X pipe H pipe or side by side

We were discussing scavenging,tuned Try Y,optimizing lengths,volumes,diameters
how 2 into 1 compared to 2 into 2 could work with some excellent analogies.
...Now we are "new Mustangs have them" , "Nascar run 9200 rpm" and "Hensley got
20 hp", that's O.K. and suggests they work but we stopped trying to "figure out how"