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Thursday, August 6, 2009

What is Horsepower?

How do you define Horsepower? A good way to effectively explain this word is to give a glimpse of history back to the time of the British inventor James Watt (1736-1819). It was Watt who coined the term “horsepower,” to increase sales of his improved steam engines. He had previously agreed to take royalties of one third of the savings in coal from the older Newcomen steam engines. This royalty scheme did not work with customers who did not have existing steam engines but used horses instead. Watt observed that a horse could turn a mill wheel 144 times in an hour, or about 2.4 per minute. Watt calculated that the horse pulled with a force of 180 pounds (just assuming that the measurements of mass were equivalent to measurements of force in pounds-force, which were not well-defined units at the time). So:

James Watt and Matthew Boulton later standardized the figure to 33,000 ft. – lbs. / minute, the figure we use today. Horsepower is defined as work done over time. The exact definition of one horsepower is 33,000 ft. – lbs. / minute. Put another way, if you were to lift 33,000 foot-pounds over a period of one minute, you would have used up one horsepower. Engine horsepower is the relationship between known engine torque at specific engine speed divided by Watt’s equivalent unit of one horsepower. Simplified, the equation is:



This is the old process that American manufacturers used as a guide for rating their cars. It was in place prior to 1972. SAE gross horsepower also measured at the flywheel, but with no accessories to bog it down. This is the bare engine with nothing but the absolute essentials attached to it; little more than a carb, fuel pump, oil pump, and water pump. SAE J245 and SAE1995 define this measurement.


In 1972, American manufacturers phased in SAE net horsepower. This is the standard on which current American ratings are based. This rating is measured at the flywheel, on an engine dyno, but the engine is tested with all accessories and standard intake and exhaust system. Both SAE net and SAE gross horsepower test procedures are documented in Society of Automotive Engineers standard J1349. Because SAE net is so common, this is the standard we will use to compare all others. This is what most automotive manufacturers publish as SAE net horsepower. Because SAE gross ratings were applied liberally, at best, there is no precise conversion from gross to net. Comparison of gross and net ratings for unchanged engines show a variance of anywhere from 40 to 150 horsepower.


Prior to electronic bench testing, horsepower was quantified as the amount of resistance against flywheel brake. Although the method is no longer used, the term remains an industry standard. Often road test magazines will measure as “bhp”. This is just another way to talk about SAE net horsepower.


This is a standard, DIN 70020, for measuring horsepower that very closely matches SAE net. The conditions of the test vary slightly, but the required equipment on the engine and the point of measurement (flywheel) remains the same. Because the test conditions are similar, it is safe to divide DIN horsepower by 1.0138697 (metric horsepower) to arrive at SAE net. This value is so close to equal that for all but the most technical purposes DIN and SAE net are interchangeable. However, be aware that DIN “horsepower” is often expressed in metric (Pferdestärke/ps) rather than mechanical horsepower.


In 2005, the Society of Automotive Engineers introduced a new test procedure (J2723) for engine horsepower and torque. The procedure eliminates some of the areas of flexibility in power measurement, and requires an independent observer present when engines are measured. The test is voluntary, but engines completing it can be advertised as “SAE-certified”. Many manufacturers began switching to the new rating immediately, often with surprising results.


This rating is measured at the flywheel. The engine is tested on an engine dynamometer or engine dyno by the manufacturers based on SAE net standards. Those horsepower figures presented in advertising materials are often based on engine tests under ideal environment to be able to advertise the best results. These figures are merely a baseline figure taken from the most powerful engine tested or an average of the engines tested.


In automobiles, effective horsepower is often referred to as wheel horsepower. Most automotive dynamometers measure wheel horsepower and then apply a conversion factor to calculate net or brake horsepower at the engine. Wheel horsepower will often be 5-15% lower than the bhp ratings due to a loss through the drivetrain.

The most common chassis dyno, the inertial dynamometer (popularized by DynoJet), measures the horsepower as delivered to the wheels – whether front (fwhp), rear (rwhp) or both (4whp). This kind of machine measures the horsepower produced by the engine and computes for the torque from the engine speed. In other chassis dynos like Bosch, Dynopack, etc., the torque output measurement is rather based on the relationship of horsepower and the wheel speed. That is why the torque figures in the first, second and third gears are completely different.


Factory ratings are all well and good. But manufacturers, measure horsepower at the flywheel (test on an engine dyno in a controlled environment). Ironically many enthusiasts measure horsepower at the wheels (test on a chassis dyno), mainly because they are not interested in ripping the engine out of their car to have it tested on an engine dyno. And, to the surprise of many, the horsepower numbers presented in advertising and brochures aren’t always accurate.

The dynamometer is a measuring machine, in order to make a proper comparison, it should be made using the same type of measuring machine.

Well, without taking into consideration the circumstances under which it is measured, testing wheel horsepower (whp) makes it difficult to convert from what the dyno says to what the manufacturer says. All that equipment between the engine and the wheels – the transmission, driveshaft, differential, and axles – introduce friction and inertial losses summarized as “power train loss” or “parasitic losses”. The efficiency of the driveline can greatly affect the amount of the power train loss. Some may incur more power loss, some may incur less. It really depends on the design of the drivetrain and the weight of the parts. That is why there is no real formula for converting wheel horsepower to engine horsepower (contrary to popular belief).

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