The only reason leaded avgas still exists is to deliver high octane cheaply and the only reason for octane is to prevent detonation in high-power, high-compression engines. But octane isn't the only way to quench detonation, something engineers have known for years.
This is a companion discussion topic for the original entry at https://www.avweb.com/features/the-return-of-anti-detonation-water-injection-adi