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PostPosted: 27 Aug 2010, 12:05 
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OK so it's not a loudspeaker, but it is a tuned enclosure for controlling the acoustic output of a sound generator...a stretch I know but perhaps you can humor me. More so than music, I love cars, fast ones. And just like everything else, I like to tinker with them. I recently bought a new Chevrolet Corvette and and while it's stock exhaust note is music to my ears, I want to change it. Now I can spend $1500 dollars and buy an aftermarket exhaust, but where's the fun in that? I closely follow the Corvette boards and recently saw a thread where several of the members (who also love DIY challenges) were modifying the mufflers in their cars. You could dismiss this as simply gear heads who like loud obnoxious cars, but to those of us who enjoy this sort of thing, there is more than just a desire for greater SPL. Anyway, in this particular thread they dissected a stock muffler and determined that the easiest way to get what we gear heads like to call a bad a$$ exhaust note is to drill holes in the interior pipes of the mufflers, which allows a portion of the exhaust gasses to bypass the central muffling chamber and exit directly out of the tail pipes. From those that did the mod, the result is a "throaty growl" at idle with an overall invrease of about 30% in loudness and a greater bass response throughout the RPM range, in other words "it sounds like a 400 hp car ought to." On the downside, there was a bit of a drone at 1500 RPM (think of the Hondas you hear with the hollow sounding tuned mufflers.) We muscle car aficionados do not like that sound, and it gets tiresome on long road trips. Now these guys are mechanically inclined, but few of them were willing to do a lot of experimentation by drilling 3/4" holes in the mufflers of their $50K cars. So, I thought I'd pose the question to the knowledgeable folks on this board who might be able to take a stab at it from a theoretical perspective. What I'm wondering is if there is a more studied approach to the modification from an acoustic point of view that might produce better results. Such as the geometry and placement of the holes, or perhaps a better understanding of the way the gas flows through the various chambers and it's resulting effect on the exhaust note. I have pictures to illustrate the internal plumbing of the mufflers, as well as the mod that is done, but I don't really have any place to post them. If someone is interested in helping out or posting them for the board, I'd gladly email them to you. I also do not have any sound measuring equipment. By and large, the modification of drilling holes in the internal pipe is the easiest , cheapest way to get a better sound (you can do it without taking the mufflers off the car) So, ease of modification is also a big consideration (I mean if I wanted to I guess I could fabricate my own mufflers designed to be exactly what I want.) So, I apologize for posting this automotive question on an audio forum, but if there is anybody who is interested in offering an opinion please feel free to comment or contact me directly. Thanks for your consideration. - mark


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PostPosted: 27 Aug 2010, 22:54 
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Location: Incheon, South Korea
This is a subject I've also been long interested in, but personally, I think if you really want to tackle it, hacking up a stock muffler isn't the way to do it (though a fun weekend experiment perhaps).

The first place I'd start would be the Edelbrock website. Generally not my favorite brand (personally), but I know that a lot of their mufflers use different baffles and vanes at different angles to reflect the sound waves, and they seem to have put some research in to it. I have no experience with their mufflers, but it might be an interesting read to get you started.

I've scratch built a few full length exhaust systems from mandrel tubing, straight pipe and different mufflers. Drone is tough to minimize and still have that raspy sound when you're on the throttle, but it's doable.

If I were going to tackle this project, I would probably start by finding out what my high-way idle engine RPM and load were and then doing some research to determine how I could dampen resonation best at that speed and load, and incorporate the information into designing a full length exhaust system with adequately sized pipe and catalytic converters/resonators/mufflers with minimal back-pressure (I won't even touch scavenging or tuning headers).

That research is the hard part, and will probably require more than just cutting open and drilling holes in mufflers... you'll probably have to start building them youself. I used to have a couple good links that talked about some of the math behind tuning resonator chamber size for different engine RPMS and exhaust manifold types (equal and unequal length headers, "shorty" exhaust manifolds etc). I'll see if I can track them down.

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PostPosted: 28 Aug 2010, 01:58 
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It just occurred to me, the people you really need to talk to here are the room-treatment guys. How different sounds and wavelengths react to different angles/materials etc.

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PostPosted: 28 Aug 2010, 05:23 
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Some reading for you:

http://en.wikibooks.org/wiki/Engineerin ... r_Mufflers

http://en.wikibooks.org/wiki/Engineerin ... top_Filter

And just because I KNOW it's going to come up at some point, some information about back pressure and delta pressure:

http://my.prostreetonline.com/forums/sh ... php?t=1639

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PostPosted: 28 Aug 2010, 14:57 
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Cold Mac, thanks for the info! You've given me a lot to think about (more than I really understand)

Here's a link to the cross section of a stock C6 muffler. Tell me if I understand this correctly. The sound waves enter the muffler from the top right pipe. It looks like the first chamber is a Helmholtz resonator, and is the largest of the three, so I assume it attenuates lower freqs. The central chamber acts as both an absorption chamber and perhaps a mid-range resonating chamber as well as passing a small amount of energy directly into the exit pipe. Finally whatever sound energy is left enters the last resonator (far left) which is the smallest and presumable acts on higher freqs.

Image

What everyone does for the mod is drill two 1/2" to 1" holes (depending on how loud you want it) in the exit pipe just about at the point where the sticker says "C6" Then they seal up the access hole in the outer skin of the muffler.

Image

You know what...never mind. I really don't understand all this. I think I've got it all backward. I'll repost after I do my homework. The Corsa site is also informative.

BTW, are you still in the ROK? I'm in Beijing, we're only one time zone apart! It's late, thanks again - mark


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PostPosted: 29 Aug 2010, 00:01 
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Joined: 18 Feb 2010, 05:14
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Location: Incheon, South Korea
The large chamber on the right does indeed appear to be a helmholz resonator, however it only seems to be setup to act on whatever frequencies escape from the perforated sections through the middle chamber.

I'm afraid this is where I'm not going to be much help to you. I've pieced together a couple exhaust systems from mandrel elbows and loose mufflers, and I feel like I did a pretty good job matching them to the vehicle (except for that kid who just wanted straight 3" pipe coming out the side of his primered hoopty), but I've never had the time or inclination to look farther into the physics (I may if I ever own another highway vehicle). There are some pretty good mufflers already out there in the aftermarket.

Yeah, still in the ROK. I'm in Incheon (north west) right now, teaching English (well technically I'm also a supervisor, but "supervising English" just sounds dumb). What are you doing in Beijing? More specifically, what are you doing with a new 'vette in Beijing?

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