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.