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But if your car has a knock sensor and the ECU controls the ignition advance I thought that the higher octane fuel meant that combustion is more efficient/closer to TDC - meaning that although there's no extra energy, there is a better conversion to kinetic energy?
Higher octane fuels have a higher flashpoint temperature than the lower octane fuels. Burning efficiency does not come into it when talking about fuels. The efficiency of fuel burning depends on the shape of the combustion chamber and pistons. I built up a Chevy 350 V8 for my Cobra and went against the general consensus of opinion that I needed domed top pistons to reach the desired C/R, as the flame would need to travel around the dome, thus acheiving uneven fuel burning and retarded peak pressure, not to mention high emissions. I went for flat top pistons and changed the heads with lower cc chambers. I reached a C/R of 10.3:1 calculated, which for a large street engine is more than enough. And I was happy in the fact that I was getting complete burning and acceptable emissions. For racing applications, domed pistons will achieve the desired C/R but then fuel ecconomy ceases to become an issue.
