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The 'regular' octane scales (there are more than one) compare the knock characteristics of fuels to pure iso-octane. So in a sense it's a percentage measure comparing a fuel to iso-octane. This is done with a device similar to an engine but with much more constraint. Plus it's goal is not to produce power but to create a measure of the properties of a given fuel. For fuels that exceed the knock characteristics of iso-octane, a new scale is used known as a 'supercharge' scale. This scale expresses the power gain allowed by supercharging. So if for a given fuel you can gain 20 percent power over using iso-octane (under increased manifold pressure from supercharging) it would be rated 120 octane. Small aircraft regularly use 110LL (LL stands for low-lead). It's unclear to me that lead is needed to make a fuel over 109 octane. However, it's important to remember that leaded fuels can not only foul oxygen sensors in your exhaust, they also leave deposits on your intake valves. Old heads built before the advent of unleaded fuels needed the lead as a lubricant for various reasons (I vaguely remember something to the lack of treatments for the valve seat area). Anyway, hopefully this clears some things up for you.
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