Bill - the PentaStar is 4 valve cyl OHC engine and it was a entirely new design from 2011Well, it's about engineering (no pun intended). The 3.6L, with 2 fewer cylinders is 305 HP. So the 5.7L with 2 more cylinders and 2.1L only gets 70 HP more. Mind, with the 3.6L the max HP doesn't begin to tell the story as it's only achieved at 6,400 rpm and only 269 lb-ft at 4,175 rpm. But the point is, displacement isn't the be all of making power.
The real question is why doesn't FCA wring more performance out of the 5.7L like Ford does with their Coyote engine? Hmmm?
the 5.7 has been around since '03 and is a 2 valve pushrod layout. Keep in mind the 5.7 (then was 340hp) and that was a 90hp improvement over the old LA series 5.9 (which dates back to '64 when the original 273ci engine came out).
cylinder head flow rate, cam profiles and compression ratio factor in - as well as displacement
for comparison the former 3.5 was 4 valve / cyl and OHC but peak power was 250hp.
The 3.6, slightly larger but significant boost in output and efficiency compared to the former 3.5 (dates back to early 90s when the LH series were out)