Maths Help!

[A.FreeMan]

OK so it's not strictly a wokrshop issue, bt it could lead to one. I am trying to work out an equation to relate resonant frequency to the volume of the airbox, the length and area of inlet pipes.

I used this guys method:
https://www.calsci.com/motorcycleinfo/Airboxes.html

Can someone just please check that he is the moron, and not me. He has got his value for atmospheric pressure wrong right? By a factor of 10. PLUS he makes disregards too many terms (for being negligible i presume).

anyone know anywhere I can find a correct derivation for this equation. I know it is universally appreciated that the rpm is proportional to the three properties A,V and L.


Thanks

[stinkwheel]

He's using odd units.

Don't trust someone using non-SI units for physics calculations. It's hard enough as-is. Centimetres have no business being anywhere near it.

Pressure is measured in pascals (netwons per square metre). Mass is measured in Newtons. Volume is in litres. Distances, areas and volumes in metres. Especially when dealing with pressures and volumes. Convert everything to SI units first, THEN do the maths. If you come up with an unmanageably long number, convert it to some other unit at the end of the calculation, not forgetting to cube or square the conversion as appropriate. To do otherwise is to incite madness.

Atmospheric pressure is 101KPa 101,000 pascals. The units he's used, while unfamiliar to me, appear to be an equivalent to a KPa.

This is what happens when 'muricans use metric.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[MCN]

3.78541 litres in a Gallon gives one clue.
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Disclaimer: The comments above may be predicted text and not necessarily the opinion of MCN.

[A.FreeMan]

[stinkwheel]

[spottedtango]

[stinkwheel]

[talkToTheHat]

The whole thing reads like a set of assumptions made by someone who never studied fluid dynamics or even enough engineering to use consistent units.

He talks about volumetric flow estimates of a compressible gas. He doesn't account for flow restrictions in the throttle body. His dimensions seem off, or the vstrom has a snorkel eqivalent to an 8cm radius (thats 16cm or six inch diameter). He doesn't account for the shape of the airbox when talking about resonance. Also, the intake cycles from each cylinder on a v-twin are not equally spaced so all kinds of crazy things happen.

It reads like an essay written by someone who read the intro notes to a proper book and skipped over the hard bits.

I'd say it's a problem for fluid dynamics software to model accurately. Note orientation of snorkel to vehicle motion, variable length plenums and water content of the air all play a part. It's hard. I didn't like fluid dynamics. I'd rather be writing the code to solve the problem then thinking about resonant gas flow.

Start with expantion chambers. They are easier.

Google helmholtz resonance. That's where the maths is coming from. It's going to get you reasonably close. Just work through it in SI units, there is a good reason everyone is shouting this.

Peak flow at the 3500ish rpm torque flat spot? The one usually attributed to emissions and noise test requirements? Skeptical. More likely tuned for peak airflow somewhere up around where peak torque and peak power are. Note how huge the airboxes are on bikes that rev to the moon.
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Bandit. does. everything.