Basically it takes a certain amount of fuel and air within a range to make x amount of hp of course there’s other variables.
Just off the top of my head guess say 400 hp is about 550 cubic feet of air per minute, cfm.
now you got a 318 and 360 how are those engine gonna displace that amount of air and fuel ?
let’s just say were operating at 100% VE so a 318 is .184 cubic feet and a 360 is .208 cubic feet per cycle that’s how much of the 550 cfm they can process in one cycle which is two revolutions. So how many revolutions “rpm” does it take to process 550 cfm for both engines? 318 = 5978 rpm and 360 = 5388 rpm for both to move 550 cfm.
Of courses there variables so not gonna be the exact out come but why they do operate at different powerband.