What is the difference between Net and Gross horsepower?
Prior to 1972, dyno testing was done with an open exhaust, no air cleaner, best fuel and timing for power, high octane leaded gas, and no accessory drives. To correct the observed data, a standard correction factor (29.92 inches of barometric pressure at 60 degrees Fahrenheit dry air) was used. This is defined as the Gross Horsepower rating. In 1972, with the crunch of the fuel crisis and insurance crack-downs, the SAE called for a switch to a Net Horsepower rating. This required the engine to be dressed with all the accessory drives (altenator, power steering pump, AC compressor, AIR pump) and full production exhaust system. Also, the engine was tuned to safe fuel and spark curves, and the standard correction factor was changed to 29.24 inches of barometric pressure at 77 degreees Fahrenheit dry air. All this works to reduced the horsepower measurement on the dyno, and thus the HP measurements advertised by the manufacturers. While both net and gross horsepower readings are taken at t
