Heres a thought on passing emission test

[Power Hungry]

The reason it's best to use the knock sensor as an "emergency relief valve", so to speak, is that when the knock sensors detects detonation the PCM pulls out a HUGE amount of timing in one shot to protect the engine. This will have a significant impact on performance and economy. It is much better to go with a slightly less aggressive curve that doesn't cause detonation than it is to push the limits.

One other thing to consider is that some of the earlier engines DON'T have a knock sensor to compensate for sloppy timing curves. Obviously, things can go wrong quite quickly in this type of situation.