datadavid said:
Lots of voodoo around this, stock airboxes use acoustic resonance to increase volumetric efficiency.
Nothing us lot would understand without the proper degree in engineering unfortunately, and we dont need to. Its all been done a million times already.
This is true, but the V.E. Is only for certain rpm's.
Its like a Helmholtz resonator, or when you open your car window going down the highway.
The airbox is designed for the rpm range the engine is most likely to run at.
When the valve opens, the vacuum is there and the air rushes towards the intake, as the valve slams shut, the air is carried by its momentum and pressure builds up at the intake.
This is when you want the valve to open for max V.E.
At some point, the momentum dies and the air shoots back into the air box, creating a low pressure zone at the intake.
If the valve opens then, the engine will not get as much air/fuel as it could have
The pressure wave will at some point hit the entry of the airbox and push air out, before bouncing back to the engine intake. The wave losses amplitude over each oscillation
These waves travel at the speed of sound, so it's some fun calculations to determine how long it takes for each pressure oscillation.
The higher the velocity of the air, the more momentum it will have, the higher pressure that will build up at the intake valve, the more time the pressure will be there before bouncing back.
Edit: thats why stacks are much better when used in a high revving engine