Issue link: https://digital.nexsitepublishing.com/i/118656
the difference between the two horsepower definitions. This difference, however, is quite small and is often ignored in comparing plotted curves. Over the years many owners have noticed that the differences between the U.S. and German ratings as well as differences between various model years for the same engine may be much greater than this small definition difference. In the 1960s and early 1970s before the first Mid-East oil crisis, horsepower advertising was popular among the domestic manufacturers. The U.S. values quoted were usually, but not always, qualified by the term, SAE, or SAE Gross horsepower, indicating the Society of Automotive Engineers standard being used. No one paid much attention at the time except car enthusiasts, but these SAE values were based on dynamometer tests in which the liquid-cooled engines had little or no power extraction for cooling fans, water pumps, electrical generation or other accessories. This resulted in inflated values compared to what the installed engine could do in the car. The air-cooled Porsche engines were traditionally tested with the cooling blowers and generators or alternators installed and operating, so there was a real apples and oranges problem in comparing the SAE Gross numbers with the Porsche DIN numbers. During that period, Porsche assigned SAE Gross values to their U.S. specifications that could be as much as 10 to 18% greater than the DIN values to better match U.S. domestic practice. For example, the 60 DIN hp 356 Normal had U.S. Advertised Horsepower 70 SAE hp, the 75 hp Super had 88 SAE hp, and the Super 90 had 102 SAE hp. The first 911 had 130 DIN hp and 148 SAE hp. All of these values were before any emission control equipment entered the picture and the differences were obviously a lot bigger than the 1.4% difference due to the U.S. and metric definitions. In 1972, all this changed, with the domestic U.S. car industry converting over their horsepower rating system from SAE Gross to SAE Net. The SAE Net rating included the cooling system power consumption with more of the normal installation losses and was supposed to be a lot more representative of an engine installed in a car. At that time all the U.S. Porsche engine specs were also changed over to SAE Net ratings and the horsepower values fell. In order to compare the ratings before and after the change in 1972, a summary of air-cooled Porsche engines and their published horsepower is presented in Figure 2, showing the DIN ratings as well as SAE ratings that were used. The SAE data in Figure 2 come from a number of Porsche brochures and new car reports. Data for all three ratings in the 1972/73 model years were obtained from Reference 2. For example, Figure 2. Porsche Air-Cooled Engine Horsepower Ratings. February 2011 37