth of the carb. As air is sucked in by the engine, the velocity will increase as the air is drawn into the venturi, both at the carb and at the air boot. Flow will drop, but as long as the system can provide more air than needed, the velocity is more important. If the flow capability is too high and velocity too low the rising piston during the intake phase will push the mix out.

At the right balance the mix velocity will create a ram effect that will not only hold mix in the cylinder, but can actually pack more in. Lots written on it in porting articles and something the MotoMan runs on about incessantly - but he's backed up by a number of reputable sources. Fact is that is what has led me to taking a more serious look at the exhaust set up for the bike. Flow does have to go two ways and that same velocity in the exhaust helps extract the exhaust, but then stop up flow. In other words, there is a flow limit in exhausts as well, otherwise we'd be running downspout size pipes!

What would be really interesting would be if someone had access to a flow bench to hook up the air box and inlet tube (to the carb) and run it up to see how much it actually has the capability to flow. Then figure out what the actual cylinder filling requirement is at say 10,000 rpm or more (above the common hp peak - significantly). If the air box flows more than enough there is pretty much nothing to gain in the air box except ease of maintenance - which I am absolutely all in favor of.

 

Though the paper filter in the air box was much larger than any snorkel or empty snorkel hole, it was still too small to flow as well as my box had been flowing with no lid and aprx 7 holes already drilled in it.

Throttle response was way up due to increased velocity I guess, but I would have had to drop down to a 132.5 mj or less as it was too rich at WOT. I don't think it would have been as fast with the jetting adjusted to match the paper filter's abilities. It sounded down on power and the pitch was higher.

I have other options coming up for increasing velocity anyway.

I wanted to try it without the seat on to check for flow restrictions to the air box but even if that proved to be true-the only option would still be to open up the holes I'd taped over anyway, and I'd have been sitting right on top of the paper filter..

No matter how I look at it, my best bet is no lid and 7 open holes in the air box which seems to have been proved out by this test using this larger than all other lid options paper filter which was choking off power.

I have some future plans to cut out most of the lid and use the perimeter of the lid bolted in place to hold down a sheet of Uni filter bulk material cut a bit larger than the air box opening, AND incorporate more BF1 or BF6 Uni filter material elsewhere in the air box in an effort to be able to remove the K&N bolted up to the air box outlet ala superbike, but like all good things I have to wait a few months till the box with those supplies shows up.