Actually Sam, I was wondering if your methodology was taking into acount interference between cylinders or you calculated your system frequency on the assumption that the three other valves were closed during the whole cycle, thus not having pressure waves from the other cylinders interfering with the wave you're actually computing (from what I understand, you only used one cylinder as the wave source). I'm new to acoustics but to me, it seems we could have a more precise model if it was time-based which mean we could compute the interference effect between cylinders over an engine cycle (maybe using the acoustic pressure and velocity equations you presented in your work). After my post, I got your model running but still have some differences with your results.

The curves for the runner-plenum interface for close and open valves look good, but they still are offset which means the only thing that could move my curves on the x-axis would be my equivalence between frequency and engine speed. I'm using w=2*pi*(2*N/60); because i figured that the frequency had to be twice the engine speed divided by 60 (twice because the wave travels twice to go from the valve and back to the valve and divided by 60 to transfer rpm to revolution per seconds). I can't really figure out what goes wrong here. Also, does someone has an opinion on using mean cross sectional area of convergent or divergent tubes to approximate the speed the waves travels throught them. It just seems logic to me. Don't worry Sam, for the time I spent working on this, I'm really not looking for a quick turn-key. Right now, it just became personal between me and acoustics, haha.