The devil is in the details with unleaded avgas
from General Aviation News
In the 1970s, the automotive world switched from leaded to unleaded fuels and the oil companies did a lot of research on knocking and how to prevent it. One of the big projects involved octane requirement increase. In this program, cars were rated for octane requirement when new and then every 2,000 miles. The octane requirement increased until it leveled off at about 20,000 miles. Usually the ORI was about 6 to 10 numbers.
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