273 or NOT 273?

It's (HP) all about air flow (Displacement over Time) = CFM.

At 0.5 lb/h of fuel per hp x 400 hp = 200 lb/h x 12.5:1 air fuel ratio = 2500 lb/h of air divide by 60 minutes = 41.7 lb/m divide by 0.075 lb per cubic foot = 556 cfm.

556 cfm x 3456 = 1,921,536 / cid = rpm

Ballpark 400 hp at same VE% these engines sizes at those RPM will/should be able to displace the same amount of fuel and air (HP).

273 @ 7039 rpm

318 @ 6043 rpm

340 @ 5652 rpm

360 @ 5338 rpm

408 @ 4710 rpm

Obviously with different amounts of lb of fuel per hp and VE% etc.. will change rpm.