gross verses net HP

[Rev]

Before about 1971, manufacturers used gross HP figures to rate the engines. They wanted the highest numbers they could get for advertising purposes. To do this they put a "seasoned" (well broken in) engine on an engine dyno. Dyno headers didn't even have to fit in a car. Large srtaight exhaust with no mufflers and no bends was used. No parasitic accessories were attatched. No fan, alternator (genrerator) and sometimes not even a water pump. Certainlly no AC or PS. The only drag on the engine was the oil pump and the fuel pump. The engines were often "super tuned" by some racing guru like Smokey Yunick if they really wanted the highest numbers.

The Feds and the insurance industry started giving the manufacturers grief about "over powered" muscle cars and heavy pollution, etc. So in 1971-72 the manufacturers capitulated by using net HP figures that are lower. This net figure represents flywheel HP that the engine would produce as installed in a new vehicle. All engine accessories except AC and PS are attatched. Full factory exhaust including cats (if applicable), stock mufflers, and pipes with all the bends are installed. The engine is just like it came off the assembly line with no special tune up.

The net figures are usually about 80-85% of the gross figures.

Rev