Once again, you need to make a different comparison. Why a 273 and a 408? Why not look at a 360 verses a 408? Or a 340 verses a 408? Then you are looking at engines with similar bore sizing.
I could do the same as you and compare your 408 to a say, 520 inch engine. The 408 has a .400 larger bore than the 273 and that 520 has a .400 bigger bore than the 408.
That 408 would get shredded simply because the 520 has a bigger bore and wider bore centers. That means you can use bigger valves, which means you can use bigger ports. And THAT makes more horsepower.
That’s why all the paper tiger arguments set up like you did skew the results to the favor of everyone who thinks displacement rules. It does IF the rules are set up in its favor.
If you are talking which platform has a higher horsepower density (I don’t like that phrase but it’s what everyone uses so I’ll stay with common vernacular here) the smaller engine would win. In other words, if the 408 made 550 hp, and the 520 made 718 hp, they would have the same horsepower density. I say it horsepower per cubic inch. In the above case, both engines made 1.38 hp/cid. It would be unlikely that the 520 would make that hp/cid if both engines were built for the same purpose.
I won’t even get into BMEP but it’s an important number that should be carefully considered.
