Re: All HP still not created equal....???

Posted by Steve - IN on January 06, 2003 at 08:59:16 from (12.222.17.160):

In Reply to: All HP still not created equal....??? posted by Wayne on January 05, 2003 at 22:03:31:

Wayne,

I think you might start by blaming Napoleon, or maybe Nelson at Trafalgar, or Wellington at Waterloo, or maybe Bismarck for not being born 100 years earlier, or maybe Patrick Henry.

The French spread the metric thing, but they never conquered England - so we still have feet and pounds, but we'd revolted by then anyway. Meantime the Germans never unified until the mid 1800's, so the German pfund (pound) was about 14.8 ounces in Stuttgart but 18.1 ounces in Vienna. They did measure horsepower, though, so the pferdstarke - ps - (horse power) which became the DIN rating (Deustch Industry Normal) that grew up a few hundred miles away from Watt in Germany is based on a slightly different idea of what a horse can do (and even a different horse) plus a bit of a fuzzy idea of what a pound weighed, not to mention the size of the King's foot. Today German, and most European cars and tractors are rated in DIN horsepower (which itself changed a bit in metric translation), American ones in SAE horsepower. Two different definitions based on the same formula concept -- just the units of measure going into the formula are different so you've got Stuttgart and Detroit gas engine horsepower as two different animals.

In Asia you'll see Japper, Korean and Chinese engines rated in kW, which is based on 1 Watt - specified as 1 Newton Meter per second (what a mess - two famous Limeys get stuck with a French linear measure and shipped off to eat sushi in Yokohama after their deaths, when they had nothing to say about it.) 1 Newton meter/sec works out to be about 1 hamster power, by the way.

Even in the good ol' USA, you can't compare SAE horsepower ratings pre and post 1971. The scale changed. After '71 they started adding stuff like alternators and air cleaners when they took the measurement on engine dynos. Same engine made less horsepower. Guys who loved fast cars or powerful tractors knew all along that an engine dyno didn't tell you what you really wanted to know, so they measure horsepower on a chassis dyno -- where the driven wheels spin the big load wheels in the floor, or a thing that turns off a tractor PTO. Same engine, same car or tractor - two different ideas on how much horsepower it has.

The next thing to blame is the thing being measured itself, because they don't all behave in the same way, or lend themselves to the same measurements as the 283 V8 in your '57 Chevy. Steam engines on boats or locomotives predate Chevies and dynos, and don't easily lend themselves to any kind of dyno we've ever seen anyway. So long ago they started measuring the gas pressure in the cylinder at various points of piston travel and backing into a horsepower number with that data. Things like electric motors behave differently than your gas or Diesel tractor engine - for example, they have full torque on their first revolution. So it never made sense to measure them in the same way as a gas or Diesel piston engine.

When you try to translate all these other measurements into an equivalent for good ol' American SAE, cruisin' down route 66 horsepower - you end up with oddball stuff. A German or Metric horse is about .986 of a Detroit 33,000 ft. lb horse, so it's satisfied to lift 325.38 pounds of coal 100 feet in a minute. It takes about 745.99 Asian Newton Meter hamsters to equal 1 Detroit horse. Even with the same language, different industry definitions don't jive because of the differences in the power plants they were trying to measure over the years. So now if you take a GE electric motor rated at 30hp continuous and stick it in a car, it will perform about the same as a 40 to 50 hp peak rated GM engine.

In short, the answer to your question is that a lot of smart or powerful guys have worked years to put their spin on things based on where they were from or what kind of stuff they were working with and turn a fairly simple concept into as convoluted a mess as possible. Good luck, and have fun sorting it all out.

Steve

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