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Old 04-11-2008, 12:40 AM   #20 (permalink)
Frank Lee
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Blue - '93 Ford Tempo
Last 3: 27.29 mpg (US)

F150 - '94 Ford F150 XLT 4x4
90 day: 18.5 mpg (US)

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From the Autospeed Brake Specific Fuel Consumption link:

Firstly, why should the SFC be lowest at middle revs? Or, to put this another way, what causes an increase in fuel used per kW at both low and high revs?

At low revs, SFC suffers because there’s increased time for the heat of combustion to escape through the walls of the cylinders and so not do useful work. At higher engine speeds, the frictional loses of the engine rise alarmingly (especially in this case with 12 cylinders!) and so the energy of combustion is again being wasted, this time in heating the oil.
Maybe this is why 1000-1200 ft/sec is better than lower speeds?

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