At first it seemed as if we built the perfect exhaust system. At the exhaust shop and the first 2 mile ride home I didn't hear a drone inside the car. I know now thats because it doesn't drone at full throttle or idle, which where the two positions of the gas pedal on the way home. As it turns out it drones bad inside the car at many postions in between.
I installed a 12" resonator behind the axle and in front of the crome tip of the noise. The only thing it did was make the neigbors a little happier, but the drone is still there. Its a little different tone but still bad, maybe worse inside.
Next I took a 8" piece of 3" exhaust tube and cut it 3 times on the end about 2" deep. I folded these tabs in at various depths to block some air flow to build a little back pressure. Slid this piece into the 4"x 16" crome tip and drilled a self tapping screw through the bottom of the tip and into the inner sleave. This solved the highway drone but a touch over idle (parking lot driving) the drone is back.
My next plan is to make a spring loaded flap that I can change springs on to adjust the back pressure. Of course this isn't good from a performance stand point, but something has to be done until I think of something better.

As it is right now, you can't hear yourself talk in there without a baffle installed.
You guys got any ideas?

Clifford shorty headers
2 2-1/4" pipes that Y together 4" behind the trans cross member.
3" pipe through a cutout and into a 3" 50 series Flowmaster
2.5" over the rear and into a 12" resonator and out a 4"x16" crome tip

http://tinyurl.com/ywxaks This picture dosen't show the resonator, but its beside the gas tank now.

_________________
67Cuda,FAST EZEFI,340cu,CR=10.25,RollerCam&Rocker (XR268HR,#20-810-9)(#1622-16)(EddyRPM#60779,#7576), (MSD6AL,#6425) A904, GearVendorsOD, 8 1/4,3.55:1, ClassicAir

Your exhaust pipe is too large in diameter. It is going to drone no matter what you do or how many resonators you add.

_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.

Maybe 2 exhaust tail pipes to keep the flow up and cut down the diameter of each?
Where does the drone come from? It sure seems like its coming up through the floor under my feet.

_________________
67Cuda,FAST EZEFI,340cu,CR=10.25,RollerCam&Rocker (XR268HR,#20-810-9)(#1622-16)(EddyRPM#60779,#7576), (MSD6AL,#6425) A904, GearVendorsOD, 8 1/4,3.55:1, ClassicAir

The drone's coming primarily from your overly-large undercar exhaust pipe and muffler. If you have room under the car, you could replace the longest straight length of pipe with a lonnnnnnnng round glasspack, and that'd help materially...

_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.

Replace the muffler. The drone comes from sound resonating at a frequency that is displeasing to the human ear- not because it's loud.

Oh, believe you me, there is very definitely such a thing as loud exhaust drone. Muffler characteristics, muffler placement, and exhaust pipe diameter all influence the presence, intensity, and RPM range of exhaust drone.

_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.

flowmasters are noisemakers. There are way better mufflers. Didn't you know that flowmasters are loud? http://www.magnaflow.com/02product/shop ... ain&id=303

_________________
63 valiant v200 Wagon 9.5, Schneider cam, E.I. 22 mpg all day

As well as air cleaner style (open vs. closed/tuned...), intake manifold style and type (open plenum vs. dual plane low runner...), position of your crossover pipe (H vs. X...)

-D.Idiot

Those'll influence induction noise (which can be loud under some circumstances) but not exhaust drone.
Definitely on dual-exhaust systems...not so much on single-exhaust systems

_________________
一期一会
Too many people who were born on third base actually believe they've hit a triple.

The 2-1/4" tailpipe I've got has the drone. I didn't have that with the 1-7/8" Tail. This is 2-1/4" from Manifold to the bumper.
Try a smaller tail pipe. It shouldn't cost you power. Everything is cooled off back there, so maintaining velocity should drop the drone.
The exhaust note will be sharper, similar at WOT blasts but not as low pitched.
Intake sound can contribute as DusterIdiot says. At certain RPM I can hear the intake and not the exhaust. (Open Element.) It makes a drone of it's own. Some of the remote intakes and Ram-Air intakes can get rid of that. Just money!

CJ

_________________
Part of Tyrde-Browne Racing

Which is where I think Bren will have to go to change the note a bit and still keep his 'freeflowing' exhaust... something like cut the uni-tube off, making it the 'x' pipe, then going to two smaller tubes to change the resonance frequency a bit, then using a good flowing muffler... not glass packs...

FYI... an old Bosch Automotive Handbook used to define 'resonator' as "the resonance chamber, or resonator operates by reflecting sound back toward the sound source and by multiplying the number of sound emission points"... so the resonator can't be a big empty tube.. you got to get a pipe with some smaller holes inside it to work like that (unless you're a Honda Accord... LOL)






There's a certain 'pushover' point for the Hpak Duster with 3.55's.. and definately after 23-2500 rpm in any gear it just get's nice and loud under the hood... I'm pretty sure Bren had a certain 'point' driving where that also occurs... (after we solve one problem, the other's going to crop up anyway...)

-D.Idiot

I've heard people talk about standing waves in exhaust systems that cause this drone. If I was able to break up that wave maybe that would help.
My idea thats cheap to do.
Cut along the bottom of the 3" pipe from the cutout to the muffler with a wizzy wheel, insert by fishing in a "wave disrupter", and weld along the bottom to seal it and hold it in place.
http://tinyurl.com/yteg7k 12or14GA

Shouldn't slow the flow to much. The design shown for after the cut out and a different angled design for the back half of the Y-pipe and the cutout. (likely not inserted from the bottom but 90* up on the passenger side) This design would keep use of the cutout as not to block it if open.

_________________
67Cuda,FAST EZEFI,340cu,CR=10.25,RollerCam&Rocker (XR268HR,#20-810-9)(#1622-16)(EddyRPM#60779,#7576), (MSD6AL,#6425) A904, GearVendorsOD, 8 1/4,3.55:1, ClassicAir

Nope

_________________
67Cuda,FAST EZEFI,340cu,CR=10.25,RollerCam&Rocker (XR268HR,#20-810-9)(#1622-16)(EddyRPM#60779,#7576), (MSD6AL,#6425) A904, GearVendorsOD, 8 1/4,3.55:1, ClassicAir

http://www.flowmastermufflers.com/50_delta.html

Flowmaster: "It is milder than the 40 Series mufflers on the outside of the vehicle, and is significantly quieter inside the vehicle"

Guess not


Those that helped design this system, what muffler are you using?

_________________
67Cuda,FAST EZEFI,340cu,CR=10.25,RollerCam&Rocker (XR268HR,#20-810-9)(#1622-16)(EddyRPM#60779,#7576), (MSD6AL,#6425) A904, GearVendorsOD, 8 1/4,3.55:1, ClassicAir

What RPM is resonant? It should be a fairly narrow RPM range if it's really a resonance issue. If it's just too loud at most speeds it's not resonance per se.

I was thinking a Helmholtz resonator might be just the thing. Since you have the cutout I would make up a closed end pipe to bolt on. I would make the length easily adjustable until you find the right length to kill the resonance. An open pipe with a movable plug would be ideal for testing. The plug could be another pipe with one end closed off slip fit inside the 3" or whatever size comes off the cutout.

Otherwise I'd use a Dynomax Ultra Flow. It was great with a single 3" exhaust behind my '74 Satellite's 318. http://tinyurl.com/2726jj

_________________
Joshua