The net horsepower of an engine does not tell the entire engine power story. How horsepower is calculated through the years have changed, as you may notice that a much older vintage vehicle has a high horsepower, say 300 hp. Yet your modern day car with a horsepower rating of 150hp is much more powerful. Understanding the difference between the net hp and the gross hp can make all the difference, and change how you use horsepower ratings.
The net horsepower is actually the measured output of the car with the alternator, power steering, AC and other peripherals included as well as smog control. This is measured at the engine crankshaft. This was the standard rating used in the auto industry until 2005. Gross horsepower was the original measurement and tends to produce a higher horsepower rating. The gross horsepower rating is taken when the engine has yet to be put into the car, and all the different peripherals attached. There is no load attached to the engine so all of the energy can go into creating horsepower. The gross hp is normally measured using a dynamometer which tells how much horsepower gets to the wheel axles. So in reality these are 2 completely different measurements. In all cases the net horsepower should be less than the gross horsepower. Most vehicles that were built pre-1972 will have a gross rating. Anything after this point will have a net horsepower rating.
Horsepower and Torque
Horsepower is most closely related to torque, as both of these refer to the power that can be achieved from a car engine. However, it is the horsepower and rpm that will dictate the torque. Horsepower itself is a measurement of work over time, and work relates to the force to move an object over a distance. However, it is normally better to compare the torque rating, as this is the twisting force that the engine can produce. The speed tends to be limited by the torque rating.
High end performance vehicles tend to have high torque and high horsepower ratings. In this way, all the horsepower can be transformed and used to go very fast. Heavy duty vehicles are classified as needing a lot of power, but they do not need high speeds, so the torque ratings will be much lower than the horsepower. However, the actual weight of the vehicle also will affect the cars performance. Just because it has a high rating does not mean it will go fast. A vehicle that is lighter will be able to use that power for performance and acceleration. Additionally, how measurements are taken has been changed as of 2005. This has shown a slight decrease in the actual rating numbers, though the vehicle has the same horsepower and torque as before. The SAE, or society of Automotive engineers, have tightened the rating guidelines. There are now standardized conditions for engine load, airflow and fuel. The vehicles that use this rating will have an SAE certification. While many potential buyers like to compare the ratings, there is more that goes into performance than just horsepower. Make sure to give it a test drive to feel how the car performs.