Ports 1&4 and 2&3 are always paired in 4-2-1 headers so that the pressure pulses in them are evenly spaced. That is for performance. Sound is a byproduct.
One of the top Alfa engine builders in the US has a set of modified IAP headers that he uses on motors on his engine dyno. He has shared data that shows how the HP peak varies with the length of tube from the collector to the open tail pipe. An engine can make upwards of 5 HP more tuning that length.
One of his customers paid Burns big bucks for a custom 4-1 system with an engineered collector and it made less power than his modified IAP headers. He was able to adjust some pipe lengths to get the Burns unit to equal his standard setup.
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