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Efficiency means BSFC, and if you study the factory figures, you will find a decrease in consumption for each increase in compression. Curb idle timing does effect NOx (not nitrous oxide) at idle, but it has nothing to do with output or fuel efficiency which occur at an advanced timing. The main thing to understand about 911 timing is that the max advance spec went from as high as 38 BTDC to 25 BTDC max because of the knock limit imposed by the factory specified fuel. A hemi headed engine with a wide VIA and domed pistons does not make peak output at 25 BTDC.
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Paul
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