For normally aspirated engines, in simple terms, torque (a WORK measurement) is going to be dependent on displacement (PLAN) * efficiency, so for a given BMEP value, the torque value is the resultant.
Since any power calculation is a function of work performed over a time period, rpm utilized affects the calculation.
IE:
1/ X torque @ 5,252 rpm = X bhp
2/ X torque @ 10,504 rpm = 2X bhp
3/ X torque @ 15,756 rpm = 3X bhp
etc . . . . . .
This is why F1 engines were running 20,000 rpm several years ago . . . . . . . .
AND, why NASCAR now has "the gear rule" . . . . . . .
If three differing engine types are all the same displacement, and achieve the same BMEP, then:A/ a pushrod, 2 valve NCF, "bathtub" chamber, making peak power @ 5,252 rpm = X bhp
B/ a 4 valve, DOHC, pentroof CF, (Cosworth) making peak power @ 10,504 rpm = 2X bhp
C/ a 5 valve, multi-cam, roller brg crank, CF bike, making peak power @ 15,756 rpm =3X bhp
You get the idea.
For accuracy, peak bhp rpm must be figured into the equation when comparing bhp/liter or bhp/cubic inch.