May 20, 2019, 05:21:38 AM

Author Topic: Branch and Helmholtz resonators  (Read 244 times)

Offline Exidous

Branch and Helmholtz resonators
« on: March 29, 2019, 11:36:26 PM »
I had some pretty bad drone around 2100rpm. I've added two 25" side branch resonator to the dual exhaust. When I first drove the car the first thing that ran through my mind was this has to be Voodoo despite the fact that I understand the physics behind it.

The issue is that it has a pretty narrow bandwidth. See the attached. At 145hz there used to be a big peak, now it's a decent valley. I want to broaden the bandwidth.

Queue the Helmholtz resonator. It's not quite as effective at it's fundamental frequency but it has a lot more bandwidth.

I've found the formulas to calculate it however I cannot find a differentiation between an amplifier and attenuator.

With the side branch, 1/4 wave and 3/4 wave are attenuators and 1/2 and full are amplifiers. Easy math. 145hz at 1/4 comes to about 23.3" tube.

Having typed this I think I just realized the answer.

My fundamental frequency is 145hz or 94.2" wavelength. I just need to tune the Helmholtz for 580hz which has a wavelength of 23.3".

Maybe someone else can use this later on. Back to tinkering. Just have to fit a pair of 3-4" canisters in the back of the car somehow.

Side note, I actually made it adjustable with about 2" of play. I think it's actually a tad long. The tone increases noticeably around 2500rpm now that 2000 was brought down so much. Even the idle was tamed a bit. Maybe I'll shorten it an inch to raise the frequency.
94 BB SBE LS1 Full Mamo top, MS3ProU with TC, RONIN 8.8 and another custom dual 2.5" stainless exhaust. Too loud, need bigger muffler....

Offline Gunnytron

Re: Branch and Helmholtz resonators
« Reply #1 on: March 30, 2019, 06:04:21 AM »
Thanks that's some cool info. Did you simply measure the frequencies with your phone?

Do you have pics of the setup?
http://www.norotors.com/index.php?topic=12880.0
FD, LS1 running 220 3.90 Trickflow heads, Comp Ultra-gold Arc 1.72 Rockers, 7.550 BTR Pushrods, BTR Stage II NA Cam 227/234 .614"/.576" 113+2, ARP Head Bolts, BTR Platinum Springs, Ported FAST 90, NW 90mm TB, FAST 60lb Injectors, Textralia Clutch/Flywheel, T56, Cobra 3.55 Diff, Holley HP EFI, Racelogic TC.

Offline Exidous

Re: Branch and Helmholtz resonators
« Reply #2 on: March 30, 2019, 08:15:26 AM »
I started with the math. V8=4pulses per rev. 2100 RPM and such. Then verified the math with the phone. It was all close enough to be considered right.

I have a picture of the unfinished product but forgot before I installed it. I'll get one today.
« Last Edit: March 30, 2019, 01:48:10 PM by Exidous »
94 BB SBE LS1 Full Mamo top, MS3ProU with TC, RONIN 8.8 and another custom dual 2.5" stainless exhaust. Too loud, need bigger muffler....

Offline Exidous

Re: Branch and Helmholtz resonators
« Reply #3 on: March 30, 2019, 10:51:52 PM »
http://hyperphysics.phy-astr.gsu.edu/hbase/Waves/cavity.html

To have a site do the Helmholtz math for you. At the bottom.
94 BB SBE LS1 Full Mamo top, MS3ProU with TC, RONIN 8.8 and another custom dual 2.5" stainless exhaust. Too loud, need bigger muffler....

Offline kinger

Re: Branch and Helmholtz resonators
« Reply #4 on: April 17, 2019, 11:22:04 AM »
They work fantastic and called a J pipe in the G8 world.  I put one on my car and did the same process as you measuring and adjusting the length of the J pipe and now, no drone just pure V8 Borla bliss! 
93 Touring, 6.2L, 415/425 Whp/Wtq, 8.8, samberg everything, AC, PS, TC, Cruise, LED Tails, HID head lights, immaculate interior and paint.

Offline FC3S Murray

Re: Branch and Helmholtz resonators
« Reply #5 on: May 11, 2019, 01:12:39 AM »
I am intrigued....... How far downstream of the exhaust are these  J pipes installed?


8 years and still running STRONG!

Offline MPbdy

Re: Branch and Helmholtz resonators
« Reply #6 on: May 11, 2019, 12:26:40 PM »
I’ve been lucky enough never to have any serious drone in my car. Smart to go for a branch resonator that works so much better than the bullet resonators people throw on hoping they’ll actually do something lol

Offline Exidous

Re: Branch and Helmholtz resonators
« Reply #7 on: May 15, 2019, 06:56:24 AM »
I am intrigued....... How far downstream of the exhaust are these  J pipes installed?

The closer to the engine the more effective they are but mine are just behind the rear subframe and work well. I was going to mount them just in front of the diff if I couldn't for them by the rear muffler.

I still would like to try a helmholz to increase the bandwidth but these are fine for now. They made a huge difference even at idle in the car.

I’ve been lucky enough never to have any serious drone in my car. Smart to go for a branch resonator that works so much better than the bullet resonators people throw on hoping they’ll actually do something lol

Prior to this exhaust I had always gone with a single out the back. Never had any drone issues. With the dual it was quite prevalent despite the three mufflers and two cats. Long tube may have changed everything enough to not have any issues but the sikky mid length give so much ground clearance back. Always a give and take somehow.
94 BB SBE LS1 Full Mamo top, MS3ProU with TC, RONIN 8.8 and another custom dual 2.5" stainless exhaust. Too loud, need bigger muffler....