Thanks gents.. I kind of figured all to do with equalising pressures etc, but given they’re so far down the system and practically next to the atmospheric opening I was having trouble believing they did much. Would a larger diameter pipe / and or a cross-connector near the head not perform better wrt scavenging active pressure?
Interesting topic as to what's going on in exhaust system.
Remembering from schoolboy physics Newton's third law " to each action there's an equal and opposite reaction" or similar, when the exhaust pulse exits the end of the pipe (this is the most prominent junction) that "pop" causes negative pressure wave to travel backwards up the pipe toward the combustion chamber, effectively helping to clear the pipe as further gases are flowing into a vacuum of varying power.
Length of pipe controls when the pulse arrives at combustion chamber, with long pipes that's at lower rpm and so affects the base regions of the rpm range.
A single pipe (no connection) will likely be tuned to most assist combustion chamber clearing just above idle and around normal low speed running. Joining them together can induce pulse from it's piped pair and may improve the cylinder clear while it's idling by getting that inverse pulse to the combustion chamber just about as the exhaust valve is closing, this improves dynamic cylinder filling as it pulls a more clean start vacuum to incoming mixture.
Shorter pipe as headers go from straight into silencer flair location will work at increased rpm, most likely as the power is really getting going and probably covers the peak torque.
Bore/flow etc at maximum rpm is more usually straight volume effectively.
Right up close to the heads is probably out of range for the road engines as the pulse arrives over that short distance almost instantly. This is probably more useful on very high rom engines with big flow requirements like atmospheric formula one engines.
Put simplest, long thin exhaust dimensions favour lowest speed cylinder efficiency assistance, short fat the highest rpm use.
The realisation that this does also work with 4 into 1 systems (understanding the links that are useful) moved bikes more that way. Still the best system for a four cylinder use 4 into 2 into one with tailored primary and secondary bore/length to tune these same linked aspects that the four cross linked originals accomplish.
