Learn me: eliminating exhaust drone with a side-branch resonator

So I had a new exhaust made up for my SE-R with what I hoped would be a decently quiet muffler and resonator:

It looks great and sounds awesome at full throttle, but WOW is the part throttle cruising drone brutal to the point where I don't want to drive the car! I guess this is one of the drawbacks to a DIY exhaust, and i'm kinda stuck with it. At first I had looked into ways to insulate the cabin, but it seems it is really hard to cut out the low frequency stuff with insulation. So I was on the internet and found this:

branch resonator discussion

And it seems like that would work pretty well to fix my problem. Anyone out there ever try making one before?

Nice looking exhaust.

I've never messed with a branch resonator, but when I did the exhaust on the RX-7 it would drone pretty bad. I ended up cutting in a second resonator made out of a glass pack that solved a lot of the problem.

I do like the idea of the branch resonator. In theory, it should work very well by changing the resonance frequency of the exhaust system. Even if you could only move the drone to a different RPM it would help. Might be tough to package under some cars. I'll have to keep this in mind when I do the exhaust on the Roadmaster.

Go ahead, be the guinea pig and then let us know if it works.

Assuming it's sized properly, it should work well.

Anecdotal evidence from S2000 owners is positive. The factory exhaust has a small branch resonator and duplicating it on aftermarket pieces supposedly eliminated most, if not all, of the droning.

Try a google search. this seems interesting: http://www.planet-9.com/cayman-boxster-modifications/50861-exhaust-drone-resonator-fix-design-construction.html

To tell the truth, I've neve heard of a drone problem on a single exhaust - only dual exhaust systems. Everything from different sized mufflers on each side to adding a resonator on one side to wrapping the exhaust or using thicker pipe.

Please let us know if you learn anything!

They come standard on S2000s and work well. Everyone who removes them gets drone. Put a pipe in a pipe that lets you slide the outer pipe in and out until you find a length that gets rid of the drone then weld it at that spot.

Carguy has the right idea,I've read on a bmw forum about a guy that played around with this using an app on his phone to find the frequency and then came up with a spread sheet to plug the required info into(engine size,# of cyl,rpm,gearing etc etc)which then spit out the lenght of tube required to cancel that frequency. Short of messing with all that math a sliding piece of pipe to experiment with should work just fine,based on what he was doing the pipe will need to be maybe 18" to 30" long.

I'll measure it later, but I'd almost bet the S2000 pipe is only about 6" long.

I'll post length and diameter this afternoon and that should be a good starting place.

BTW I love the 2 90 degree bends in your exhaust flow. I'm sure those help a lot.

carguy123 wrote: I'll measure it later, but I'd almost bet the S2000 pipe is only about 6" long.

What RPM does the S2000 cruise at 60-70?

carguy123 wrote: BTW I love the 2 90 degree bends in your exhaust flow. I'm sure those help a lot.

Hey! I like having a gas tank!

http://forums.corral.net/forums/general-mustang-tech/496793-exhaust-drone-completely-gone-12.html#post10903602

I've got a nasty drone on my Mustang since installing the Magnadrone catback so I've kept this corral link around for when I decide "enough is enough".

My dongle is 6" long by 1.75" OD.

I'd post pics, but this site won't let me. I have to first post them elsewhere and then link which causes all kinds of useless threads when the pics disappear.

I can't remember what the car cruises at, but I'm thinking at about 3,500 prm

carguy123 wrote: I'd post pics, but this site won't let me. I have to first post them elsewhere and then link which causes all kinds of useless threads when the pics disappear.

Slightly less useless than when you don't post them at all.

This isn't a factory exhaust, but it is for an S2k.

Ok, here we are a couple months later, and I've been pretty successful. I calculated the resonator to need to be ~37" for 3000rpm, and had one made up:

It did a pretty good job of lowering the tone at the calculated rpm, and the exhaust was a good bit quieter overall for daily driving, but it was ineffective with the throttle open more than about 25%.

From there I did some reading, and it seems like the C4 corvette guys have issues with drone when using a dual-outlet muffler. Some experimenting with a wad of aluminum foil found that the exhaust got a whole lot quieter with a tip blocked off. So it was back to the welder for one last go, and the finished product is great. My android phone's dB meter shows about 69-71db while cruising at 70, which is just about where it was with the stock muffler. I decided to give it a coat of bbq paint:

When the muffler on my now-ex wife's Talon rusted through, I went with a stock replacement Walker Quiet Flow muffler over some cheap Ebay ricer piece... It had one tip open and one almost closed off. It worked great... The pic is of a Walker Quiet Flow for a C4 vette but you get the idea...

To reduce drone on my v8 240sx I put extended tips on it about 4" past rear bumper. It helped a lot at cursing speed.

stan_d wrote: It helped a lot at cursing speed.

I guess cursing speed can vary quite a bit from vehicle to vehicle .

I had the Megan Racing cat-back on my '91 SE-R and it was LOUD. I added a generic ebay "silencer" that fit inside the 4" tip and reduced it to 2" or so and essentially baffled the straight through muffler. It was completely liveable after that.

There's some decent tech in this (long) thread about 1/4 wave tubes to balance out the harsh resonances. http://forums.corral.net/forums/general-mustang-tech/496793-exhaust-drone-completely-gone.html

cghstang wrote:

stan_d wrote: It helped a lot at cursing speed.

I guess cursing speed can vary quite a bit from vehicle to vehicle . I had the Megan Racing cat-back on my '91 SE-R and it was LOUD. I added a generic ebay "silencer" that fit inside the 4" tip and reduced it to 2" or so and essentially baffled the straight through muffler. It was completely liveable after that.

so you made your high flow muffler into a not-so-high-flow muffler and it made it quieter.. interesting.. is this why all the Hondas with the big exhaust tips only have a 1.5" opening in the middle of it?

Nice work on the exhuast. Moving the resonator farther away from the cat might have lessend the beat freq. That branch res work looks good too. Bet someone will think it was a dual exhaust set up that when unfinished..