Digging up this thread in hopes someone could help assist. Read every page and like to get a little more confirmation.

Looking to do this mod to the new Invidia CBE I've installed (that I'm already regretting coming from a Motordyne CBE). It's droning horribly from 1900-2800.

At around 2000 RPM it seems to reverberate across three peaks, strangely 50hz apart... ~100, ~150, ~200.

Past this point 150 becomes the dominant droning frequency.

Photos attached of both occurrences.

According to the formula in this thread I get a length of 22.5"... but that seems quite small compared to the 28-30 I'm seeing thrown about.

Does this still seem right?

Speed of Sound / ( (frequency) x 4 )

1125 / ( 150 x 4 ) = 1.875ft > 22.5in

I think the best bet is to make the tubes easily adjustable for length. In theory we can crunch the numbers but you won't know until you try it. Exhaust systems are not created equal.

This was going to be my approach. It was already suggested earlier in the thread to have an adjustable style system with a clamp.

But still was hoping to hear my math was on point or missed something.

I love every note that my Fast Intentions non res exhaust makes. Even so called drone.

That's great for you... but not for the loads of people who've commented and contributed to this thread. :rolleyes:

On topic, happend to come across an article regarding using 1/4 wave tubes to tune out certain frequencies in Diesel engines but the concept is transferable here. Kind of annoyed I didin't think of it before coming across this paper.

Rather then asking a shop to tack on a tube with a clamp system to vary the length of the tube, work with the shop to tack on the longest, open-ended, straight tube you can manage... and fab up a resonably thick disc to act as a piston to vary the internal length of the pipe to alter it's resonant frequency. The piston would have a rod attached at the end of it to push in or pull out, that can be extended or shortened... surely can't be all that difficult to extend, even temporarily as it's only for tuning purposes.

Once you've settled on an ideal length, mark it, take it back to the shop and get them to cut it down and weld an end cap on.

Personally, I'm going to have 2x 39 inch pipes put in and work my way down. This'll allow me to try negating as low as 100hz drone.

Great article by the way for more info.

Toyota Supra MKIV : Powercruise Taupo 2009

Yeah it's for a Supra, but the concept is absolutely the same. Key thing to note is how the temperature affects the length needed.

120hz @ 20c (68f) length needed is 710mm (28")
120hz @ 80c (176f) length needed is 780mm (30.7")

I'm in the process of having a shop install this for me. They have the tubes and are welding them and capping off at 28 inches. As soon as they get their lift freed up they will call me so I can bring my car to them. They have done several jobs for me and are very good fabricators. I'll post the results and pictures when its done.

My wife hates the sound of my car now. Super Charged with FI cat back and TP with MilSpec cats before the Y pipe. It does drone a lot between 2K & 2.8K rpm. It was okay until I exchanged the stock cats for TP and the Milspec HF cats.

So the Milspec HF cats aren't long enough to fill the gap left by the OEM cats, and require TPs too?

The TP are used to get the Milspec cats further away so that they don't get overheated. This is what FI recommends.

With all of the things I have done to my Z the exhaust system which so sweet at first had become a drone monster. From 2K-3K rpm, the exhaust would drone at highway speeds and get really loud during regular driving. I did some searching on the forum and saw an article about adding Helmholtz resonators to reduce the 130-160Hz noise by pressure wave cancellation. Well, I added them in and they DO REDUCE the drone.

They are quite simple in design but a bear to install. The fab shop I use came up with a solution that was way easier than T-ing off the exhaust tip area. The system clears everything under the car and the drone has been reduced by at least 50%.

The resonators are 28 inches long and 2.25 inches in diameter. All 304 stainless.

Are those pre bent 45's or did your fab shop bend them. What degree?

Yes, pre-bent at 45. Got them from Siliconeintakes.com

Do you have a direct link to the product? I can't find it on their website.

Here's a link to the 45's that I bought.

Mandrel Bent Thick Stainless Steel Tubing

So you welded a 16" straight pipe to this 12" bent pipe for the 28" overall resonator and welded the whole thing onto your exhaust? I wonder if Fast intentions could do this for people and how much it would cost.

