BSFC must be calculated based on OBSERVED hp, not corrected hp.

I disagree.  If BSFC was based upon observed HP, then no two BSFC curves could be compared.  It would always be apples to oranges.  Standard day corrections are used for everything and are not "junk science" (as for "snap HP," well, that's a different matter).  Bottom line is that you would like for every engine to be dyno-evaluated in exactly the same manner for making valid comparisons.  FAA engine tests are one way in which that is achieved.  Car manufacturers dyno-test to specifications published by SAE.  The local speed shop or aircraft engine rebuilder . . . who knows?  

Moral of the story is to rely upon manufacturer curves where available.  Second moral of the story?  

BSFC is useful for comparing engines and not much else.  If you want to know how efficient your airplane is with a given engine (i.e., "system efficiency" composed of airplane aerodynamics, engine efficiency and popeller efficiency components), the only realisting way is to run a closed course and measure the fuel use and then correct to standard atmosphere (ala CAFE) if you want to use the data for comparison purposes.  As we all know, a "standard atmosphere" is an engineering fiction.  I doubt there's ever been a standard atmosphere here in Houston.

Rick Price

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