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.