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?!
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:
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.
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.