now that I understand his view, I can debate on Joe's behalf.
No offense man, but lemme give it a try, lets see.
One must remember that while we are looking at a pressure trace, there is also velocity. Pressure converts to velocity, which converts to pressure, back and forth, and that's basically what it's doing when it is resonating.
When the pressure is crossing it's mid point that's probably when the velocity is near it's peak, inward or outward depending on which way it's going.
You could say that what is driving the cycle is the piston going down, because this results in FLOW, and stopping the flow is what generates the pressure spike and resonance that follow.
A third view is, that a period positive flow should be TIMED to the piston going down, and this DOES happen regularly every 720 degrees.
For the mild mannered carbureted IR system this won't be the best approach, because like I said earlier the flow will be peaked for some time during the middle if the induction stroke, and this period of time will increase with rpms, making counting from IVC to TDC the more practical target. The target is to get positive pressure at TDC, not necessarily to time the positive flow to the piston going down. IC is used instead of BDC because we are generally taking about the top end of the rpm range, and the pressure spike WILL be at the IC point, because we are in the top of the RPM range.
If the intake pipe is quite large so flow does not peak and "reset" the timing every stroke, then you may want to time it so that the positive flow will time to the piston moving down. In which case, will want to count 720 degrees. And....actually in that case it won't matter what you count from, because whatever you choose will happen regularly at 720 degrees
