bcdavis

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.

hb4

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.

Lizard928

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.

hb4

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.

Lizard928

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).

jcorenman

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,

Ispeed

I have tried every combo and am running a rear muffler. No drone and great at WOT.
Lomas X, no cats, small resonators and '89 GT rear muffler.

oceaneer

Any updates or progress??
Thanks
Dylan

hb4

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.

Mongo

I still think a single exhaust conversion is the only real way to tone down the drone and refine the sound on these cars...

bcdavis

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

Speedtoys

You can always solve a noise problem, by corking the flow...that wasn't on my list of results.

Mongo

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.

Speedtoys

Mongo...you have dual undercar pipes, or single pipe?

Mongo

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.