After reading yet another 6speed thread comparing the melodic tonal qualities of after-market exhaust systems for 996TTs, with little discussion of performance differences, I though it might be worthwhile to share a few interesting articles, listed below, regarding exhaust theory. Certain of these articles address the differences between chamber mufflers [reverse flow] and straight through mufflers. Given that the vast majority of the after-market exhaust systems marketed for 996TTs use power sapping chamber mufflers, which restrict air flow, I thought these articles might me interesting to those seeking to increase performance.

http://www.lcengineering.com/TechNotes/TechNote68.htm

"Straight Through Versus Reverse Flow Mufflers [aka Chamber Mufflers]

Having a optimally free flowing exhaust all the way from the manifold would not do much good if the restrictive stock muffler is still used. The inlet and outlet diameters of the pipe in the muffler should also be as large as feasible, so as to allow free flow of exhaust gases. A straight through muffler would be preferred to a reverse flow muffler mainly because the process of air re-direction in the reverse flow muffler is too restrictive. A straight through muffler design would allow exhaust gases to be expedited out as efficiently as possible, although the muffling abilities would not be as efficient as that of the reverse flow design. Therefore it will be inevitable that the exhaust will sound louder than before, but as mentioned before a couple of times, an aftermarket straight through muffler uses noise suppressing material that tones down the sound to that of one that's deep and throaty and not irritating. However, as will be discussed in the next section, a new generation of mufflers may be able to tackle this."

http://www.sportcompactcarweb.com/te...scc_ssbbpart2/

“To produce the most power, an exhaust should have the least amount of restriction to the exhaust flow. Restriction hampers the burned exhaust gases from exiting your engine, causing some charge dilution with the incoming fresh fuel/air mixture. (In other words, exhaust left in the cylinder is mixed with the fresh intake charge.) This causes a loss of power. With greater restriction, back pressure is generated, making the engine work harder to pump the exhaust out of the cylinders. This is one source of pumping losses.

Some stock mufflers have up to 18 psi of power-robbing backpressure. A well-designed performance exhaust typically has about 2 to 6 psi of backpressure. For comparison sake, an unmuffled straight pipe usually has 1 to 3 psi of backpressure.
To get the least amount of backpressure, most of the good, high-performance mufflers available today have what is called a straight-through design. These type of mufflers quiet the exhaust by the absorption of high frequency vibrations in a heat-resistant packing usually consisting of stainless steel mesh and heat-resistant ceramic fibers. They typically have a straight inner core with no baffling at all, much like a straight pipe with many small holes. The pipe is louvered or perforated when it passes inside the muffler's shell, allowing sound energy to pass through the holes but leaving the exhaust gas flow unimpeded. You can see straight through these types of mufflers. The louvered or perforated core is usually wrapped with either Fiberglas wadding, hence the old-school term Glass-Pack or on the better mufflers, stainless steel mesh to help further absorb the sound. On straight-through mufflers, the longer the muffler, the quieter it is. The length usually has no effect on the backpressure, just the noise output. These mufflers work in the same manner as the silencers used on guns. If a silencer impeded bullet travel, you would definitely have problems!”

http://www.geocities.com/MotorCity/7...ory.html?20052

http://www.sportcompactonly.com/catb...ent-guide.aspx

"Stock mufflers use closed designs with several internal baffles to deaden the sound. While this approach works well from an acoustic standpoint, it causes considerable backpressure and tends to degrade performance. Most aftermarket mufflers for sport compact vehicles use a straight-through design without internal baffles or chambers. Instead, they channel the exhaust flow through a perforated pipe surrounded by a sound-absorbing material, such as fiberglass."

The following is also a good analysis relating specifically to Porsche exhausts (albeit for marketing purposes):

http://www.fabspeed.com/993mufflers.html

The following are a few photos of my straight through mufflers:









There is simply no denying that straight through mufflers are superior to chamber mufflers from the perspective of air flow and power (although admittedly not nearly as melodic in tonal quality).

Craig

 

...and this is why running the stock tips is not the best solution for power gains. If you have a 3-4" pipes in your exhaust that taper down to a 2-2.25" (I forget the exact measurement) pipe at the end so that you can put your stock tips on, you are creating backpressure.

You really need a minimum of 3" all of the way out in my opinion.

 

I completely agree. That is why I modified Tubi tips to match up with my 3'' piping, with no taper at the meeting point.

Craig

 

I'm running the Turbo Exhaust v8.0 that was put out by David L. in Virginia w/ 4.5" tips.

Specs per his site:

* Modified 2.5 inch single catalytic converter
* 83 mm Exhaust Port (3.27")
* Stainless 4.5" exhaust tips
* Secondary catalytic converter removed
* Weight reduced by 10 lbs
* HP Gain 40
* TQ Gain 35
* Back pressure pre-cat at 6700 RPM 0.01 BAR

 

One thing that should be emphasized for those who may think that simply putting on a less restrictive exhaust will give them more power---doing so will ALWAYS, ALWAYS (did I say always) reduce the engine power and torque unless fuel supply to the cylinders is increase proportunately to the wasted mixture exiting the exhaust. You cannot increase the amount of energy exiting the exhaust with each cycle and gain power without increasing the overall energy in each cylinder.

 

this is what I have:

 

I believe Craig's thought process is right. Fabspeed Supersport exhaust is based on the same concept and showed better performance than competitors in Stephan's test a while back.

 

I remember David's exhausts. They were so different compared to what was available at the time.

Excellent write-up Craig.