Horsepower, gross and net

Some many weeks ago I put forward a notion that there is something known as engine gross horsepower and net horsepower. One of my cars was rated as 270 gross horsepower and the next year, the same car with exactly the same engine was rated as 200 net horsepower. One or more lister suggested the notion was lunacy (my word) and maybe I was a lunatic. (I prefer nut case, but generally only after I've had half a dozen pints) Well, I have just come across an article in the July 1999 Car and Driver magazine that puts this all straight.

Csaba Csere in his column "Power is power, right?" speaks of the outrageous power claims by U.S. auto makers in the 60's. He says "Truth is, horsepower in the '60s was a crock. Sure, there were fast cars with big engines, but they rarely generated the power touted in ads. That's because power was measured by a procedure labeled SAE gross, and it was the grossest standard ever associated with the Society of Automotive Engineers.

"SAE gross horsepower was measured by removing the engine from the car and stripping it of its exhaust system, air cleaner, air-conditioning compressor, and power-steering pump. The engine itself was usually a carefully selected example with its compression ratio at the high end of the tolerance and some porting and polishing. For the testing, a set of headers was bolted on, and the spark timing and the carburetor were fine-tuned to maximize output.

"To illustrate how this procedure exaggerated an engine's power, I will quote from a pair of 'stock-engine tests' conducted by Ford in the late '60s, which I fished from the garbage when I was an engineer there 20 years ago. Report No. 31253 covered a 1968 Oldsmobile 455 cubic-inch V-8, advertised with 365 horsepower at 4600 rpm.

"The Ford procedure first measured the engine's output exactly as it was in the car, complete with the exhaust system and all the accessories. In this form, the engine developed 226 hp at 3600 rpm -- 139 hp or 38 percent less than its claimed 365 hp. Then, with all the SAE gross power-inflating tricks applied, the 7.4 liter V-8 still only made 319 hp at 4400 rpm.

"The truth was, these SAE gross ratings were essentially meaningless. But it wasn't that way everywhere in the world. In Germany, power was measured to a strict standard called the Deutsche Industrie-Norm, or DIN. The engines were outfitted exactly as they were installed in the car, and the measurements were corrected to prescribed standards of temperature and pressure.

"There could, of course, be no consistent conversion between DIN and power and the nebulous SAE gross figures. For U.S. purposes, most of the German manufacturers simply added between 10 and 15 percent to their DIN figures.

"The U.S. manufacturers came around in 1972, when they began quoting SAE net rather than SAE gross hp. The net figure, just as the DIN figure, was measured with the engine in its as-installed configuration. Quoted power figures plummeted dramatically, especially since, at the same time, compression ratios were dropped in response to unleaded fuel. For example, the Corvette's high-revving LT1 V-8 went from 370 hp SAE gross in 1970 to 330 hp SAE gross in 1971, when compression dropped from 11.0:1 to 9.0:1. That 330 gross hp translated into 275 net hp."

So, it looks like I wasn't a lunatic after all. :-)

Relatedly, in the same article he also discusses the horsepower being quoted by BMW for the 328i. "When BMW introduced its redesigned 328i last year, the official power rating was 193 horsepower, three ponies more than its predecessor. There was nothing unusual about this increase; the '99 engine was upgraded with several potent features.

"The problem is that the European 328i is rated at 193 PS. PS stands for Pferdestarke [with two dots above the "a" since my keyboard can't do that], which literally means 'horse strength' -- German for 'horsepower'. [Perhaps a German speaking lister can speak to this] Not surprisingly, a PS is a metric horsepower, equivalent to 75 kilogram-meters of work per second. Engineers can convert that figure to 542.5 foot-pounds per second, which is slightly less than the 550 foot-pounds per second in an American hp. Therefore, a 193 PS equals 190 hp, not 193 hp.

"..........the Germans corrected to 68 degrees Fahrenheit and 30.0 inches of Mercury of dry-air pressure. Meanwhile, the Americans corrected to less dense conditions at 77 degrees and 29.3 inches of dry air. The result was that the DIN corrections produced slightly higher numbers than the American method did. Combined with the difference between hp and PS, SAE horses had suddenly gone from being a joke to being actually about five percent bigger than DIN ponies. That's' why a 1978 Porsche 911SC, which was equally powerful in the U.S. and Europe, was rated at 172 SAE net hp and 180 DIN PS.

"In the mid-'80s, the European carmakers adopted a new European Economic Community (now known as the European Union) power-measuring standard, which standardized the test conditions as the SAE values. There are still minor differences in the correction formulas, but generally, the power calculated by SAE and EU methods should be within a half-percent of each other.

"Which still leaves us with the puzzling identical U.S. and European power quotations for the BMW 328i. Turns out that in order to meet our Low-Emissions Vehicle standard, the U.S engine has a different intake manifold and a larger, freer-flowing catalyst than the German version. As a result, it develops three more hp than the Euro motor, which is why it is rated at 193 hp -- three hp stronger than 193 PS.

So, for those lister(s) who had previously said "power is power", or "horsepower is horsepower" perhaps it's not really as simple as that; you have to name your terms.

By the way, horsepower is Torque * RPM / 5,252