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

NBZ34

I think everyone is pretty aware by now that you're not happy with your exhaust (as you've posted this in several threads already). It's a matter of taste. The majority of us love the sound of our FI exhaust.. you dont. Get over it or sell it already.. but please.. quit bitchin about it!

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CHRIS 開田
2012 370Z MB Touring/Sport/AT/Nav | Takeda | Fast Intentions | SWIFT | H&R | UpRev | Michelin PSS |

Mr&Mrs

I dont see how with the countless videos you didnt pick up on it either. Quit blaming us for your problem.

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Project Z <--Click - Check out the link too many mods to list!

NismoZ70

We know....we read your same exact post in your other thread

ped

Videos can't/don't pick up on the fact the interior of the car becomes a low-frequency resonance chamber. And with people claiming in nearly every thread (and in fact on FI's website) that their exhausts have no drone at all, it's easy to feel misled about it.

ped

A public service that might save someone some aggravation and money. It's pretty clear FI is being less than fully forthcoming since I think their site even claims no drone.

If you guys don't want to read it don't come into the thread. As you can see I predicted this response. You guys are too predictable.

bacalhau1669

Yea dude, get over it. "Oh man, my aftermarket exhaust is loud and drones in a car notorious for such."

No one cares

bacalhau1669

If you werent so naive while reading about exhausts, then perhaps this wouldn't have ever happened.

You clearly are picky about exhausts and should've known this. The stock exhaust was perfect for you.

ped

You have an interesting definition of "naive" if it includes reading and believing FI's own website plus nearly every review/fan on this forum who almost universally claim "no drone". You might be OK with that but people who don't want drone shouldn't be.

cv129

Guys, to the very least, this is good knowledge. Some people care.

However OP, you may wanna change the title to just say "Truth about drone" as the current title suggests a direct attack on FI, and that's not the right battle

bacalhau1669

or maybe thats why im not an idiot and always kept a stock muffler on my car.

If you watched videos of this car with different exhaust setups and such, its easy to see that being THAT loud, will absolutely come with drone.

cv129

Actually I'm looking at exhaust right now...so for everybody else other than OP, y'all are saying FI has lots of drone now?

I think that's the issue here...he kept reading ppl singing "no drone", then when that's not the case, he's being called stupid for believing other members?

Let's be a little fair now.

Disclaimer: I've never met OP despite being in same city.

ped

Then why not just say as much in the reviews on here and on FI's site? Why misrepresent the situation and then expect people to be OK with it when they find out differently?

ped

A fair point, but do the ther makers claim "no drone" also?

megalapagas

It just varies from person to person OP, I like drone/ deep growl noise thats why i went Non Resd CF CBE with TP even though being this setup while cruising its like stock....some ppl here in the forums go LTH with my same CBE etc etc....I believe maybe for your pickiness you have either stayed stock or gone nismo, just my 2 cents. But I do agree though you cant blame us for what we picked and liked and gave out info on the setup we F.I Customers picked.