The TRUTH about aftermarket exhaust drone

ped

Other manufacturers are probably guilty of this as well, but FI is the one I (now) have experience with, so unfortunately I have to pick on them. I just had a FI catback with 18" resonators installed two days ago. Despite claims all over this forum this system has drone and a lot of it. 7AT drivers who leave it in D will find themselves in the tallest gear most of time - pressing the accelerator in this situation (but not enough to force a downshift) will bring on drone like crazy, turning the interior of the car into a low-frequency resonance chamber! Some people may not care about this, but the fact remains that all the claims of "no drone" are not true, and I wish someone had made this more clear around this forum - it would have saved me a lot of wasted money. So new people researching an exhaust for your 370Z and considering Fast Intentions consider yourselves warned.

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. The 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 feet / frequency / 4 in length 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 into/through 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 in concept.

Why FI and possibly others are leaving this out? I'd guess it's the inability to determine the proper frequencies needed 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 is 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 flamed 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 put 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 here is how to get the length resonator needed to counteract it:

1100 / 120 / 4

= 9.16667 / 4

= 2.29167 feet

(times 12 for inches)

= 27.5 inches for the length of he Helmholtz resonator tubes