Drone free exhaust system-solved!!!!!!

[Borys]

Hope this helps with this issue.
Had my system nicely done recently but of course Ii did not happend without droning. Some probably experienced this at different noise levels. Measuring mine with frequency app was around 100mhz. So off I go to explore youtube and google. Solution I found was at US forums. Widely practiced and effective. Simply it is based on heimholz resonator and it carries a proper math behind it. Around forums they call it "j pipe". Now there isn't a garage in a UK does it!! Which puzzles me as being an exhaust shop owner surely would like to have a solution at hand. I guarantee there isn't a system with after market boxes, straight pipes with out a drone. On topic. Car was buzzing/droning beetwen 1100rpm to 2000rpm. That range is commonly affected. With help from US forum calculation was made. J pipe to install needed to be 30" long to get rid of the drone across affected rpm range. This is a nice video explaining drone
https://m.youtube.com/watch?v=cILip2l0J6o
And where to place it
https://m.youtube.com/watch?v=afwdv3Qwdpk

Made a trip to exhaust shop made my set up. I prepared a presentation like for high paid job interview and owner agreed to try it. We installed the pipe at the back of the exhaust where the muffler usually sits. If I knew this before we would design a totally different pipe work. Its not a neat job as on the rest of the system as it has been done for the first time.

I don't think guys at shop believed that this will work.....test drive and boom no drone whatsoever only sound is left is a perfect v8 burble.
Just to add j pipe will not affect the sound it will eliminate sound waves which cause the drone. Also it can be any shape
Welcome to a drone free system!!!!!

[MikkiJayne]

Very useful to know So thats a 30" long tube which is capped at the end?

Many intake systems use such a thing to reduce induction noise - I think the D3 S8 has it in both intake tubes iirc. Never seen it used on an exhaust before but it makes perfect sense it would work there too.

Could it be that its only of real value on V8s, hence popular in the US but not here? We're not the usual target demographic of companies like Powerflow I suspect but I would think the likes of Milltek would understand it.

[Borys]

Yes that is correct 30" capped at the end.
Induction yes that is true. Some Japs they have similar system.
It is "the drone killer" :-)

[David's8]

GH77 (Gregor) had a modification done to his which sounded pretty good when he came to the 2015 meet. I exchanged a few pms with him concerning what he had done and he used a resonator tube but in this case fitted at right angles to the exhaust tube. This is what he said and there is a link to a photo:

"I do warn you that the system is rather rudimentary and while there is no additional noise at 80-85 it is fairly brutal at 60-70 under light load. All to do with the speed of the exhaust sound waves at certain RPM. The resonator helps a bit but it is all very experimental still as the resonator can only cancel out the drone at a set RPM. I'll explain in detail when we meet up."

https://app.box.com/s/ztqjfkuy6o50yu2mtzyf2dce8wivbeha

I didnt end up meeting up with him and have never progressed any mod myself.

[Borys]

Straight away can say it will not work from his picture.
It has to be on both pipes. If you look at the videos it has to be a j shape parallel to exhaust pipes as much as possible. Even if a bent done like in my case it is still parallel to existing pipes
Also it very important to remember that sound waves will cancel each other out at 180 degree not 90 like on the pic. It is explained also with speakers. If you face two speakers to each other there will be no bass.

[David's8]

tbh with your arrangement the pipe/sound wave enters the main pipe at 90deg - its a matter of making sure that the reflected wave is in opposite phase when it enters the main pipe however that is achieved. As Gregor said, it was a fairly rudimentary arrangement, arrived at (I believe) by trial and error and, of course, it was a trifle(!) resonant at certain revs!!
Of course your comment on the fact that it must be on both pipes is totally relevant.

[Borys]

Specialist for this are on the other side on the pond :-) There are lots of pages to read through. I never saw a set up on US forum with 90 degree pipes. Think the name given "j pipe" got meaning too. Sound wave need to travel parallel to exhaust-thats what I understood
I'm glad mine is perfect now and I hope this will help others
Droning can be quite annoying

[HPsauce]

Actually the resonator doesn't need to be in any direction at all, certainly not parallel. Nor any particular shape, many are actually spherical!

What is critical is its resonant frequency, calculated using maths/physics (or trial/error!).
A tubular dead-end is just convenient for fitting under a car along with the exhaust and cheap to make/fit using materials familiar to exhaust makers.

But as noted above, on a dual-pipe exhaust you do really need two.

Note that resonators are often used on car air intakes....

[Borys]

Either I was lucky then or my math is quite good

[David's8]

Originally Posted by Borys

Either I was lucky then or my math is quite good

Yes, its the length or, indeed volume, and any restriction which determine its efficiency at producing a wave form in the correct phase (180deg) and at sufficient amplitude. A mouse fart even if 180deg out of phase would not cancel out an elephants fart. So I guess that the pipe has to be of similar bore and you need to tailor the length to the frequency you wish to cancel out. And thats where your maths comes in.