have you tried this to eliminate exhaust drone?

[Elombard]

I recently installed a used set of B&B headers with heat on my 3.2.

the system came with a crude looking flow master type (not a flow master) 2 in 2 out muffler ('merican V8 style) with Supertrapps welded on the outlet pipes.

In spite of the looks (and weight) I was excited because I figured I could change disc's and quiet the exhaust down and then remove the STs for track days and have almost an open exhaust.

The muffler is quiet enough out side - not even too bad when I remove the caps on the Sts but it drones like a muttha from ~2K to 3K rpm. Initially I figured 'no worries' I will play with the disc's. I tried two or three set ups, including capping one outlet and removing the ends (open system). I was very surprised, the drone moved around slightly but did not dissipate to an acceptable level.

So I went to the super trapp website and blog and found a 'guru'. he said that drone is generally not affected much by the end of pipe application, its more a function of the pipes. he suggested I try wrapping the pipes.

This made me think - drone is often associated (based on minimal Internet research on these forums) with '2 out' header systems (although I had SSIs with 2 in 1 out banana on a 3.0 and I don't remember any drone?) so maybe its the long header pipes 'vibrating' at a certain frequency? perhaps some of the exhaust pros will weigh in?

I am looking at 'cheap' header wrap to experiment - wondering if any one else has noticed a difference in drone after installing heat wrap?

Also wondering if a support bar from each header would change the frequency and move the drone way up, way down or eliminate it?

Also wondering if the B&Bs are particularly susceptible with their relatively long primaries.

Your thoughts appreciated.

[wayner]

With a certain miata 2 out exhaust, the common fix those guys use is to insert a baffle in just one of the outlet pipes. It works on those cars in that configuration.

I wonder if you car needs on on an inlet pipe instead?

[Joe Bob]

I used one on my single 3 inch outlet...worked well.

[Elombard]

Interesting, I wonder if inserting a baffle in the end of one of the headers where it bolts to the muffler (in the inlet to the muffler as wayner said) would change the harmonics (if that is what we are talking about). I wish those things were less than $100, I would rather spend the $100 on a track day :-).

I also want to try just removing the muffler for a short time and see how that changes it. Like to find some cheap mega phones for track days.

[KTL]

Do a search for Jack Olsen. He ended up cutting the collector and shortening up the header itself, as well as changing tail pipe layout. He'd found that resonance was a function of both in his research?

I don't doubt the tail pipe exit style (straight, angled, square, whatever) helping as well. I recall a lot of GM cars back in the 90's had a scalloped end on their tail pipes and it made for a unique (crappy IMO) sound. Also the 4th generation Camaro Z28 that came out in '93 had a strange rectangular tail pipe tip. It also made for a unique thumpy sort of tone.

[Elombard]

Gives me chills thinking of cutting up the pretty headers.

I did not remember when JO had B&B on BB1 he worked on this issue. I will get searching.

[175K911]

Back when I ran the B&B system with 2-out on an old 911, the only way to get rid of the resonance was to cap the right side outlet for the street.

And when I had SSI's and a Flowmaster 2-in 2-out muffler on a previous Carrera, same thing. Only this time I experimented. Maintained the 3.5" outlet on the driver's side, but welded a restrictor (essentially a washer with a 3.5" OD and a 2" ID) into the tailpipe on the right side. still looked like a 3.5" tip but was choked down internally. Resonance range narrowed and amount of resonance was significantly reduced. Ultimately I made a cap for the right side that still looked like a 3.5" tip but with the cap at the muffler end of the tip essentially the tip was a dummy. Voila, resonance gone.

In a conversation I'd had with John Daniels at SSI some 20 years ago he suggested that the 911 engines are very sensitive to outlet diameter sizing. The factory sport muffler was a pair of 2" outlets, and much more exhaust pipe volume will resonate. He's actually the person who suggested putting a washer as a restrictor in the right tailpipe. B&B likes to make the mufflers with two really big tips so I suppose that's why they have a resonance point.

I'm not an expert on sound wave nor pretend to be, just passing along my own experiences.

[Elombard]

Thanks Ed,

Yah I was really suprised when I capped off the one side that it still droned. you can see straight through this muffler on the out lets, I wonder if the inlets and out lets were switched if it would help. I have an old flow master, I may try home brewing the pipes to see if its better.

I wonder if I capped one side and restricted the other side if it would make any difference?

