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Thread: Air intake design

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    Question Air intake design

    Hello,

    Ive always wondered what this black box behind the air intake "horn" does/is? i can't find any reference to it in the service manual and it doesnt appear to have any hoses/cables coming out of it.


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    Still Plays With Cars Loren's Avatar
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    Helmholtz resonator.

    It helps reduce intake noise (resonance), and by doing so, it also improve airflow at or near its resonant frequency and harmonics of it. So, it might be tuned to the frequency of all of the intake valve noise at 3,000 rpm... and would also be fairly effective at the first harmonic frequency of 6,000 rpm. It is very likely tuned to somewhere near the torque peak of the engine and often makes a verifiable difference in torque around its resonant frequency.

    This all goes hand in hand with the intake manifold design to give you the broad and flat torque curve that you want out of the engine.

    The best way I can explain it is...

    Think of your intake tract as one simple tube. On the intake stroke of a given cylinder, air is sucked in. Now you have a column of air in your tube that is MOVING into the cylinder. Then the intake valve closes and that column stops. But, it doesn't just stop. It bounces back! Now, your column of air has reversed direction (and probably has some turbulence). What an intake resonator does (and the intake manifold plenum sort of does the same thing) is gives that reverse pulse someplace to go so that it doesn't keep trying to reverse the flow in your tube.

    In this case, it's probably just working to keep that small intake tube off of the filter box pulling air relatively constantly in the correct direction AND reduce intake noise.

    Intake design is pretty complex stuff!
    Simplify and add lightness.

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    Hello Loren, thanks for the fast reply, your my first ever message on this fourm. Do you have any photos of what the inside of this resonator looks like?

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    Still Plays With Cars Loren's Avatar
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    It's almost always empty! Could have some foam in it or something like that. Maybe some strange internal tubing.

    I'm a night owl. Welcome to the forum!

    The way Helmholtz resonators work is a function of the volume of the chamber, the diameter of the neck of the opening, and the LENGTH of that neck. The chamber itself can be any shape to allow for packaging. And the shape of that neck can be complex, as well. If you think of the resonator as a bottle... the "neck" of the bottle could actually be inverted as a tube INSIDE the chamber. In the age of CAD... they can do all sorts of weird things.

    https://itstillruns.com/air-intake-r...-12156112.html
    Simplify and add lightness.

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    Senior Member AtomicPunk's Avatar
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    Quote Originally Posted by Loren View Post
    Helmholtz resonator.

    It helps reduce intake noise (resonance), and by doing so, it also improve airflow at or near its resonant frequency and harmonics of it. So, it might be tuned to the frequency of all of the intake valve noise at 3,000 rpm... and would also be fairly effective at the first harmonic frequency of 6,000 rpm. It is very likely tuned to somewhere near the torque peak of the engine and often makes a verifiable difference in torque around its resonant frequency.

    This all goes hand in hand with the intake manifold design to give you the broad and flat torque curve that you want out of the engine.

    The best way I can explain it is...

    Think of your intake tract as one simple tube. On the intake stroke of a given cylinder, air is sucked in. Now you have a column of air in your tube that is MOVING into the cylinder. Then the intake valve closes and that column stops. But, it doesn't just stop. It bounces back! Now, your column of air has reversed direction (and probably has some turbulence). What an intake resonator does (and the intake manifold plenum sort of does the same thing) is gives that reverse pulse someplace to go so that it doesn't keep trying to reverse the flow in your tube.

    In this case, it's probably just working to keep that small intake tube off of the filter box pulling air relatively constantly in the correct direction AND reduce intake noise.

    Intake design is pretty complex stuff!

    Nicely done.

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    Loren (02-10-2021)

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    Agreed. I think Loren explained that about as well as it could be explained.

    I think the only real reason for removing it, may be some reason like, you were going to race the car and it was determined that the resonator main impact was at (for example) 3,000 rpm. But if you were going to race the car and it would never see below 4,000 rpm while racing ... AND dynos showed more power from 4,000 to 6,500 rpm with it off (I kind of doubt that in reality), then it would be worth removing it. However, if you were racing a Mirage for example, and the catalyst(s) and mufflers would likely be removed and such would change the flow characteristics, making the orginal "tuned" size of the resonator not accurate any longer and probably not useful either.

    I like intake sound. But I'm not sure I would like the kind of sound that may happen from removing the resonator. I'd have to check it and see. But quite honestly, like with Loren's explanation, I would imagine at certain rpm, the engine will have less power. And if Loren's guess that the resonator is probably most effective near torque peak, then around 75 to 80 mph cruise (5 speed) would likely net a good bit lower mpg.


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