How in HECK to you figure HP on a B&S engine?

Yeah well hp is still figured the same, namely; Torque times rpm over 5250;
so 8 ftlbs at 3100 is 4.75 hp.
The bigger the piston, the more power it will have,
so don't fool with the mini pistons.
The biggest decks will have the most power.
Self-propelled will be more powerful
Baggers will be more powerful.
19" and smaller decks with no extras will be the least powerful. However, these will move off your yard quicker cuz you can price them down into Kid-affordable, for the enterprising young fella that wants a summer job.
Best of luck to you.