Technical air box mod revisited


New member
Nov 1, 2003
United Kingdom.
Hi, whilst feeling sorry for myself and looking for inspiration i read through this entire thread.
I recently removed the connecting pipe from the intake and noticed the engine note changing. As a sound engineer, all sound but especially sub-low frequencies intrigue me. I made a mental note to revisit this nice new sound and this thread set me off on a bit of light relief this morning.

Just removing the rubber connecting pipe and leaving the elbow exposed.... what a sound especially under load!!!! Sounds a bit like a mustang or trans-am :)
I first thought that removing the intake "scoop" would reduce performance as there would be a certain amount of ram-air effect and the air would be at its coldest. Cold air is goood as cold air is denser i.e. more of it. This combined with the speed quotient should be good. And then i looked at the shape of the airflow, that first bend will cause a lot of turbulence and actually the back pressure "impedance" may possibly restrict the airflow effectively bouncing air turbulently back out. Then of course there is the change in profile as it joins the elbow. hmmm
From an acoustics and engineering point of view the secret here to releasing greater bhp might be to have a tuned induction system. Exactly the same as an exhaust but in reverse. Changing the air intake pipe properties will have a similar effect as changing the exhaust components to match the back pressure etc.
How does it work?
Well, simply blow across an empty beer bottle and you will hear a note. this is a function of the width and length of the neck coupled to the volume of air inside the bottle.
The volume of air dictates the fundamental frequency of the note, the more volume of air the lower the frequency of the note. (colder lower also)
What's happening? air blown across the open end of the bottle pushes a plug of air into the bottle compressing the air. Air is elastic, so it will only squeeze so far and then it says, too crowded in here and it pushes the plug of air back out. But because it's elastic it over shoots, (especially after a few he he) and the air pressure inside the bottle is reduced, sucking the plug back to equilibrium, where as long as you blow over the top the cycle repeats and produces a note.
These cycles happen in excess of several tens to hundreds of times a second.

With me so far? good.... changing the neck diameter reduces the frequency but it also reduces the elastic resistance (impedance) and makes it difficult to produce a note without increasing the air flow pressure significantly. Try blowing over a narrow glass and it becomes impossible.
Larger pipes, think digiridoo, exhausts etc, have a narrow end and a wider end are much more difficult to get a sound from compared to a bottle unless the air (pressure) flow is increased significantly they need several bar of pressure to achieve the same effect.

Now the easy bit....resonant systems
Without back pressure (impedance) it becomes incredibly difficult to do any work. The best analogy i can think of is a bouncing ball.
Drop a ball and it will bounce until it comes to rest each bounce getting smaller. Input more energy at the right moment repeatedly and you will find the resonance of this system where it becomes incredibly easy to get the ball to bounce with minimal energy input as long as it is timed well.

So, an engine spluttering out exhaust gases works better when it has some resistance that causes sympathetic resonance, which is why when my front pipe cracked, it sounded like an awsome dragster but it had next to no power whilst driving it to my friendly welder.

Timing gents is everything, try saying
suck squeeze bang blow
with equal time allocated to each word. The rythm becomes easy, now say squeeze slower and your rythm (efficiency) dissapears.

The engine exhaust cycles set up waves of pressure that have a frequency and harmonics, if the length of the exhaust pipe is "tuned" so that it is similar to the length of the wave produced, the back pressure (impedance) that governs the sympathetic resonance of the engine cycles can now be used to increase the efficiency, like tapping the ball in time.

So what the *&^$ has this got to do with the induction side? well......
That extra roarty sound is caused by air being sucked into the airbox through a pipe, albeit bent but that never stopped a saxophone, it then goes through a box (resonant system) and then through another few pipes (resonant systems.)
The deeper sound without the air intake is largely due to the wider "neck" of the bottle. Sucking air over the neck of a bottle is the same as blowing, just a bit more difficult.

OK so i'm no motor engineer but that little thing that fits on the inlet air pipe that causes turbulence is good that it would remove the possibilty of standing waves that might mean a concidence whereby the air entering the intake manifold actually has minimal velocity due to the rate at which it enters the system.

What actually might work better is if the air intake pipes were designed to take advantage of the resonance.
Problem is, as any tuned pipe bhp seeker will know, is that there are only a few optimum rpms where this works and some where it's a huge disadvantage.

This is now getting into slightly more difficult acoustics. Standing waves are good as no wind or brass instrument would work without the phenomenon but u don't recommend sticking a trumpet on your exhuast.

Oh yes Polecat, isn't it likely that the reduced mpg is simply due to the increased occurrence of flooring the pedal to induce this grisly noise? grrrrrrrrreat! :)

If you look at Neptunos turbo 20v you will notice the air filter sans box

Ok so its open to the bonnet gril but i suspect the airflow over the bonnet does not make for a ram-air effect and in fact it will be low pressure compared to the engine bay.


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