I haven't done mine yet as I'm still waiting for my mil spec cats to come in so I can have the lead pipes and this mod done at the same time. It appears that's the way Tbatt did it by looking at his pics.
I'm sure Fast Intentions could it, you might also check with Seb (Specialty Z). I showed him Tbatts pics and he thinks it's a great idea.

https://www.siliconeintakes.com/dsm-...ng-p-1159.html

https://www.siliconeintakes.com/dsm-...l--p-1587.html

I used two of each of these to make the resonators.

Would this make much difference if I already have 18" resonators?

I hope so because that's what I have.

I have said the drone is horrendous, sold mine after 3 weeks...everyone else says sounds great, ur either deaf or stupid..$1500 for a FI and then u have to get it modified by a muffler shop to get it acceptable,i dont get it..

I have been going back and forth with Fast Intentions to try to understand all this, and figure out how I can get s/c-compatible cats while retaining the OEM Nismo muffler. I'm embarrassed to say I still don't get it. I've crawled underneath the car with a flashlight, and this is what I think I see:

cats -> H-pipe with resonators in very back (?) -> muffler

I can't even seem to find a Nissan diagram that identifies the different sections of the exhaust. What I think Fast Intentions is telling me is this:

resonated test pipes -> lead pipes with 100-cel mil spec cats -> X-pipe with resonators -> muffler.

The lead pipes won't fit the OEM system, so you're stuck ordering the whole exhaust which I don't want to do. I want to have cats, but how do I get them downstream in the OEM H-pipe?

Please correct me if I'm not understanding this correctly!

TBatt - What did the work cost you? Also, are the ends of the resonators attached to the mufflers in any way? Was wondering about the stability of the tubes sticking out.

Thanks

Why not go with high flow catalytic test pipes? Berk, Fast Intentions, AAM, Kinetix, Z1, etc.?

I am trying to understand your requirement for no cats in the stock placement (down/test pipe), but cats needed somewhere else downstream in the exhaust.

The shop charged me by the hour so the total bill was around $300 with me supplying the parts.

Yes, the tubes are attached to the muffler so that they are supported.

High flow cats are just cats that are less restrictive, not made for supercharged engines which produce not only more exhaust gasses but at a much higher temperature. When I talked to Tony at FI he said that what they had found is that going to a HD high flow cat (the MILSPEC) and moving it further away from the stock location makes for a very reliable setup.

So, I followed his recommendation and went with the test pipes (removes the stock cats) and installed new lead pipes with the MILSPEC cats much further downstream of the exhaust but still before the H pipe section. All I can say is that it works. Loud, but it works.

That is what my system has too, 18 inch resonators in the H pipe section. What these resonators do that is very different than a pass through type resonator. A Helmholtz resonator is tuned (by length) to cancel out certain frequencies and in this case the 130-160Hz (2000-2800 rpm) drone. Works pretty well.

How to get rid of exhaust drone? Either turn up your radio or downshift!

Reviving an old thread.
I'm going the Helmholtz route for my FI exhaust. Just wondering if there are any lessons learned out there since this thread was created.
Does it matter where along the exhaust you place the chambers? Do the chambers need to be the same diameter piping as the exhaust? The equation people generally use probably assumes the same diameter pipe, I would guess.

Did you check out post #130. Tbatt gives some good info

I want to actually thank you for reviving this from the dead. I was planning to buy FI exhaust but that drone would annoy the crap out of me.

Motordyne always seemed too loud to me. But now I know about the Ark Grip so I might buy that.

I've got the HKS Hi Power. It's quiet, has a great tone to it and no drone

Did you have the FI and ditch it? Did you attempt the Helmholtz?
It'd probably be cheaper to sell my FI and start over, versus pay a shop to fab a Helmholtz for me. Today's fab prices are crazy where I live.
I'm torn.

I had the FI on a different z. I installed Art pipes. They helped with the drone

Probably hard to find someone that's done both but any differences(both power and drone) between ART pipes/resonators at the front vs at the mufflers?

I have ART test pipes and they make little difference in the cabin resonance I'm having.

You have HKS on your supercharged Z? Must be happy then.

Had Art pipes on the supercharged Z. Supercharger and Art pipes are sold. Fi tt kit being installed :tup: