Horsepower relation to speed of vehicle?

Answer 1

Horsepower has no "direct" relation to the speed of the vehicle with the exception of a ram air intake.

-Ram air intake/hood scoops use the vehicles speed to force more air into the engine, increasing it's horsepower. This effect is only measurable at high speeds >60mph.

Horsepower's relation to the vehicles "aerodynamic drag" has a direct affect on the vehicle's speed. Meaning it takes a certain amount of horsepower to sustain a certain speed of a vehicle with a certain amount of drag. It's a "non-direct" relation because a car with the same horsepower can go faster if it's drag coefficient is lower( aerodynamics improved). Horsepower's relation to vehicle drag can be seen in that it takes 4 times the current horsepower to double the current top speed.

An example: If a car goes 100mph with 125hp, it would take the same car 500hp to go 200mph.

"An engine doesn't produce horsepower", horsepower is just a calculation of the amount of torque an engine produces over time, in the form of "engine speed".... not vehicle speed. If you can maintain/increase torque output of an engine as rpms increase... Horsepower will also increase. The equation to calculate Horsepower is Torque x rpm / 5252.

An example:

An engine that makes 300 ft-lbs. of torque at 4500rpms 300x 4500/5252 = 257hp at 4500rpms.

If the engine can maintain 300 ft-lbs at 7000rpms the horsepower would be up to 400hp.