Good point about Comp’s use of .015” for duration and I agree with their reasoning. It has a lot to do with viscosity, turbulence, and flow. I am not a CFD guy but I’ll make a futile attempt to sum it up. When you push a fluid through a tiny orifice you reach such a high point in velocity that the turbulence generated slows, or even stops, any further increase in flow. (kind of a Bernoulli thingy). Comp is probably right in assuming there is little to no flow at anything less than .015” of the valve curtain. Thus mechanical duration is not the same as flow duration.
Sorry about my trivial treatment of detonation i.e., (pepper on the insulator). When a cylinder starts detonating the first thing that gets knocked off by the shock wave is any residual carbon or other crud that has accumulated in a dirty cylinder over time. These particles attach, or get imbedded onto the plug insulator. They usually look like tiny pepper flakes. They are a sure sign of detonation.
If you are a racer and have clean cylinders due to frequent tear downs you will still see pepper flakes. In that case they are parts of your piston! The shock wave is hammering your piston and very tiny bits of aluminum are being blown away. While you would think they would be shiny and white because they are aluminum, usually they are not. AlO2 is naturally a grey color and combined with carbon from the rings the stuff still looks like pepper flakes but maybe without the extreme black color. The key is to look for small specs of anything on the insulator. That is a clear sign of detonation. By “specs” I mean particles not to be confused with other junk that can be accumulated on the plugs as well. That is why it is critical to be able to make a correct plug reading. Of course, if you are at the track and can make repeated runs you can just keep increasing timing until it starts to nose over. I have greatly simplified everything but you get the point. The engine will have it's very own WOT number and you have to test to get it right.
Thanks for the Zeitonix link . . . interesting.