I'm fairly certain I understand the theory behind Helmholtz resonance but haven't figured out which distance I should be working with. As I understand it, Helmholtz resonance allows the pressure wave that is formed by the closing of the intake valves in one runner to bounce back up the runner, with some amount of that pressure traveling to the runner where intake stroke is happening. The ideal case is for the wave to reach the valves of the piston in the intake stroke right before the valves close for increased VE, and this distance can be set by selecting an RPM to optimize to and calculating the length of the runners to allow the pressure wave rebound to occur at the right time.

Now correct me if I'm wrong, but the pressure wave must travel from the closed valves, up the runner, across the bellmouths, down the intake-stroke runner, past the open valves, and into the combustion chamber. Would it make sense to be working with the shortest distance from the valves of the closed valves to the open valves because the pressure wave only provides a quick burst of pressure right as it arrives at the open valves? Or the centerline of the runners with the same valves to "average out" the entire pressue wave? Or should I be using the distance from the closed valves to 1/4 (or 1/3, or 1/4...) of the stroke length because the air must actually enter the combustion chamber? I'm sure Ricardo could plug and chug some numbers but I think understanding why the 1-D case works prefers certain assumptions is more helpful.