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I am looking for information on the effects of changing certain criteria in an exhaust system relating to its performance and its sound.

I am unable to find any resourses relating to this on the net or even in print.

Is it a protected black art?!

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2 Answers 2

Is it a protected black art?!

Protected? No. Black art? Not really. Art? Almost definitely.

There are actually quite a few references on the design and optimization of exhaust systems for race engines. Here is quite a nice overview. Be aware, however, that optimization may be out of the reach of most DIYers. For example, let's consider this pithy section referring to Figure 2 in the linked article:

Figure 2: Intake Port, Exhaust Port and In-cylinder Pressures with Effective Tuning

The second component is the result of the pressure "spike" which occurs at EVO, shown by the peak in the red line in Figure Two, just after EVO. That pressure spike, or pressure wave, moves down the pipe at the sum of the local sonic velocity plus the particle velocity of the gas flow. Whenever the pressure wave encounters a change in cross-sectional area of the pipe, a reflected pressure wave is generated, which travels in the opposite direction. If the change in area is increasing (a step, collector, the atmosphere), the sense of the reflected pressure wave (compression or expansion) is inverted. If the change in area is decreasing (the end of another port having a closed valve, or a turbocharger nozzle, for example), the sense of the reflected wave is not inverted. The amplitude of the reflected wave is primarily determined by the proportionate change in cross-sectional area (area ratio), but the amplitude is diminished in any case. For purposes of approximation, the particle velocity can be ignored because its effect is self-canceling during the round-trip of the wave. However, highly-accurate simulations must take it into account. These waves are sometimes called finite difference waves, because of the finite difference numerical modeling techniques used to calculate their propagation characteristics.

Note that reference to "finite difference numerical modeling techniques": if those words are Scary Math then optimization is likely out of reach.

However, putting together a quite nice exhaust for a street car is absolutely in reach of a DIYer with decent welding equipment, welding experience, a lift and the patience required to test-fit all the components together multiple times.

Full disclosure: I currently have none of the above.

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The answer and links given by Bob Cross offers a good and real insight into the considerations for building an exhaust system. Whilst MagnaFlow and Cherry Bomb deliver exhaust boxes to go after enhancing the sound of an exhaust I have never been able to find a tutorial to fully embrace the subject. In my dealings with exhaust companies it was always an elaborate process of pressure volume graphs, gas velocities, valve timings and component temperatures and pipe length, at an optimum engine RPM. Loads of rolling road/dyno time to prove everything. In todays street legal world, a world where CO2 is increasing the oceans acidity and nitrous oxides are taking the place of CFC's in depleting the ozone layer, there is a fortune to be made in exhausts that truely have clean emissions. This leads on to exhaust manufacturers and designers being very secretive with their developments.

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Quick tip: Porsche uses butterfly valves to keep their modern cars making that distinct Porsche sound. –  Juann Strauss Nov 28 '13 at 10:16

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