Let me preface this by saying I understand the basics of work, power, and energy. I know how horsepower is calculated from torque. I'm confused over how engines are designed for horsepower. I've read many mechanic books, many car magazines, etc. but never come across how to specifically design an engine for specific horsepower. I've heard all of the general answers - no replacement for displacement - and all that other stuff, but can someone please tell me the relationship between airflow, cams, etc. to produce specific torques at speeds? Essentially, I'm trying to figure out if you want x amount of horsepower at x speed, how do you know which parts to pick? Thank you.