Another look at Exhaust Systems. (m)
12-19-2002, 11:08 AM
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Another look at Exhaust Systems. (m)
As we know, they come in many configurations and fabrication qualities. Here are some principles to consider in assessing an exhaust system's backpressure potential for all out performance:
1. Backpressure in a pipe results from friction and turbulence, converting kinetic flow energy into thermal energy (entropy).
2. The larger the pipe diameter, the lower the flow velocity and resulting friction for a given mass flow.
3. Sudden increases or decreases in pipe diameter and pipe direction (welded kinks etc.) cause local flow separation and turbulence (entropy).
4. A tapered tube avoids turbulence and is cheaper to make than bell-mouthed transitions.
5. For the tube lengths in the S4, tubes should be tapered only to match flanges.
6. Mandrel bent turns are a must to avoid turbulence.
7. Welds should exhibit minimum penetration roughness inside the tubes.
8. The Y tube is a bottle neck. It should merge the flows as smoothly as possible.
9. Catalyzers are designed to produce minimal backpressure and entropy but do nonetheless.
10. Mufflers are deliberate turbulence (entropy) generators.
11. To minimize total system entropy growth, connective tubing up to, and between catalyzers and mufflers should observe above points 2 - 6, and be carefully matched at every flange.
For those who buy an aftermarket exhaust only for the sound, just open up or eliminate the muffler.
Happy Motoring :-)
1. Backpressure in a pipe results from friction and turbulence, converting kinetic flow energy into thermal energy (entropy).
2. The larger the pipe diameter, the lower the flow velocity and resulting friction for a given mass flow.
3. Sudden increases or decreases in pipe diameter and pipe direction (welded kinks etc.) cause local flow separation and turbulence (entropy).
4. A tapered tube avoids turbulence and is cheaper to make than bell-mouthed transitions.
5. For the tube lengths in the S4, tubes should be tapered only to match flanges.
6. Mandrel bent turns are a must to avoid turbulence.
7. Welds should exhibit minimum penetration roughness inside the tubes.
8. The Y tube is a bottle neck. It should merge the flows as smoothly as possible.
9. Catalyzers are designed to produce minimal backpressure and entropy but do nonetheless.
10. Mufflers are deliberate turbulence (entropy) generators.
11. To minimize total system entropy growth, connective tubing up to, and between catalyzers and mufflers should observe above points 2 - 6, and be carefully matched at every flange.
For those who buy an aftermarket exhaust only for the sound, just open up or eliminate the muffler.
Happy Motoring :-)
12-19-2002, 11:22 AM
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sounds like
he'd buy a straight large diamter pipe that only tapers if it has too, has no Y tube, is mandrel bent and has no mufflers in the catback area (heck, no cats either).....keep in mind, th efirst sentence or so says ultimate performance...
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12-19-2002, 11:28 AM
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its a trade off.....
2 pipes, you lose the turbulence caused by the Y...however, now you have more surface area (and thus srface boundary effects...like friction) PER cross sectional area...
now the trick is to see which one of these factors dominate...
my guess would be teh Y....
howver the fluid mechanics behind this are VERY dependent on surface finish of the interior of the tube as well as the actual dimensions, as well as the geometry of the Y....
the points he mentions are all good IMO, the trick is optimizing it for our particular constraints....and that is the beauty of a tuned exhuast over a custom one somebody just fabbed up...
personally i htink as far as catback...take the AWE one and hack out the mufflers....
now the trick is to see which one of these factors dominate...
my guess would be teh Y....
howver the fluid mechanics behind this are VERY dependent on surface finish of the interior of the tube as well as the actual dimensions, as well as the geometry of the Y....
the points he mentions are all good IMO, the trick is optimizing it for our particular constraints....and that is the beauty of a tuned exhuast over a custom one somebody just fabbed up...
personally i htink as far as catback...take the AWE one and hack out the mufflers....