The recommendation to "pull the snorkel" comes up all the time in various moto forums, most of the time such advice is not backed up by anything but same unsubstantiated recommendation read somewhere else in the interwebs.

A tight airbox with a "snorkel" type air inlet tube is a Helmoltz resonator and at its resonant frequency (and to lesser extent at the harmonics of the resonant frequency) it can boost the engine power by 10% to 15%.
(Kevin Cameron; Sportbike Performance Handbook, page 50)

Well, I took a KLX300 airbox and snorkel and took some key measurements of them to calculate the resonant frequency, and the corresponding RPM, of said airbox.
Airbox volume: (including the "air boot" up to carb inlet level) 4.4 liters
Air inlet cross section: 7.5 cm^2
Air inlet length: 18.5 cm

Using those numbers I calculated the resonant frequency of the KLX300 airbox as 165Hz which translates to 4971 RPM. Wouldn't be too far fetched to guess that engineers at KHI set the target at 5000RPM to fill the unavoidable mid-RPM torque dip.

On the other hand, at of 8000 RPM if we assume an unrealistic 100% volumetric efficiency the mean air velocity at the air inlet (snorkel) is only 22.3m/s. That is mere 6.5% of the speed of sound and as such causes only negligible head loss at that air speed.

For the fun of it took also the popular KDX snorkel and measured that one.
That snorkel pushes the calculated resonance frequency to 332Hz which translates to just about 10.000RPM for a FOUR STORKE ENGINE.
In the original 2-smoke the first resonance frequency again corresponds to 5000RPM. Pretty cool, huh?

Until someone produces a pair of dyno charts with same motor, same day, same dyno, showing more torque from anything else than than stock airbox + stock KLX snorkel, I'm not going to remove mine.
The theory and every relevant experiment so far shows that to the give best engine performance at mid and high RPM's on these KLX bikes.

--
Mikko

 

I know nothing about your math. I pulled the snorkel to see what it did. I was hearing popping and have a flat spot at 5,000 to 6,000 rpm. So flat that sometimes I get an actual misfire. I have inspected the spark plug. I ran the bike that way for a while. Air noise was much louder, so I put the snorkel back in. Power difference was not noticeable by my seat of the pants dyno. Once I re jet the carb, I will try again. If the popping and misfire go away with snrokel in, I will probably leave it that way.

Guys that ride V twins seem to think more noise is more power.

The only way I could tell is with a top speed run and that would tell me nothing about 5,000 to 6,000 rpm.

09 KLX250SF Box Stock

 

Interesting theory. You don't need a dyno, though, you can use a stopwatch on a given course, or, to reach a given indicated speed. Dead stop to 60 mph, or roll-on from 30-50 in 4th gear, etc.

 

FlyingFinn, you sound like you know what you're talking about so I'm wondering how your equation might be affected if the engine had a larger header pipe and high flow exhaust along with re-jetting the carb? Seems like the air flow restriction is reportedly the greatest at the header so if that opens up, the airbox/snorkle may become more restictive respectively. Any thoughts?

 

Well, wouldn't your result indicate that the jetting was lean after removal of the snorkel, therefore indicating more air was entering the airbox and therefore your engine? Without richer jetting to match the increase in air, the popping and the flat spot would seem logical. I would agree with your decision to leave the snorkel on if you're not going to rejet to match the air intake.

The airbox debate is always an interesting one, but it's frought with variables that are often overlooked and/or don't apply to the KLX...ram air, dead air space volume, CV carb slide disturbance, etc, etc, etc. There is also the principle of the internal combusion engine just being a big pump, and while that's basically true, engineering manipulation by many means...turbo, supercharger, variable cam timing, etc...will alter the efficiency of an engine. Generally the more air an engine can take into the cylinder and mix with a proper amount of fuel, the more power the engine can produce. But again, variables of all kinds can influence that result too...timing curve, compression, combustion chamber shape, etc. The KLX relies on fairly simple and proven engine technology without many creative engineering tricks. I would tend to bet on more air and more fuel delivered in the correct manner to result in more power. The biggest hurdle for most KLX owners seems to be matching the fuel and air ratio to achieve that performance.

Finn, you make a good point that without a dyno comparison or a flow bench test, a person cannot make an absolute claim of increased performance on a KLX by removing the snorkel or airbox lid...but...you certainly can't prove that it doesn't by just using a calculator. There are lots of scenarios that we know to result in a benefit that we can't qualify without extensive testing and comparisons. I think the KLX benefits from the conventional more-air-more-fuel concept because it's a very conventional internal combustion engine. The stumbling block for many can come from their efforts to match the fuel delivery to that additional air. The snorkel design on the KLX is surely a sound control device to meet decibel requirements on a street vehicle...at least as its primary function...while also functioning as a water/debris diversion as a secordary function. That's my story, and I'm sticking to it.

 

ohhh boy!!. Did a can of worms just gotten re-opened with this. While your airbox math seems sound. In reality, in may not work out. There is a lot of different mechanics going on. In my opinion, what math may occur before the carb/tb is not so important as the math after the air has entered the carb/tb. Keep the intake open, so the engine can breath easier. And change the dimesions of the manifold to best fit the required engine performance.

 

Great Job on the "math" aside of the performance calculations the airbox design also takes into consideration niose emissions. Not saying that many have not made improvements to the bikes, but on the other hand, many do not understand many of the systems and why they are designed for a reason / purpose.

 

I did the usual mods, bigger snorkel, header and muff, jet kit, etc.

Bike ran great after the intall, more power and a lot more snappier.

One day I was bored and pulled out the kdx200 big snorkel and stuck in the stocker. It resulted in a huge loss of power, the bike couldn't get out of its own way. It slowed the revs way down.

Again, this is just me driving it. I don't have any math or science to back up my results, but you could notice a huge difference.

 

I'm no motorist, yet I think there's much more to it than a simple Helmoltz resonator that Mikko mentions. What I see at first is that the airbox acts as an acoustic short circuit for the vacuum wave that is created at the cylinder head at the start of intake, resulting in resonance with attendant overpressure at end of intake at about 7000 rpm. That's the reason the torque curve peaks at that rpm, I believe.

The air box does not act as an ideal infinite reservoir of air, so there are secondary resonances in that space, and in the snorkel, that can affect the intake process. I don't think it influences the final amount of air inducted to a great extent. But, it can influence significantly the amount of fuel delivered by the carb. I predict that if one could adjust the jetting at will to maintain the same A/F ratio at all rpms, there would be no noticeable effect of snorkel and backfire screen on the output power.

At least this is what my testing revealed when working on my KLX conversion. I went down to having only 1/4 of the lid opening left, while still having no significant change to A/F ratio relative to full open. Acceleration times also did not change, within a few percent (30-100 km/h, 3rd gear tests).

It's definitively a shame that I did not have access to a dyno when performing my FI conversion, but I expect this will be remedied next spring.

 

This thread is giving my head an inside oowee; due to lack of exercise I suppose ...