AIR INTAKE RESONATORS
The photo above is of a black plastic air intake resonator as found on many Mitsubishi and Proton engines. It has a couple of functions. Mostly people think its to reduce the volume of the air intake noise, but it actually serves another more important purpose - :huh:
Design and Construction - The resonator itself couldn't be simpler in design; it's basically just an expansion chamber or wide spot in the otherwise-smooth intake pipe. It may or may not contain some kind of baffle or plate, depending upon the design and the intent of the designers. Resonators come in two types: In-line resonators are open chambers that sit in the intake tube, while side-branch resonators are chambers that sit next to the tube and are connected to it via a small duct or channel.
The Common Misconception - Most hot-rodders and car enthusiasts think of intake resonators as simple mufflers in the intake tube, devices designed to reduce all the awesomeness out of a car's induction sound. That makes it a prime candidate for the "chuck-it" school of auto modification [hands up all those guilty as charged!]. After all, it's basically just a plastic tumor growing off of a tube that should by definition be as smooth and free flowing as possible. While sound control is indeed part of the resonator's job, the sound control itself is really more of a side effect of its primary purpose.
Pressure Wave Harmonics - Air flowing into your cylinder head's intake port doesn't move in a straight line while the valve is open, then politely stop in its tracks to await another valve opening. When the valve closes, the moving column of air slams into it, then compresses and bounces back like a spring. This pressure wave travels backward at the speed of sound until the intake runner opens up or it hits something, and then it bounces back toward the cylinder. This is the "first harmonic." The pressure wave actually bounces back and forth two or three more times before the intake valve opens again.
Intake Tube Pulses - The resonator in your intake is technically known as a Helmholz resonator, an acoustic device used to control pressure wave harmonics. Air bouncing back out of your engine and into the intake tube doesn't do it in a single pulse the way it would in a single intake runner; the multiple pistons put out pressure waves at their own intervals, and some of those are going to try to bounce back in while others are going out. The result is a "clog" or high pressure area in your intake tube that ultimately limits airflow through almost the entire rpm spectrum.
The Resonator - Adding an expansion chamber to the intake tube forces air coming back out of the engine to slow down to fill the cavity, thus expending a great deal of its energy and slowing the pressure wave reversion. This slowdown allows fresh air to flow toward the engine without fighting pressure reversion waves the entire way, thus aiding in cylinder filling. Since these pressure waves are essentially sound, giving them a place to expend their energy before exiting the air filter box ends up dampening the intake noise and quieting the engine. Thus, the resonator helps to make the engine paradoxically quieter and more powerful.
