Air box design questions

[SpookDog]

@ninja
I don’t know enough to form an opinion, but I’d look towards the design on bikes like YZ Rm’s, ect. Bikes that are fully tuned with no eye towards restriction. They probably chucked about as many Yen as possible into R&D

[Calum]

@spookdog The RS 125 was raced competitively alongside the Honda RS125 in the Superteen GP Series.

So disregard the fact it was a learner legal 125, these things were well spec'd.

[Stevie Wonder]

Something I learned from her and then from a couple of the 2t R&D groups

Velocity and Density.
Your air box is more then a reservoir, if you want an example of that then look at what the YIES does.
Imagine your air box not as an active space doing stuff but a channel between the areas that make a difference to the stuff that’s happening.

I feel like I can kinda get where your friend is coming from on the idea that you need to have a consistent air flow or a “settled air” but the principles with air and engines seem pretty universal regardless of what it is

[NINJA]

AAAHHH!! So I'm guessing that the snorkel acts as a funnel to push air into the box, which creates a sort of vacuum for air to be rammed in at a higher velocity then? With the actual air box acting as a reservoir, or volume of air for the carb to suck air in from? Does that about sum it up?

And does this make the air more denser or not then? Or is air just plain ole air???

I found this really interesting thread for more info;

https://www.125ccsportsbikes.com/forums/index.php?showtopic=104889

[Glynn123]

Cut the lid off and send it

[Calum]

I'm not terribly convinced. I long ago ditched the airbox on my bikes, My RS is fairly rapid.

Airbox is good on MX to avoid getting dirt/grit into/near the filter.

For a road going daily it makes sense to avoid the filter getting wet on your daily commute.

But it terms of real world performance? I doubt it's much if any. In fact, probably better gains to be had binning the airbox in favour for a pod filter just in terms of weight savings alone

I have no facts to base this on. For me it just comes down to if I really wanted more power. go bigger on displacement.

For me it's just a convenience factor. It's easier to service and rejet on a pod filter when compared to an airbox. It also allows easier access to the important things when it comes to servicing too.

The portgual lot aren't bothered with airboxes and their bikes are far faster than ours.

My guess is that due to their better climate, they don't have to put up with the crappy weather we get here.

BARacing

[NINJA]

@calum It's all very interesting stuff for-sure, thermo-dynamics obviously play a big part in performance. Along with air quality, density and temperatures as well. My friend posted this reply which is all very interesting;

"I do believe that air box volume in the still air portion is important to lessen engine acceleration issues from inadequate immediate airflow across the filter. Books written on the subject indicate suggested air box volumes needed.
Resonant tuning a two stroke, and even the four strokes brings the discussion of fuel stand off in the carb air inlet bell and extending even some short distance up the air boot. Fuel stand off is a function of resonance. The engine pulses generate a reverse wave through the carb and since not all fuel is consumed on each intake function, the resonant wave tends to carry atomized fuel back to the waves break point. The break point is a function of throttle opening, engine intake timing, and the mass of the air being moved.

On a steady throttle operation, such as the examples posted earlier, a known setup can be made and tested to optimize resonance and fuel stand off. At low throttle settings, and changing throttle settings, resonance seems less important.

I do consider the rubber air boot between the carb inlet airbell and the air box important. Not in a sense to gain a resonance performance gain, but rather to minimize premix spray into the still air box."

[SpookDog]

The only thing I know for sure is that the air box is a giant pain in the ass to fit and remove. If I was working regularly on the engine I’d use a pod...

[DTR+NSR]

Ninja's friend is very much right, typically the bigger the air box the better. But a manufacturer can tweak the volume of the air box, to change its resonance, to help fill a flat spot in the rpm range. To make a engine more flexible, not something that important on a engine that will be running a full tilt most the time.

Most 'ram air' intakes are just cold air feeds, I believe the Yamaha thundercat was one of the first bikes to run a true ram air setup. And that is far more than just sealed air feeds to the airbox. It has various air chambers under the fairings, and even uses the air to pressurise the float bowls on the carbs to increase fueling to match. And with all that, it's claimed to only add 3hp at over 100mph, and that's on a near 100bhp engine.

Unless your trying to chase every last 0.5hp, I think your best off with a air box, especially on the street.

[NINJA]

@dtr-nsr Thanks for the input guys. It's a very interesting topic, which will have many viewpoints and will surely be a hot debatable one for many years to come!!!