Originally Posted by edo
It can't. Such statement the "expert" made is nonsense. ECU adapts ignition constantly, depending on various factors (temperature, humidity, load, etc.). So an aftermarket catback just can't harm your engine.
Oh, and the aftermarket aftermarket software that the "expert" mentioned usually is off the shelf for cars with slightly different mods. In other words usually it is not custom tuned to the mods of the specific vehicle. But it also adapts ignition to a certain level.
If an aftermarket software is too aggressive for the mods, then it results in knocking and can harm your engine.
The computer only adjusts the timing in response to knock. The design tolerances are for fuel, not what some monkey might do to their well engineered system. Changing anything in the exhaust means immediately the "correct" timing advance for any given RPM or load point has changed. Unfortunately, freer flowing exhaust means better exhaust evacuation from the cylinder, which means the optimal timing point is retarded compared to it's previous location. This means the first guess by the computer is much too advanced. What happens when you run too much timing? RIGHT.
You have to be a scientist and a computer engineer to understand how it really works, there is no explanation that will be satisfactory for any layman, but if you're curious as to how these things work: http://forums.openecu.org/