Looking for help with exhaust gas accoustical questions

[allblowdup]

I have a muffler that I have built for a a sport boat. Problem is, it works to well. The boat is way to quiet. Some how I think i have actually got the dimensions correct to cancel out most of the desirable exhaust note. I have look around and cannot find anywher to get any answer on what I should change. I though that maybe someone here might know.
It is a v8 engine and the exhaust goes into a 6" dia tube that is 3 ft long that is mounted accross the transom of the boat. It has two inlets that are 4" in dia and are 28" apart c to c. The oultlets are 4" dia alos and are centerd on the tube and are 12" apart.They are all fluch mounted. Have I fluked out and got the distancese so that the exhaust waves are colliding and cancelling each other out? I can shorten the inlet ends of the tube 2" and the outlets 4" if needed. Or I can add length also.

Mark

 

[MiketheEngineer]

So what's the problem. Most boats make way too much NOISE as built.

Figure out what you did right, market it and sell it!!

 

[allblowdup]

Yes I would agree, It seems as though the correct sound and the amount of it is just as important as the look of the boat.
From what I can tell it is the equal distance from the inlets and teh distance between the oulets that are the reason.

 

[MikeHalloran]

What do you do about the heat radiated from the muffler?


Mike Halloran
Pembroke Pines, FL, USA

 

[allblowdup]

It is a wet system so there is an amount of water flowing through it.

 

[MikeHalloran]

Ah. The way that the water flows, and pools, can have a major effect on the way the muffler works. Wet mufflers, on the inside, tend to look very different from dry mufflers.

A drawing, showing the interior details as well as the connections, might help us better understand your problem.


Mike Halloran
Pembroke Pines, FL, USA

 

[allblowdup]

here is a very crude drawing. The inside is completely open. The inlets and the outlets are flush mounted. I know that by moving the outlet tubes toward each other the noise increases. In this application I am not able to do that though. If in the center is where the wave fronts collide and cancel then I could weld another smaller tube from the center out and make a U turn to exit at the same point as the outlets on the inside?

 

[allblowdup]

Here is a top view with my proposed modification.

 

[MikeHalloran]

There's a fair chance that the exhaust is unusually quiet because it's flooded. That would not be good for the engine.





Mike Halloran
Pembroke Pines, FL, USA

 

[SomptingGuy]

At the exact centre, waves at all frequencies will reinforce. But that's not generally where you would be measuring radiated noise. In the far field, the superposition of waves will depend on their wavelengths and the distances from the source of the measurement location. ach frequency will superimpose differently because of the different wavelength. If the sources are nearly coincident, they will act as a single source with double the amplitude.

- Steve

 

[allblowdup]

I would agree although because to the sound sources supply the pulse wave at 180 deg intervals would it not not cancel instead of reinforce?

 

[allblowdup]

For some reason the picture ended up upside down. So the exs yube does not flood although it does have water through out it. The risers from the headers on the engine are much higher and the outlets are lower that the inlets by at least 2 ".

 

[MikeHalloran]

L/R combustion events are not normally simultaneous, so acoustic cancellation doesn't really seem to fit for me.

Maybe you've got some kind of cavity resonance, or anti-resonance, going there, but it's hard to evaluate, since we have no data on the gas flow, the water flow, the noise vs. rpm, the system geometry in 3D, etc.




Mike Halloran
Pembroke Pines, FL, USA

 

[allblowdup]

I see. On this v8 engine the cylinders fire one cylinder on one side then fire one on the other and so on. The frequency or rpm shouldn't matter if I have 180 deg out of phase waves colliding. I guess what I am asking is this plausible? If so what can I do to change it to 1. verify what is happening 2. change it to achieve my desired effect.

 

[MikeHalloran]

That's a very unusual firing order.

Mike Halloran
Pembroke Pines, FL, USA

 

[allblowdup]

Nope that is the GM LS firing order. 18726543
1 2
3 4
5 6
7 8

 

[MikeHalloran]

... which puts 3,1 and 2,6 firing in sequence.

Mike Halloran
Pembroke Pines, FL, USA

 

[allblowdup]

Ah I see your point. I knew about 2,6 but didn't think about 1,3. So I guess I should drop that train of thought.

 

[allblowdup]

I am going to try a little experiment. I am going instal a third ex outlet in the middle between the two others. I should be interesting if it makes alot of difference when I either plug it or leave it open.