There seems to be some confusion over pipe-length......Remember, we're dealing with SOUND...Take a guitar, pluck the string, it will vibrate with maximum amplitude at the centre (skipping-rope)
Now lightly touch the centre of thestring and pluck in the normal place, the string will now have a s-shape...2 opposite-phase vibrations at twice the "free" frequency.......repeat at the quarter-node and again,the string will vibrate, snake-like an 4 sections.
The same principles apply to brass instruments didgeridoos and aircraft exhausts!....the shorter pipe has a much tighter-defined frequency-range from "into" and "out of" resonance, than a longer pipe.....all to do with Fundamental Frequency and Harmonics.
Also ties in with vibration, resonance and stuff vibrating itself to bits!
Regarding the "burnt-valve" issue.....draw a circle and plot yopur cam timings on it.....you'll see a large number of degrees where both inlet and exhaust are open simultaneously.....Why doesn't the "new"charge get ignited by the previous one, and why doesn't it all go straight downthe flue-pipe?- Indeed, why doesn't the flame-front go back up the open inlet valve and set fire to the whole inlet-manifold and carburettor?
lots of questions, a complex answer of interacting physics.