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Horsepower S.A.E. and Horse Power?

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Not sure I know what “S.A.E. horsepower” is. In the really old days there was N.A.C.C. Horsepower which was based on the bore and stroke and assumed something like a 1900 era engine design (low RPM and low compression ratio). It was woefully out of date by the time my 1933 Plymouth was built. The NACC horsepower was sometimes called "taxable horsepower".


There is Brake Horsepower (BHP) which is based on putting the engine on a dynometer and using a brake to put a load on the engine. And that is what I think most US build cars from the late 1920s through the 1960s advertised. The issue there is that they did not include the losses in the driveline nor losses to accessories. By the 1960s with automatic transmissions, air conditioning, higher electrical loads, etc. a lot of the engine BHP never made it to the tires. So I think current cars are supposed to be rated by how much power actually makes it to the wheels.


With all that, my 1946 through 1954 Plymouth factory service manual shows the 217.8 cu.in. engine putting out between 95 BHP in 1946 and 100 BHP in 1954. Looks like much of that gain can be attributed to the increase in the compression ratio from 6.6:1 in 1946 to 7.1:1 in 1954. For all those years the taxable horsepower is listed at 25.35.

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Engines/cars are still rated by crank HP. The deal today is that accessories that are standard equipment, like power steering must be installed on the engine during the testing. The standard is SAE J1349.

Edited by lostviking
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