Horsepower
Horsepower is a standard of measurement used in the automotive industry to measure the power output of engines. There are two types of horsepower measurements used in the automotive industry, brake horsepower and SAE horsepower. One horsepower equals 33,000 foot pounds of force per minute of work. This number was derived in the late 1700s as a method to compare the work of steam engines to that of a horse, which was a common form of power at the time.
Brake Horsepower
Brake horsepower (bhp) is the measurement of an engine's power output with no water pump, gear box, transmission, or other power robbing accessories. The amount of power going to the rear wheels is often significantly less than an engine's rated brake horsepower. Previous to 1971, most automakers rated their cars by brake horsepower, which provided greatly inflated ratings during the height of the muscle car period. While the engines are capable of producing such numbers, the actual amount of power that the car can use due to power loss through the drivetrain and accessories is somewhat less. However, automakers would often under or overstate horsepower claims for a variety of reason, which ultimately means the only true test of horsepower is to put a vehicle on a dynamometer that measures actual horsepower at the drive wheel(s).
SAE Horsepower
SAE horsepower is a standard developed by the Society of Automotive Engineers (SAE) which rates an engine's power output at the flywheel, including all standard installed accessories. This model does not take into account power loss through the drivetrain, but is an more accurate estimate of 'real-world' horsepower. SAE horsepower become the standard measurement for most automakers starting in 1971, partially due to consumer interest in fuel efficient automobiles, and partially to appease insurance, safety, and environmental concerns.
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