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I tried to look back in some old books for a brake horsepower definition. What I found was insufficient.
My understanding of the use of brake horsepower with respect to an automotive meant that it was the mechanical power out from the crankshaft with the engine block stripped of any other loads. What this means is no water pumps, no fan, no generator, and no muffler, and today no airconditioner compressor, or power steering pump, etc.
In "The Gasoline Automobile", by Hobbs and Elliott, 1924, on page 35 ---
" We can determine the power developed by the explosions in the cylinder, in which case we have what is called the indicated horse power (i.hp.); or we can attach a brake to the flywheel and measure the power that the engine actually produces. This is called the brake horse power (b.hp.)."
When comparing these two measurements it is logical to eliminate all unrelated loads on the engine. Further for evaluation of the engine from a thermal conversion perspective it is useful to not have to subtract a lot of unknown values. Thus, it makes sense that brake horsepower is defined with no accessories on the engine.
In 1929 to 1932 when Ford was developing a low cost monoblock V-8 much of the secret development work was done in the Edison Fort Myers Laboratory in Greenfield Village. To do dyno testing of the engine Zoerlein used the steam engine in this building as the brake load, probably as an air compressor.
In the early 30s after the V-8 was in production and durability tests were run on the regular dyno stands, then with the engines running full throttle the exhaust manifolds would glow red.
When you have an engine in your car you really aren't interested the brake horsepower, but the actual power available with all the extra junk on the engine. So a brake horsepower rating of an engine is greater than what you actually get at the drive shaft .
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