graham_mitchell

A current thread raised a question that I have been mulling over for some time. What aspects of a muffler affect engine power and sound?

The Fabspeed website has a section which tears apart the construction of the OEM mufflers and other aftermarket mufflers, but there is no information on the design on the Fabspeed mufflers themselves. Typical!

Here is Fabspeed's image of the OEM mufflers:



Does anyone know what the inside of a Maxflo or Supercup muffler looks like?

A welder friend of mine has TIG equipment and has volunteered to modify my mufflers for nothing, so I would like to learn from some successful designs before we do anything. I assume that removing the fibreglass will help (and reduce weight) but what next - remove the inlet pipe capping?

graham_mitchell

Kim, do you mean that the outer muffler housing is just there for show, and it's really a muffler bypass?

poorb0yw

I believe the housing isn't even there on the supercups.

Poon

here's a picture from the fabspeed website, doesn't look like any muffling is going on!

graham_mitchell

Thanks, I found a photo and I see what you mean. In that case, I am not afraid to remove a lot of the insides of my muffler. The only concern really is creating resonance. I assume that's not easy to predict!

ruffy

removing all the stuffing is definitely going to give u droning
repack it with steel wool... will definitely give you the deep throaty sound

Q

graham_mitchell

you may find this page on my website of interest.
http://www.qplanet.org/993ex.html

I rebuilt the internals on a set of 993TT mufflers
Droning was not an issue, as we used good quality ss packing.
They were an acceptable volume.

I did however find the noise to be hollow
....infact my TT ended up sounding like a naturally aspirated RSCS
*weird*

All the data and .wav files are on the site

cheers
Q

pzull

If you're modding the existing muffler then a perforated U-bend would be best with details as foolws:

1) Use a u-bend or 2 90degree bends with as large a bend radius as would fit the housing

2) Perforations should only be on the straights and not bends as it will cause blow-out of stuffing and also added turbulence.

3) As you are essentially building a helmholtz resonator (ie. attenuation thru resonance) The size and number of holes you drill (or perforated pipe you use) will determine the adsorbtion frequency. The smaller and fewer holes the lower would be the absorbed frequency. There;s a formula for this but do not think its worth while, just find a smooth perforated tube for least resistance.

4) The tubes you use should be between 2.25 to 2.5 inch diameter

5) The bends should be mandrell bent

6) Wrap/wind steel wool tightly around the perforated pipe. Fill the rest of the cavity with heat resistant mineral fibre or fibre glass (former is better)

Good Luck.

BTW, magnaflow makes cheapish mufflers (about $95) of the above design but you'd need some joining work to connect to cat and tail pipe but certainly alot easier than modding the original. Very well built, they are, as well. No affiliation, just using them myself

Chris W. - '96 Targa

Just a quick thought re: steel wool packing in a muffler:

Try to find stainless steel wool for the packing. Regular steel wool will rust with the condensation that forms in the exhaust system as it cools. This in turn will cause the steel wool to crumble and fall apart. If you're lucky it just gets blown out the tailpipes, but if you're unlucky it can clog, increasing backpressure in the muffler (= reduced power) or even get sucked up into a cylinder through the exhaust valve with the momentary vacuum that occurs when shutting off the engine.

-Chris

Ron

Good source for stainless steel wool is http://www.gervois.com

You could probably email Thierry. I believe the cost was about 68 euro for 10 kilograms.

graham_mitchell

There's some very interesting information here. Thanks!

Gervois link is especially interesting.

Pzull, your recommended using 2.25 - 2.5" diameter tubing, but I'm under the impression that the existing tubing it 2". Where is the best place to make the transition? At the wall/inlet? Or the join with the rest of the exhaust system?

mlincoln

Hi, I used to polish my model railroad tracks with steel wool when I was a child, in order to remove corrosion and make the train pick up electricity better. The residual strands of steel wool left behind would burn quite nicely when I applied power and electrical resistance caused it to heat up. I could even burn a giant wad of steel wool, with a nice flame, for fun. It would seem like steel wool could do the same in a hot muffler. I don't think that stainless steel wool would necessarily be any better....

Mike

Ron

Hi Mike,

Not all stainless steel wool is created equal.

The stuff Gervois sells is Stainless steel wool AISI 434 and is heat resistant to 700 degrees C, for even high temperature they make knitted wires which are stainless steel wool AISI 309, AISI 314, and inconel.

I'm no expert this is just the information I got from them for a project I have been working on.

pzull

Graham,

The inner pipes in the muffler may be 2inch i dunno but I think the pipe connecting from cat to muffler is 2.25inch

Anyhow 2.5 inches if you want better high end....gave me peak bhp at 6500 rpm instead of stock 6100rpm on 964. You may lose low rpm but I doubt it. Mine didn't but that was because it was an entire exhaust change including headers and crossovers.

2.25 inch would be safe but may not maximise last 0.25bhp at higher rpm

Dont fill the entire exhaust cavity with steel wool...it'll weigh a ton! Just use it to wind around the pipe.

See This: