Originally Posted by Mguar

American octane is motor method plus research method divided by 2
British octane is just research method. So British 95 octane is a point or two higher than our combined divided by 2 method.
When you add alcohol to the number, it’s rated lower technically then it is. I say pure ethanol has a 105 octane but it’s not really. Alcohol is both higher and lower but not rated by octane.
The engine will ping less with ethanol and run cooler but that’s because it’s a slower flame front.
That means the expansion of gases occurs further down the stroke of the piston.
Long strokes ( 4” ) develop more torque with alcohol all things being equal.