UPDATE IV: Exhaust Drone - X-pipe flute?. See Post 72
#16
Drifting
Join Date: Jul 2002
Location: Los Angeles, CA
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I had this problem on my 928, and went through 3 exhaust setups before I was happy.
Too loud, too quiet, too raspy, drone, etc...
I never tried the J pipe, but I fixed the drone with a straightforward resonator tip.
Having owned other cars since then, I have seen people struggle with the same
problem on other models. And many times it comes back to resonator tips being
the answer to the problem. So sure, you can try all these fancy experiments,
but you really might want to try some resonator tips, and see if it helps.
They aren't that expensive.
Too loud, too quiet, too raspy, drone, etc...
I never tried the J pipe, but I fixed the drone with a straightforward resonator tip.
Having owned other cars since then, I have seen people struggle with the same
problem on other models. And many times it comes back to resonator tips being
the answer to the problem. So sure, you can try all these fancy experiments,
but you really might want to try some resonator tips, and see if it helps.
They aren't that expensive.
#17
Racer
Thread Starter
Reporting back for the record.
Based on the frequency plot....
I tried the 1' resonance tubes
Without much noticeable improvement.
If I really am experiencing a resonance at ~ 300 Hz, then the resonating section(s) of the exhaust should be about two feet long (half wavelength), but there's no section of the system that is that short.
I had the pipes offset so that I could easily convert to a balance tube, which will be my next stab at it - mostly because it's quick and easy.
If that doesn't work, I think replacing the aftermarket center mufflers with stock large resonators per Bill Ball's experience (since he has the GT cams and X-pipe similar to my setup) makes the most sense, as long as the new rear muffler does its job. I sure wish I knew a lot more about exhaust systems.
Based on the frequency plot....
I tried the 1' resonance tubes
Without much noticeable improvement.
If I really am experiencing a resonance at ~ 300 Hz, then the resonating section(s) of the exhaust should be about two feet long (half wavelength), but there's no section of the system that is that short.
I had the pipes offset so that I could easily convert to a balance tube, which will be my next stab at it - mostly because it's quick and easy.
If that doesn't work, I think replacing the aftermarket center mufflers with stock large resonators per Bill Ball's experience (since he has the GT cams and X-pipe similar to my setup) makes the most sense, as long as the new rear muffler does its job. I sure wish I knew a lot more about exhaust systems.
#18
Nordschleife Master
Bill,
See if you can get a sharktuning session. Then at your cruise areas where you have the drone, pull fuel and go to a lean burn, while at the same time increasing advance.
This allows the entire mixture to be burn before it leaves the combustion chamber and can drastically reduce the noise and drone I have found.
See if you can get a sharktuning session. Then at your cruise areas where you have the drone, pull fuel and go to a lean burn, while at the same time increasing advance.
This allows the entire mixture to be burn before it leaves the combustion chamber and can drastically reduce the noise and drone I have found.
#19
Racer
Thread Starter
Colin,
That's a direction I never would have imagined! Does it have to do with making sure combustion is more complete before the exhaust valve opens, or something else? Maybe resonance has nothing to do with it?
Sharktuning is in my future for sure, hopefully before summer's end; then I can put fuel pressure and the temp sensor back to stock. If it fixes most of the drone it's worth the price of admission.
That's a direction I never would have imagined! Does it have to do with making sure combustion is more complete before the exhaust valve opens, or something else? Maybe resonance has nothing to do with it?
Sharktuning is in my future for sure, hopefully before summer's end; then I can put fuel pressure and the temp sensor back to stock. If it fixes most of the drone it's worth the price of admission.
#20
Nordschleife Master
Bill,
It does have to do entirely with making sure that the combustion is more complete before the exhaust valve opens.
When you hook up the sharktuner, do this as a test, change the idle ign value from 10 deg advance to 28 deg, and watch the noise of your car almost completely disappear...... (this is why I plan to run no muffler with the turbo I am installing).
It does have to do entirely with making sure that the combustion is more complete before the exhaust valve opens.
When you hook up the sharktuner, do this as a test, change the idle ign value from 10 deg advance to 28 deg, and watch the noise of your car almost completely disappear...... (this is why I plan to run no muffler with the turbo I am installing).
#21
Rennlist Member
Bill,
I think your freq-plot is off by 2x, somehow... Your earlier post was 2200 RPM and a 4' resonant length, that matches my back-of-envelope calculation... 2200 RPM is 37 rev/sec, x 8 cyl's divided by one exhaust stroker per two rev's, is 147 Hz and an organ-pipe resonance of roughly 4'. That's assuming the two sides are mixed by the "X", while the factory system has only a small "H" there-- so less mixing left-to-right.
