[na85]

It lets you run comparatively higher manifold pressures than an otherwise identical but unleaded fuel.

[maximilianburke]

Only because the engines themselves haven't evolved to handle higher manifold pressures. Lots of advancements in internal combustion engines, like combustion chamber and piston shapes that help manage the flame front, direct injection, wideband O2 sensors, are entirely unknown in certificated aircraft and still on the periphery even in experimental aviation.

Like, Mazda is running 14:1 compression ratios in gasoline engines, but 7:1 is considered normal for a naturally aspirated Lycoming and it'll often dip into the 6's for turbocharged versions.

[sokoloff]

Mazda sold 332,756 vehicles in 2021.

Cessna has the best selling model of airplane of all time, the 172, which has sold around 45K units since its introduction in 1956.

Mazda sells that many cars in a typical 2 month period. It’s no surprise that there’s more non-recurring engineering investment in car engines than airplane engines.

[p_l]

It's not just engine evolution, but TEL was replaced by non-lead additives, not removed. And the fuel mix impacts values critical for aviation but mostly irrelevant to cars, like fuel vaporisation for given temperature/pressure (this is AFAIK major issue with ethanol added to fuel and source of limits on use of MOGAS in aircraft) as well as impact in performance which are pretty much impossible to notice for a car (except maybe if you have engine with sparkplugs designed for leaded fuel) but matter of life and death in airplane.

[throwaway0a5e]

They were designed in the 60s and the FAA certification process makes designing a replacement cost prohibitive with current and future sales volumes.

[bombcar]

More importantly most aviation engines were designed in the 50-70s and need the lead to keep the valves happy.

[p_l]

Another issue is that replacements for TEL that were fine for cars had issues with high altitudes (and high ambient temperatures)

[NonNefarious]

It's about detonation; I don't think it's about the valves.

[jabl]

Post-WWI aviation engines overwhelmingly have hardened valves (and valve seats) which work fine with unleaded fuel.

[outworlder]

And the spark plugs unhappy?