If you're doing FI catback does G3 make a difference?

ped · 09-19-2013, 08:49 PM

Ok for those who care or later find themselves in the same position, I want to let you know the answer to this problem is that Fast Intentions (and possibly other manufacturers) are building their exhausts minus a very important item - a Helmholtz resonator. This type of resonator attaches to the main pipes at a 90-degree angle, and is capped on the end. He shape doesn't matter (J shaped, etc) as long as it's the proper length - basically what you need is to know the frequency of greatest drone (say 120 Hz for example) and then shoot for a resonator that is 1100 / frequency / 4 which gets you a tube length which will have a natural resonance 1/4 of the wavelength of the worst drone. Since this tube is closed on the end that doesn't attach to the exhaust pipe no exhaust really flows, it rather a standing wave is established (like when you blow over the top of a Coke bottle) which cancels much (or all) of the drone. Similar to noise-canceling headphones.

Why FI and possibly others are leaving this out? I'd guess it's the inability to determine the proper frequencies since the catback may eventually be combined with other items like an HFC, different exhaust header, etc. Thus what would work with stock cats and headers wouldn't necessarily work (at least as well) with aftermarket replacements of these items.

My beef isn't so much that they leave them out, it that so many fans of FI in this forum make FI exhausts out to have NO DRONE which is absolutely nonsense as I can tell from having it installed two days ago. At a minimum people need to have REAL information before blowing 1100-1200 dollars on an exhaust that can induce headaches or at a minimum be highly annoying. I'll probably get flames for this since FI seems to be so well regarded here but I wish someone would have warned me before I wasted money having this installed and now will be paying labor charges again to have the stock system out back on (or to have Helmholtz resonators welded on) plus loss for restocking fee since the exhaust can't be sold as new anymore.

For the curious who want to see the outcome of the calculation above, for a drone that is worst at 120 Hz:

1100 / 120 / 4

= 9.16667 / 4

= 2.29167 feet

(times 12 for inches)

= 27.5 inches for the length of he Helmholtz resonator tubes

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09-19-2013, 08:49 PM	#34 (permalink)
ped Enthusiast Member Join Date: May 2013 Location: Dallas, Texas Posts: 258 Drives: 2013 370Z Rep Power: 13	Ok for those who care or later find themselves in the same position, I want to let you know the answer to this problem is that Fast Intentions (and possibly other manufacturers) are building their exhausts minus a very important item - a Helmholtz resonator. This type of resonator attaches to the main pipes at a 90-degree angle, and is capped on the end. He shape doesn't matter (J shaped, etc) as long as it's the proper length - basically what you need is to know the frequency of greatest drone (say 120 Hz for example) and then shoot for a resonator that is 1100 / frequency / 4 which gets you a tube length which will have a natural resonance 1/4 of the wavelength of the worst drone. Since this tube is closed on the end that doesn't attach to the exhaust pipe no exhaust really flows, it rather a standing wave is established (like when you blow over the top of a Coke bottle) which cancels much (or all) of the drone. Similar to noise-canceling headphones. Why FI and possibly others are leaving this out? I'd guess it's the inability to determine the proper frequencies since the catback may eventually be combined with other items like an HFC, different exhaust header, etc. Thus what would work with stock cats and headers wouldn't necessarily work (at least as well) with aftermarket replacements of these items. My beef isn't so much that they leave them out, it that so many fans of FI in this forum make FI exhausts out to have NO DRONE which is absolutely nonsense as I can tell from having it installed two days ago. At a minimum people need to have REAL information before blowing 1100-1200 dollars on an exhaust that can induce headaches or at a minimum be highly annoying. I'll probably get flames for this since FI seems to be so well regarded here but I wish someone would have warned me before I wasted money having this installed and now will be paying labor charges again to have the stock system out back on (or to have Helmholtz resonators welded on) plus loss for restocking fee since the exhaust can't be sold as new anymore. For the curious who want to see the outcome of the calculation above, for a drone that is worst at 120 Hz: 1100 / 120 / 4 = 9.16667 / 4 = 2.29167 feet (times 12 for inches) = 27.5 inches for the length of he Helmholtz resonator tubes