[HawgRyder]

Another old trick...if you disconnect the muffler from the header...and insert a piece of steel (thin bar stock?) into the pipe...so that it fits snuggly either in a vertical or horizontal position...imagine dividing the pipe in half by this....and try it again.
What this does is stop the circular flow of gasses in that section of pipe...and...it divides the pipe into 2 pipes so the the frequency would be twice the original.
The nice ting here...is that there is very little reduction in pipe flow capability.
This used to work on old aircraft engines and some older cars.
Bob

[175K911]

Originally Posted by HawgRyder

Another old trick...if you disconnect the muffler from the header...and insert a piece of steel (thin bar stock?) into the pipe...so that it fits snuggly either in a vertical or horizontal position...imagine dividing the pipe in half by this....and try it again.
What this does is stop the circular flow of gasses in that section of pipe...and...it divides the pipe into 2 pipes so the the frequency would be twice the original.
The nice ting here...is that there is very little reduction in pipe flow capability.
This used to work on old aircraft engines and some older cars.
Bob

Sounds like the philosophy behind the "delta flow" dividers inside a Flowmaster muffler. Supposedly by dividing the sound wave in half, then having it merge back onto itself helps cancel noise and resonance without negatively impacting flow.

[Bill Verburg]

There is some resonance that comes from the single walled construction of aftermarket mufflers, thats' why factory used double walled w/ shape reinforced/stiffened mufflers, take a look at a 997 muffler for an example, wrapping the outside can lead to moisture condensation and accelerated corrosion

There are several best practices for the muffler internals one is is to dump the gases into a mixing chamber like the this GT3 w/ an acoustic x=over between the banks


and this BMW which is mostly flow separated but has an acoustic link between the banks




another is to keep the banks totally separated like this 993



A third is to use dead ended resonators

[911pcars]

I'm waiting for someone to fabricate a current version of this exhaust system created by Steve Weiner (Rennsport). The cross-over tube joins both sides and changes the resonance of the exhaust pulses - a common strategy performed on front-mounted V8 engines.



A variation of this would be some way of varying the ID of the tube using different "donut" discs sandwiched between exhaust flanges in the crossover or using the following elec. exhaust cutout to effectively vary the crossover pipe ID. In addition to modifying the resonance level, there may be some tuning opportunities for power.



Sherwood

[KTL]

I also recall Jack saying his design involved reducing the outlets to 2-1/4 dia.

[Elombard]

Great stuff - some possible inexpensive experiments to try.

I wonder if making a large washer with a 2 and 1/4" hole that would bolt in between the collector and the muffler would help. I definitely will try the splitter technique that hawg rider suggested with some scrap metal.

[wayner]

Dead ended resonators look like the same technique used to stop banging water pipes in a house (water hammer).

Essentially a capacitor.

On one of my vintage race bikes we simply inserted a wider diameter pipe mid way down the header pipes to reduce resonance that was causing backflow through the carbs.

(Head pipes exited small, got fat and then went back to small before hitting the collector).

These dead ended resonators remind me of that but later in the exhaust path.

[Quicksilver]

I subscribe to the theory that I want my car as quite as possible because I USE the motor and I don't want to advertise to police or to idiots in a Camery that think they want to race. (Why does everyone lose there mind around this car?)

A month ago Steve Wong posted that someone had fashioned a Bursch style 2 in 1 out using a full size turbo muffler and it was very quiet and only a 4 HP drop (I believe his GT3 experimental setup had the same HP drop). I will be building one of these once I have time and some more argon for the TIG.

I have a few setups for the B&Bs:
- The Dynomax 17725 dual in / dual out. To loud for me. Capped one outlet and still a bit much so put the shortest glasspack I could find on the single out and it made it livable. You floor it and when you hit 2800 RPM everybody knows you you are on the throttle. Both uncorked is real fun on a track day.
- The Magnaflow 11385 dual in / dual out. Was told that it was quieter then the Dynomax. If it is I can't tell...
- The stock B&B 2 in 1 out muffler. Quieter but the sound is just isn't musical. I think it is just ricey and unpleasant. (Wife thought it was ok and not to loud but it isn't her toy!) (This muffler will be for sale once I get a setup that I like...)

[Elombard]

Wayne do you have any drone issues with these set ups? I might be interested in buying once you get your new set up. I dont want to attract attention and I do want to be able to drive long distance with out the drone.

[wayner]

I think you might have meant Bill?

[tobluforu]

Here's a pic of what I like to call, the Franken muffler. This muffler is now about 11 years old and looks a tad different than what you see here. When I first made this it was just the Bassani muffler with no glass packs. The problem is that it droned like crazy and was ear splitting loud. I then added the glass pack, drone gone, not loud and out the five mufflers I have had, it is the best. I have had a stock muffler, bursch triple, Dansk sport and the M&K GT3 style muffler, which droned like crazy as well. This is by far the best muffler I have had. When I went to the dansk sport I thought I had found the best muffler ever, but then I had to replace the registration sticker and the only way to do that is to remove the muffler. When it was off I decided to try the fraken one more time for the hell of it, and I never went back. Sold every other muffler I had. This muffler is now wrapped, has no more supports that you can see, and has different tips.

[Elombard]

Hmm I am suprised the glass packs did not fatigue with out support - looks like a good solution.

Wayner - I was talking about Quick Silver - his name is also Wayne.