Bonneville America Forums Forums Technical Forums Exhaust baffle packing and back pressure?

I have specialty spares and I want to tone them down a notch or two. I was planning pulling the baffles and wrapping them with some muffler packing.

two questions:
Will this work?

and

Will this change the back pressure so that I'll need to adjust my settings?

You can wrap them with no significant change in back pressure.....Angelis

packing will reduce noise but you can only put in so much, the baffels is what really cancels out the sound, check this out.
How Can You Cancel Out Sound?
The key thing about sound waves is that the result at your ear is the sum of all the sound waves hitting your ear at that time. If you are listening to a band, even though you may hear several distinct sources of sound, the pressure waves hitting your ear drum all add together, so your ear drum only feels one pressure at any given moment.
Now comes the cool part: It is possible to produce a sound wave that is exactly the opposite of another wave. This is the basis for those noise-canceling headphones you may have seen. Take a look at the figure below. The wave on top and the second wave are both pure tones. If the two waves are in phase, they add up to a wave with the same frequency but twice the amplitude. This is called constructive interference. But, if they are exactly out of phase, they add up to zero. This is called destructive interference. At the time when the first wave is at its maximum pressure, the second wave is at its minimum. If both of these waves hit your ear drum at the same time, you would not hear anything because the two waves always add up to zero

Inside a Muffler
Located inside the muffler is a set of tubes. These tubes are designed to create reflected waves that interfere with each other or cancel each other out. these baffels may be positioned so the gases and sound go thru at different areas by bouncing of walls within the pipe. In the end an appropriate muffler will have cancelled out most waves in phase and you only have left out of phase waves which cancel out noise. I'll look for a webite tomorrow that had a really good example of how it all works, I modified a set of pipes even building my own baffles from this site, sounded ok and didn't have any problems

I don't know what those pipes are like, but i packed my thunderbike pipes almost all the way to the muffle/header joint without the slightest change in performance. And it quieted them down a good bit. Just right to my ears. Now if i ride w/o earplugs it doesn't sound like the shuttle taking off even during a slow start like it did before. The t-pipes are one long tube about 2" in diameter that runs the full length of the muffler. The last foot near the end has spacers between which the stock packing resides and there are of course many holes for the sound to soak thru. The rest of the tube only has a few holes that aren't round but are called louvers. Kinda like gill slits. And there are very few and only on many 1/3 to 1/2 of the length. However, those few holes help divert sound waves into the packing i put in, but theres also i believe a lot of sound attenuation just due to the space between the muffler and inner tube being packed and therefore quieting down the resonance of the metal. Overall the effect is quite good. i left maybe 1/3-1/4 of the length unpacked only because i didn't have enough packing to do the whole length. So it could get even quieter. Anyway, on mine it works well and if your pipes are similar it will work.

I got a e-mail response from Dale at Specialty Spares about toning down the exhausts. If your baffle material is still in pretty good shape, you can tighly wrap another layer over the top of what's there. Either tape or wire in on and reinsert. Be sure you buy 4-stroke exhaust wrap not 2-stroke. The more you can wrap the more it will tone it down and still have a nice throaty sound.

Hey Bob,

Awesome, that was exactly what I was planning on doing.

thanx,
Kurt