When the Wright brothers first took to the air, the fuel they used was a straight run gasoline with only about a 37-octane rating.
Straight run is basically a light fuel that boils just below the kerosene fraction and has no additional processing done to it. Due to the poor cooling of the Wright engine, it usually started detonating or pre-igniting shortly after takeoff.
Since straight run gasoline was one of the limiting factors for aviation, especially for the military, a huge research effort was launch by the oil companies, the military, plus numerous other research agencies in the early 1900s for a better solution.
When I started working for Shell, one of my projects was working on lead scavengers. This led to a lot of looking through old research reports on the subject. I was really surprised at the scope and amount of basic research done on the subject.
The first work was refinery process changes that would reduce the knocking tendency of fuels. It was quickly determined that they could improve the fuel some, but they needed an additive to help them meet the requirement of many newly developed aircraft engines.
Then they figured out that most liquefied metal compounds worked to reduce knocking in test engines, however all of the metals tested had some negative side effects. For example, iron compounds greatly increased engine wear, the manganese additive cause red whiskers on the plugs, the lead additives would short out the spark plugs, and on and on.
But work continued, concentrating on finding scavenger compounds that would negate the bad side effects of the metallic additives. They eventually found that a lead compound used with a combination of bromine and chlorine scavenger agents (plus a phosphorus supplemental additive) could be used in high compression engines. This worked especially well with a new alkylation refinery processed fuels. This resulted in 100/130 avgas, which had no lead limit, but usually contained about 3 grams of lead per gallon.
The fuel for lower compression engines had a 0.5 grams of lead per gallon limit or 80/87 fuel. This worked well with the lower spark plug core temperature for these engines.
The 100/130 avgas was just what the high-powered military aircraft used in World War II needed. These fuels allowed the Allied aircraft engine manufacturers to greatly increase the compression ratios and performance of their engines and gave the Allied forces a significant advantage over the German and Japanese forces.
After the war, the aviation community settled on the two fuels and they remained unchanged until the early 1970s.
At that time the commercial aviation world had converted over to almost all Jet-A engines and the volume of avgas had gone down to an almost insignificant percentage of the world fuel market. That meant the total volume was too low to support two separate grades.
The oil industry reached a compromise and converted over to just one grade, 100/130 low lead. In this compromise the level of lead in 100/130 was limited to 2 grams per gallon and the 80/87 fuel was eliminated.
Unfortunately, this compromise did not work well for either sector of general aviation. The increase from 0.5 to 2 grams lead per gallon caused spark plug fouling and increased engine deposits on many aircraft.
And even though the lean rating of 100LL was greater than the old high leaded fuels, the decrease in the rich rating for the 100LL fuels resulted in a significant increase in knocking complaints.
The lead story on the automotive gasoline side was similar. The demand for increased performance caused an octane race. In the 1960s compression ratios kept increasing. By the late 1960s, there were numerous high-performance vehicles with 11 to 1 compression ratio or even higher.
Then in 1971, most of the auto manufacturers reduced the compression ratios into the 8 to 9 to 1 ratio to allow for the use of unleaded auto gas. They have now switched to electronic fuel injection and timing controls, plus many other systems, to return back to the 1960s performance levels using lower octane fuels.
The piston aviation community is now facing the switch to unleaded fuel with lower anti-knock performance in the real world than 100LL and probable exhaust valve recession in many engines. And the industry and government want to do it with no changes to the engines.
I feel this is an impractical goal without a research effort similar to that done before World War II that included thousands of very talented research scientists.
And the more important question: Who is willing to pay for it?