Contributed by Russ Muller Photography
Horsepower: We all have it, and many of us want more of it. In 1927, the Ford Model A produced 40 horsepower. In 2005, Bugatti introduced the first 1000+ horsepower production car, the W16-powered Veyron. In between, a lot of things changed, including how horsepower numbers were measured and reported. Here’s a very brief look at gross vs. net horsepower.
From the early days, through 1971, most American auto makers used a gross horsepower rating for their engines developed by the Society of Automotive Engineers (SAE). Power was measured on an engine dyno with no air cleaner, no power accessories, and no exhaust. Non-standard ignition curves were frequently used, and the output numbers were “corrected” to sea-level atmospheric conditions and 60 degree air temperature. In other words, the gross horsepower was the best theoretical output an engine could produce.
1971 was the beginning of the transition from gross horsepower to net horsepower numbers being reported. In some cases, manufacturers reported both numbers in their marketing material. This example was taken from the 1971 GM brochure made for my Pontiac Grand Prix:
The net horsepower numbers for both available engines are quite a bit lower, but why? All the “theoreticals” have been removed from the equation. Power was measured on an engine dyno with all accessories attached and functional, and an air cleaner and a full exhaust system were installed as well. The ignition curve was set the same as it would be when a customer bought a car, and the atmospheric conditions were adjusted to a more realistic 1,000 feet above sea level and an air temperature of 77 degrees. The numbers were a lot lower, but also a lot more realistic. In 1972, the SAE standard of reporting net horsepower was adopted by most manufacturers.
If you’re kind of nerdy like me, you may have looked at the chart above and started to calculate the difference between net and gross horsepower for both engines. You also may have noticed that the difference is not an exact percentage. For example, the 455 in my GP has 325 gross hp, 260 net. That’s a 20% difference. The base 400 was rated 300 gross hp, 255 net a 15% difference. The amount of power used by accessories varies by engine, so there is not a simple, single conversion from gross to net horsepower. To get a rough idea, you can multiply a gross hp number by .8 to approximate net horsepower, and vice versa. That 365 hp LS5 454 in a 1970 Chevelle, for example, translates to roughly 292 net horsepower. The 225 hp 360 in my 1979 Li’l Red Express translates to roughly 281 gross horsepower. In 2005, the SAE adopted even tighter standards that require a 3rd party be present when horsepower testing is done, and that the testing be done in an ISO 9000 certified facility to ensure accuracy. That 1001 hp W16 Bugatti Veyron I mention earlier translates to roughly 1251 gross horsepower…
That’s a very brief overview of gross vs. net horsepower. There’s a lot more to it, but hopefully this will get you curious enough to read more about it.
