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Originally Posted by Peer
Well, I'm still confused -- can't figure out how that stock muffler really works internally. Besides, comparing the pictures, there's obviously some difference between the 986 and the 986s muffler (assuming they both are stock).
>SNIP
-- peer
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If you want performance, then why not just gut the thing to get maximum flow? Any free flowing mods like that though or any of the free flowing aftermarket mufflers will give you a resonance problem (buzzy). If you've got a stock 986, then why not just get a cheap, used S muffler off the net and install that? It would give you enough flow for a bigger engine but without any resonance problems.
I don't know muffler design but I do know subwoofer box design. Basically the insides of this muffler look a lot like a ported bandpass subwoofer box. Cut open any of those Bose subwoofer boxes with 6.5" drivers that produce a decent amount of bass and you'll find the same kind of design.
Exhaust gas expands into the first chamber through the perforated holes. This probably helps to reduce any pulses in sound from the engine. It then goes through a pipe to a second chamber. This is probably really the key to the whole system is that pipe between the first and second chambers. I imagine it's the same as a tuned port on a subwoofer box. The pipe was designed to an exact length and diameter to reject certain frequencies. I think this is why you don't get resonances with the stock system.
Again, I've seen the exact same kind of thing in Bose enclosures. They try to do a lot with very little (small, cheap drivers). Mostly they do this through carefull engineering design of the enclosure, the chamber sizes, and tuned ports between the chambers. This muffler looks to be designed using the same concepts.
If you would really want to improve flow while having no resonances like stock, then you'd almost have to go with the same kind of design except use larger diameter ports and adjust the length appropriately to get the same resonant frequency rejection. Someone in the aftermarket may eventually develop something like this, but it won't be cheap. Still you'd have a lot of GHL exhaust owners standing in line to get something without all of that buzzing.
Kirk