i haven't had a chance to confirm this with the family mechanical engineer (dad
) so i may be somewhat mistaken or downright wrong:
hp is a function of torque and engine rpm. I don't know what the actual formula is, but i believe the relationship is a direct one. ie. x(torque) * y(rpm) = z(hp)
i don't know what the exact formula or co-efficients are, but it is entirely possible that torque could increase or decrease without affecting hp. it all depends on what the engine rpm does; in otherwords, the shape of the torque curve.
which is why torque and hp numbers are entirely useless unless associated with rpm. you could have an 800hp engine, but it's ability to do useful work depends on whether that's at 500rpm or 15,000rpm.