Helmholtz resonance question.

[RomanV]

Okay, well I understand the concept of H. resonance.

However, I was wondering how/if this worked, if you have open throttles?

I would have thought that in order for the pressure wave to be reflected back down into the intake runner, it would need an opposing surface to reflect off. (Back of the plenum)
If this were the case, then obviously exposed throttles wouldnt work all too well. (Although, tuning intake runner length would still help with assisting intake velocity?)


If Helmholtz resonance does indeed rely on a 'back wall' to work, then I have an (impractical) idea that I've been thinking about.
You could have a movable plate inside the plenum, opposite the intake runners, that becomes further away/closer depending on RPM, optimising the length.

thoughts?
(Apart from the impracticality of it all of course)

[Vertigo]

im not an expert on this particular subject, but i am doing a physics degree, for what thats worth....

you need to remember that air is an object, maybe not as dense as say, a block of metal, but it will resist things, even more air. so i think in effect, the air outside the tubes will give some resistance against the air bouncing off the valves.

[suberimakuri]

check out gen 2 3sge inlet manifold....... that has what you're talking about.

[RomanV]

Yeah, I've seen the ACIS system, I've got the remnants of a gen 2 3sge scattered around my garage.

However, the ACIS merely seems to increase the volume of air in the intake runners/plenum, rather than effectively increase/decrease the length that the reflected wave has to travel... Or does it?

But then again, I didnt have much of a good look at it before I gave it away with the head.

[suberimakuri]

pass. probably just volume increase.

[RedMist]

Helmholtz itself requires nothing to reflect off. However helmholtz isnt the be all and end all of intake tuning. There are gains to be made by reflecting some resonance off a back wall.

This isn't a bad explanation

Qualitative Explanation
When additional air is forced into a cavity, the pressure inside increases. Once the external force that caused the air to be forced in disappears, the higher-pressure air inside will tend to flow out through the entrance. However, this surge of air flowing out will tend to over-compensate, and the cavity will be left at a pressure slightly lower than the outside, causing air to be drawn back in. This process repeats with the magnitude of the over-compensation decreasing each time.

This effect is akin to that of a bungee-jumper bouncing on the end of a bungee rope, or a mass attached to a spring. All of these phenomena are examples of what physicists call damped harmonic motion.

Note the Bungee jumper is never required to smack his head at either end in order to get a resonance.
You want to search the web for intake reversion, as well as helmholtz. One rule of thumb for trumpet to wall clearance, I read, and implemented on the Bunderson was 1.5 x trumpet diameter.

[FMS01]

1.5 X Trumpet "mouth" or nominal throttle body bore?

Cheers,
Julian.

[vvega]

pass. probably just volume increase.

changing volume effectively lengthens or shortens the inlet trac

so your dead on teh money there


v

[RomanV]

Yes, but it doesnt lengthen or shorten the distance that a pressure wave has to travel.

eg.

you could have wide but short intake runners, or long but skinny intake runners, and they'd both have the same volume...

but they wouldnt 'resonate' at the same frequency (RPM)

Well, I think so anyway.

[vvega]

dont think to often then

do some reacherch on exactly that and then come back to me :d

v