Originally Posted by Mike94PGT
Imagine a turning fork and a speaker, side by side. Connect the speaker to a audio signal generator that sweeps from 20Hz to 20000Hz. Turn on the generator and crank the volume up, pointing the speaker at the tuning fork. Slowly wind the generator from 20Hz up...
When the frequency of the signal generator gets close to the frequency written on the side of the fork, it will start to vibrate. When the frequency of the generator matches that of the fork, it will vibrate alot and the system is said to be in "resonance". As the frequency moves on past the resonance frequency, the fork will stop vibrating again.
Now consider a flute: a hollow tube of specific length with holes placed strategically along it's side. If you blow across the end, you hear a note. If you change which holes are covered, you change the note that's played. What's happening is that the effective length of the flute is being altered by covering/uncovering the holes. This changes the natural frequency of the flute and thus the note you hear.
In the engine's intake manifold, you've got a similar setup. The engine changing speed is equvalent to the change in frequency in the speaker example. The pulses of the speaker are similar to the pulses produced by the action of each cylinder's intake valves (and chamber pressures when the valve open etc...) The tuning fork might be considered to by the medium - the air in the runners in this case. As the engine runs through the RPM range, the intake valves open and close faster and faster as engine RPM increases, just like the speaker pulses faster and faster as the frequency dial is turned. The length of the runners, their volume, manifold pressure and temperature etc all determine at what frequency the air in the runners & surge tanks will resonate -- that is, vibrate just like that tuning fork.
During times of resonance, the vibrating columns of air (which, though moving, still can support a standing wave) can actually help charge the cylinders. A vibrating column of air has dense sections and rarefied (low pressure) sections. If the standing wave is just right, a dense section of air meets the valve just as it opens. The denser air pours into the port, moreso than if resonance (and thus this standing wave) wasn't happening. The effect is a very mild "supercharging".
But the engine RPM is always changing. If there was only one runner length, there would only be one resonance point. Mazda makes the manifold like a flute that can resonate at several wavelengths by making the runners have variable effective lengths. They do this by using butterfly valves in the manifold to direct air a "long way" or a "short way" (#2 is primarly responsible for this). Closed, the path is "long" and the primary resonance frequency is low. Open, the path is short and the primary resonance frequency is higher. By further changing the frequency in each surge tank (by opening or closing #1 valve, which exposes the each bank to the others' intake pulses, effectively doubling the frequency each surge tank sees), they manage to get several points at which good standing waves are set-up through the RPM band.
In the KL03, the VRIS (variable resonance intake system) plates are controlled by the PCM, which is looking at engine RPM and loading (throttle angle) to know when to open the valves. #1 opens at 3250, which doubles each surge-tanks' frequency. #2 opens at 4250 which then shortens the average path length by 1/2. At 6250, both valves close.
This 6250 point is a bit of a let down. The natural tendency would be to further shorten the path length but the realities of aluminum casting (driven by packaging and cost concerns) rears its head: there's no provision to make things even shorter. The only recourse they have is to revert to the longer path and lower frequency and rely on the 2nd-harmonic to provide a reduced-but-still-effective ram effect. It's a compromise and one of the things I dislike about the KL03 manifold.
The North American I4 does not have this system, just the V6.
BTW, people who get a KLZE but who don't upgrade their PCMs are getting hosed: the KLZE, with it's markedly different manifold, has different resonance points than the KL03. The PCM doesn't know this and switches the valves thinking it's an '03. Meanwhile, the ZE manifold never really has a chance to do its resonance thing properly because the PCM is stupidly flipping the valves at the wrong times.
Also, people who remove or "tie" their VRIS are also hosing themselves. By disabling this feature, they're removing resonance points from the RPM band. Engine torque will suffer in the affected RPM areas and peak power will not climb. (Well, if someone completely removes the #2 valves & shaft, I suppose a bit might be gained from the removal of the restriction in the 4250-6250 range, but it won't be a huge gain and output will suffer everywhere else...)
Does that help? (Please, someone put this in the FAQ
[img]images/smilies/biggrin2.gif[/img] ) ** Mod Note: Done, Mike! Several years late, but Done!**