Exhaust resonance help - with fancy diagram!

The Challenge wagon has a sweet "new" exhaust system. It's loud. It's mean. It attracts all the wrong attention.

The only problem is it drones like hell at low RPM. It probably has something to do with my decision to use a single, rusty, large-diameter side-exit muffler. Hey, it was free!

The setup looks like this. The exhaust dumps straight out of the muffler (no turn down) just before crossing the frame rail. No exhaust tip yet. That might be why it's so loud. What do you think?

The muffler pictured is not there anymore. It was too loud, so I replaced it with an older rustier one, which is basically two offset perforate tubes. I think it's from a circle track car. No idea.

Get the exit point at least near the side of the car- you want the sound out of there, not bouncing around under the floor.

After the headers I'd want a cutout (electric or manual) and then stock exhaust all the way to the back. Best of both worlds in a wagon you want civilized then flip the switch to a race car. My $0.02.

Stock exhaust is long gone. I had to cut away whatever wasn't too rusty to unbolt.

From where the muffler pokes out the side, I have about 18" along the rocker before the rear tire to work with. Or I put little bullet mufflers right after each header. But I'm getting sick of working on my back under this thing.

Just run the pipes straight up through the hood. Dump them about 18" above the roofline and that should sort out the resonance

It looks like you have room for some glasspacks or a couple Speedway $9 specials after the headers. It would help, but you're not getting rid of the drone if the exit is under the car. How about a bosozoku kickup out from under the rocker?

Use double walled tubing so the outside stays cool enough to put matching woodgrain on the tips!

How street legal are bosozoku pipes? I've been seeing them in games and online lately, thinking they're definitely weird enough I might like them on the miata.

In reply to RevRico:

As far as I know, not any more illegal than any other exhaust modification. I would keep them relatively sane though, if only so that you can walk around the car without feeling like you're jousting with it

Yeah, just get the exit out from under the car, that will do wonders for your driving experience.

Robbie wrote: Yeah, just get the exit out from under the car, that will do wonders for your driving experience.

This and a newer/better muffler. I found a pair of used magnaflow mufflers off a mustang for $50 for the pair. They have been collectively on 4 cars now. Maybe check with muffler shops if they got anything decent in their scrap pile? You might could even get some decent piping too since scrap metal pricer are so cheap now

Getting the dump out past the pinch weld will quiet it down in the cabin, but drone is a resonance thing you'll need to deal with.

The cheap and lightweight solution is to build a resonator to cancel out the resonance drone. Good guide here http://www.performancetrucks.net/forums/gm-engine-exhaust-performance-21/how-build-exhaust-resonator-tube-eliminate-drone-489463/

Branch resonater is what u want

In reply to BrokenYugo: My 68 GTO had those tucked up inside the quarter panel from the factory.

In reply to BrokenYugo:

Those are exactly what the bosozoku guys use to eliminate low hum. I plan on running a lot of those on my 1uzfe. http://www.bosozokustyle.com/page/144/

Any chamber style muffler will resonate (Flowmaster, Thrush Welded, Spintech, etc). Getting turndowns out from underneath the car will also help. Getting the exhaust to run straight out the side will help.

Going cheap, highflow, and cutting back on drone, I would go with a glasspack(s). Dynomax mufflers (Super Turbo) have less drone and flow well, but may put you over budget.

A long time ago, I had a Flowmaster system on my 78 Z28 with the dual in/dual out 80 series that mounts behind the axle with tailpipes. Sounded good on the outside, didn't hear much on the inside. I later went to Super 40's with turndowns under the car. Sounded awesome, but the mufflers barely cleared underneath and the drone was bad down low that in the staging lanes with a helmet on I could not hear anything else around me. I recently swapped to Dynomax Bullets. This is a modified 350, about 350hp roughly.

I'm still a fan of the Thrush Glasspack. Cheap, tough, easy to mount, good flow. I found back in the salted WI winter roads they would last longer than the others.

Summit Racing has a reasonable glasspack also (it's louvered vs. Thrush core).

I'm wondering if you could run two right out the same side off of your current pipes in place of the circle track muffler that you have. I previously had a 80 Dodge Power Wagon with a 360, that I ran headers and due to the dual tanks - had to run both exhaust pipes down passenger side - through Thrush glasspacks and out straight before the rear tire. No drone, tough idle, and a bark on the gas.

My sons 2000 Impala 9C1 with a 3800 - we first put Thrush Chambered mufflers on that ran out the back, and even with a long resonator (glasspack) up front - the low rpm drone was rough. We switched to Dynomax Super Turbo's - which have a nice idle rumble and sound great in the throttle but don't resonate. They sounded good going 128mph at the Ohio Mile last year in particular.

Drone can also be affected by how the pipes are positioned relative to the floorpan. In the Jeep, when I swapped from a Flowmaster 70 to a big Magnaflow, I actually gained a drone I didn't have before, even though it was quieter overall and sounded better from outside.

Adjusting the tailpipe positioning under the cargo area cut the drone by probably 80% (there's still a tiny bit, but not enough to be worth doing anything about until I replace the whole exhaust with a less janky one).

Update:

Scientific proof that sidepipes make everything better. It sounds way, way better now! And looks kinda cool, I think.

https://youtu.be/pBXVgZNEDN8