oem muffler hack

[CJ_Boxster]

Originally Posted by Peer

Kirk wrote:
> The arrows show the flow of exhaust gas through the system.

Okay -- so I can follow the three first arrows, but arrow #4 makes no sense to me.

-- peer

in #4 you cant see it but there is another pipe that leads out to the exhaust Tip, You can see it better in Tool pants picture.

[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).

The reason why I'm eager to know how a stock 986 muffler works, is that I recently bought one to try do a hack myself... but before cutting it open, I thought that I at least needed to know how it works -- but so far no luck.

I might need to emphasize that I'm looking for performance enhancements rather than sound, although I wouldn't mind if also the sound was improved (as long as it's not too loud or sound buzzy).

-- peer

[ChrisZang]

Hmmmmmm,

for me it seems that the "do it yourself PSE" http://www.babblers.org/forums/showthread.php?t=9388 seems to be the much "cleaner" solution.
You get better sound, you can switch it on and off and you don't have to gut your exhaust

[Kirk]

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

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

[szentej]

"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).

The reason why I'm eager to know how a stock 986 muffler works, is that I recently bought one to try do a hack myself... but before cutting it open, I thought that I at least needed to know how it works -- but so far no luck.

I might need to emphasize that I'm looking for performance enhancements rather than sound, although I wouldn't mind if also the sound was improved (as long as it's not too loud or sound buzzy).

-- peer"

By labeling the chambers of the first picture of this thread from the left, one through five. The gasses from the left inlet pipe travel through a pipe and are released into chamber two. From there they travel through a pipe and are released into chamber five. Then through a pipe (which you cannot see) into chamber three and out the exhaust pipe. The gasses from the right inlet pipe travel through a pipe and are released into chamber four. From there they pass through a pipe and are released into chamber one. Then through a pipe (which you cannot see) into chamber three and out through the exhaust pipe. The exhaust gasses from the right and left sides do not interact with each other. They each have a separate path through the muffler.

You probably will not gain any performance on making mods to the muffler. Only increased sound.

Originally Posted by szentej

The exhaust gasses from the right and left sides do not interact with each other. They each have a separate path through the muffler.
.

No. They do interact in the middle of the muffler. That first pic is misleading. The left and right inlets connect in the middle; it is straight though from end to end. perforations along that length determine where the gas spills out into which chamber.


In the 986 S, there are gaps along the main length also, not sure about the base 986.

[Peer]

Many thanks to szentej for explaining the muffler flow -- so yes, I now finally get it.

szentej wrote:
> You probably will not gain any performance on making
> mods to the muffler. Only increased sound.

I'm not sure if you have seen this a dyno chart for a hacked muffler. I'm just curious what they exactly did to it, since it's quite an improvement, especially the torque.

-- peer

[Kirk]

Originally Posted by boxsterz

No. They do interact in the middle of the muffler. That first pic is misleading. The left and right inlets connect in the middle; it is straight though from end to end. perforations along that length determine where the gas spills out into which chamber.

Correct... the pipe in the very middle equalizes the pressure between the two sides that come into the muffler. Many motorcycle exhausts (like Harely Sportsters) have these equalizing pipes that connect the two exhaust outlets together.

One more correction... there are only two chambers on each side. After gases flow into the second chamber they are routed through piping out the exhaust tips. It's hard to see in the photos. I changed the brightness of the Tool Pants photo before I posted it to try to make it easier to see, but the pipe from the second chamber looks like it goes straight to the exhaust tip and not into that central chamber. I believe the central chamber does not receive any exhaust gases, but only houses the various piping. At least that's the way it is on the S muffler, you can't really see it in the picture of the standard Boxster muffler.

Kirk

[Tool Pants]

Here is a second picture of the first (base Boxster) muffler with the packing removed.

The inside of the base muffler looks the same as the S muffler in the second picture. The only difference I can see is the spacing of the 2 tubes that exit the can to the tip(s).

Originally Posted by Kirk

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.
Kirk

Yes, I think so. Those chambers tune out drone in the 2-3k range. Though I like the sound, I think the 986 design really hurts flow. My guess is, what makes it quiet is the sound wave cancellation of the opposing cylinder banks which doesn't happen in the 996, 997, 987 and Cayman designs.


I do not think a max flow exhaust is necessarily a buzzy one. It depends on the length of the exhaust pipe and the Helmholtz principle.

Here's an interesting link,

http://www.autolounge.net/calculators/exhaust/exhaustresonance.html




RE: Pedro hack

I'm not crazy about the look, but I may just go that route. I would certainly be clean-er afterwards