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February Tech NoTes Porsche horsePower raTiNgs All AllAn CAldwell Engine horsepower is the one characteristic that most Porsche owners know about their car. And if there is one topic that never fails to surface at Porsche owners social gatherings or club meetings, it is always horsepower. Although horsepower ratings are often thought of as a universal standard for automobile engines, there can be significant variations due to differences in basic power definitions, differences in various rating systems and the effects of variations in emission equipment. In addition to all that, there is the manufacturer's policy regarding standards and advertising claims. All of these factors can confuse comparisons. Understanding the various ratings is particularly important when comparing a Porsche built before the 1972/73 time period with later models, since that time period represented a major change point in the rating system for cars sold in the United States. Emission control system effects on horsepower were greatest during the 1968 to 1979 time period before the oxygen sensor system became standard and the DME engine management systems became primary. Now that the USA is entering a new era of carbon dioxide reduction and hybrid engine vehicles, it may be worthwhile to review basic horsepower definitions, standards and conversion factors. this calculation and displays the results, but the primary physical measurements being made are of torque, rpm and fuel flow for a given rpm and throttle setting. The torque is measured by a calibrated resistance device (such as a friction or hydraulic brake in its simplest form) that is attached to the flywheel and will just balance the output of the engine at a given rpm (hence, the name, brake horsepower). In the U.S. and Britain, one unit of horsepower has always been defined as 33,000 foot-pounds per minute which dates back to James Watt's steam engine. Using this definition, the well-known U.S. formula (equation (3) in Figure 1) is obtained for calculating horsepower from engine torque. Note that at 5252 rpm, one foot-pound of torque produces one horsepower. At 2626 rpm, one foot-pound of torque produces 0.5 horsepower and at 7878 rpm, each foot-pound of torque produces 1.5 horsepower. The critical factor in achieving high horsepower from a given displacement engine is the ability to have constant or increased torque at engine speeds beyond 5500 rpm. Metric horsepower is conveniently similar to the U.S. definition. Denoted as PS (Pferdestarke) on the Porsche factory engine performance charts, it has a definition of one PS equals 75 kilogrammeters per second, and is usually referred to as DIN horsepower since the methods follow the German standards. When this is converted over to English units, it becomes 32,549 foot-pounds per minute compared to 33,000 for the U.S. definition. This difference means that the factor in the horsepower equation is 5180 (equation (4) in Figure 1) instead of 5252 and a U.S. horsepower value is lower than the German PS value by a factor of 0.9863, or 1.4%, just due to Horsepower ratings provide a standard of comparing the relative rate of work output between two engines. Work is defined as a force acting on a mass over a distance or force times distance. In the case of a four stoke cycle internal combustion engine, the work done is the output torque developed by the engine over a 360 degree angular motion and is obtained by multiplying the Figure 1. Horsepower Equations and Conversions. maximum full-throttle torque the engine can sustain at a given rpm by 2 pi. The rate of doing work, or power developed, is then obtained by dividing the work by the time it takes to do one engine revolution, which is the reciprocal of the rpm. This calculation provides a value of foot-pounds per minute. Figure 1 summarizes the basic equations from Reference 1. The final basis for engine horsepower ratings are measurements from engine dynamometer tests done to a set of specified standard conditions. Note that a dynamometer does not directly measure horsepower, but measures the maximum engine torque attainable at each rpm and then the horsepower is calculated. Modern dynamometers usually have a built-in computer which continuously does Basic Definitions 36 February 2011