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November 2004 Horsepower History |
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By Bill Vance
The author is an automotive historian and columnist for DPN and other media outlets. See billvanceautohistory.ca for author books.
There is another ìHorsepower Raceî on, and itís far more potent than the original of the 1950s and 60s. The 1949 Cadillacís new short-stroke, overhead valve, 160 hp V-8 was the most powerful in the industry, except for Packardís 165 hp flathead straight-eight. But Packardís engine was already passÈ, and Cadillacís was just scratching its potential.
Chrysler countered in 1951 with a hemispherical combustion chamber (the ëhemií), pushrod V-8 with exactly the same 5.4 litre displacement as Cadillac, but with 180 hp. Cadillac responded with 190 hp in 1952, and the race was on. By 1955 Chryslerís C300 had an industry-topping 300 hp, and a Chevrolet could be had with 180. Power more than doubled in less than 10 years, culminating in 450 (Chevrolet) by 1970.
The origin of the brake horsepower used to rate engines really was based on the amount of work a horse could do. Scottish engineer James Watt, inventor of the successful double-acting steam engine in the late 1700s, needed to rate his enginesí power for prospective customers.
The principal use of steam engines was for dewatering mines, so Watt tested some horses to see how much work they could do pulling a turnstile-driven water pump. He concluded that a strong horse could do 33,000 lb ft of work per minute, and established 1 hp as the ability to lift 33,000 lb 1 ft in 1 minute (or 550 lb ft per second).
This is called brake hp (bhp) because it involves resisting shaft rotation with some kind of brake.
Bhp has been measured by several types of dynamometers, including air, water and electric. The classic one was the Prony Brake in which a brake drum and brake band are fitted to the engine crankshaft. One end of an arm is attached to this drum at 90°, and the other end rests on a scale. To test power the throttle is opened, the engine is held at the desired rpm by the band, and the force exerted on the scale by the arm is recorded.
Imagining this arm as the spoke of a wheel, the amount of force exerted, multiplied by the wheelís circumference and the rpm, yields bhp. For example, an arm (spoke) 3 ft long gives a 6 ft diameter, or 18.85 ft circumference ìwheel.î If the engine exerts a 30 lb force at 1500 rpm, it develops 30 x 18.85 x 1500, or 848,250 lb ft of work. Dividing 848,250 by 33,000 yields 25.7 bhp. No matter which type of dynamometer is used, the principle is the same.
Following the big hp rise, engine power collapsed in the 1970s. Some loss came from lower compression ratios needed to accommodate lower octane unleaded gasoline required by catalytic converters. And engines were being strangled by emission control equipment.
Another reduction in quoted power came in 1972 when the American auto industry began using Society of Automotive Engineers net bhp, as measured with the engine installed in the vehicle, rather than SAE gross figures obtained from lab engines without exhaust systems, alternators, air cleaners, power steering pumps or air conditioning compressors.
SAE gross bhp was unrealistic, but it was more impressive to advertise. In fact for regular driving, engine torque (twist) was more important than hp, but not nearly as sexy.
The change from gross to net reduced bhp approximately 30%. All these factors contributed, for example, to a Chevrolet 5.7 litre V-8 dropping from 250 hp in 1970 to 165 in 1972. The oil crises of the 1970s emphasized economy over power.
Electronic engine controls gradually enabled engineers to obtain lower emissions and higher fuel economy. Things like multi-valve cylinders, forced induction and higher compression contributed to increasing hp.
 Another hp race is emerging, and not just in 400 hp Cadillacs and Corvettes, or 500 hp Dodge Vipers, but in mainstream cars. The Honda Accordís 240 hp (above) and Nissan Altimaís 250 seemed daring enough, but now DaimlerChrysler has introduced its 350 hp Hemi powered Chrysler 300C sedan, with 425 hp available in 2005. Even small cars are bulging with power. The 230 (net) hp subcompact Dodge Neon SRT-4 engine (left) makes the original 1951 Chrysler Hemiís 180 (gross) hp look truly anaemic.
These are genuine, full-bodied, SAE net horsepower from engines that are cleaner and more fuel-efficient than ever. They are remarkable engineering achievements, but one wonders where it will end, and whether the insurance industry and government safety agencies arenít starting to notice!
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