Quote:
Originally Posted by some_other_dave
Where did you hear that? The lines cross at the same RPM, always! (Exactly what RPM depends on what units you use for power.)
In SAE units, HP is defined as (Torque * RPM / 5250). So those lines will always cross at 5250 RPM, assuming the engine was measured both above and below that RPM.
5200+ RPM is almost never the most efficient speed to spin your engine.
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Each engine BSFC map will show an island of highest thermal efficiency, spanning some rpm range.
If memory serves me, it has to do with mean effective pressure developed and coincident pumping losses.