But I am liking where Ken put his "H" for exactly that reason... the right-side exhaust path is a couple of feet longer than the left side, putting the acoustics out-of-phase at 150-ish Hz, where they are joined by the H-pipe. .
But then why didn't the two-in/two-out muffler do the same thing? Unless they carefully separated the two sides, that is. Any idea what is inside that muffler?
And we can try Colin's ignition-advance trick. I must admit to being a bit skeptical, there's still got to be quite a bit of blow-down pressure when the exhaust valve opens-- but it is simple to try once we get the ST2 hooked up, and that's pretty simple. All we need to do is find a free day
Cheers,
I think your freq-plot is off by 2x, somehow... Your earlier post was 2200 RPM and a 4' resonant length, that matches my back-of-envelope calculation... 2200 RPM is 37 rev/sec, x 8 cyl's divided by one exhaust stroker per two rev's, is 147 Hz and an organ-pipe resonance of roughly 4'. That's assuming the two sides are mixed by the "X", while the factory system has only a small "H" there-- so less mixing left-to-right.
But I am liking where Ken put his "H" for exactly that reason... the right-side exhaust path is a couple of feet longer than the left side, putting the acoustics out-of-phase at 150-ish Hz, where they are joined by the H-pipe. .
But then why didn't the two-in/two-out muffler do the same thing? Unless they carefully separated the two sides, that is. Any idea what is inside that muffler?
And we can try Colin's ignition-advance trick. I must admit to being a bit skeptical, there's still got to be quite a bit of blow-down pressure when the exhaust valve opens-- but it is simple to try once we get the ST2 hooked up, and that's pretty simple. All we need to do is find a free day
Cheers,
Last edited by jcorenman; 07-17-2011 at 12:21 AM. Reason: speling
#24
Racer
Thread Starter
Well, Jeff has some documented success with his recent resonance tube addition. Quite a simple and elegant solution if one is willing to forgo twin pipes all the way back.
Here's a link to a thread that drifted to a discussion of his exhaust for reference and photos. Basically, Jeff started with an X-pipe to a 'Y' back to a single 3" muffler and tailpipe. The resonance tube was added between the 'Y' and the muffler and is pointed aft where the PS muffler had been located.
Here's a link to a thread that drifted to a discussion of his exhaust for reference and photos. Basically, Jeff started with an X-pipe to a 'Y' back to a single 3" muffler and tailpipe. The resonance tube was added between the 'Y' and the muffler and is pointed aft where the PS muffler had been located.
#26
Drifting
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Seriously, try an actual resonator tip, with fiberglass baffling.
This particular resonator has killed the drone for me on several vehicles.
http://www.summitracing.com/parts/PSM-95-9919/?rtype=10
This particular resonator has killed the drone for me on several vehicles.
http://www.summitracing.com/parts/PSM-95-9919/?rtype=10
#27
Rennlist Member
Seriously, try an actual resonator tip, with fiberglass baffling.
This particular resonator has killed the drone for me on several vehicles.
http://www.summitracing.com/parts/PSM-95-9919/?rtype=10
This particular resonator has killed the drone for me on several vehicles.
http://www.summitracing.com/parts/PSM-95-9919/?rtype=10
You can always solve a noise problem, by corking the flow...that wasn't on my list of results.
#28
Official Bay Area Patriot
Fuse 24 Assassin
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Fuse 24 Assassin
Rennlist Member
I have 2 1/4" resonated tips I still have to put on my dual exhaust system. They are 3.5" OD and are actually designed for trucks (exhaust tips are 20" long). I believe that this will virtually eliminate the drone at 2200 I am experiencing as well.
#30
Official Bay Area Patriot
Fuse 24 Assassin
Rennlist Member
Fuse 24 Assassin
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I have dual right now with my Flowmaster 10 Series single chamber mufflers in the intermediate muffler areas. The car sounjds phenomenal right now and pisses the neighbors off with drone (who cares, they should be driving real cars instead of hybrids). I was going to do an entire single exhaust system using a 3.5" Y collector and enlarging a 3"ID cat to 3.5" with a 3" outlet for the same diameter all the way back. However, I am a bit in a bind now on this decision. If I fail smog again next year due to typical aftermarket degredation (already replaced cats with metallic ones), I am considering the full single exhaust conversion.
For now I just figure I complete the exhaust system with my tips still sitting in a box at home.
For now I just figure I complete the exhaust system with my tips still sitting in a box at home.