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Although only one grade of leaded avgas (100LL, standing for 100-octane, Low-Lead)1 is in common use today, there were formerly a number of different leaded avgasses with higher octane ratings and tetraethyllead (TEL) contents (things like 100/130 and 115/145), catering to the large, high-compression piston engines of decades past; these higher-performance blends are now produced only in small-volume special-order batches for applications where even 100LL doesn’t cut it, such as air races.

Are there differences between the various grades of leaded avgas other than their lead content, or could one convert 100LL into (say) 115/145 simply by adding more TEL?

1: 100LL is only “low-lead” when you compare it to the even-leadier avgas blends of days mostly past; it still has a considerably-higher lead content than leaded mogas ever did.

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Gasoline (both avgas and mogas) grades are distinguished solely by octane rating and lead content.

In theory, the octane rating is the amount of actual octane (C8H18) it contains, with the remainder being mostly heptane (C7H16), hexane (C6H14), pentane (C5H12), etc. As you note, a higher octane rating is needed for engines with higher compression ratios.

A century ago, the only way to get high-octane gas was via distillation, and since actual octane is a tiny fraction of crude oil, this was very expensive. It was soon discovered, however, that adding TEL made gasoline behave like it had more octane than it actually did; the higher octane rating you needed, the more lead you added. Leaded gasoline was born, including the 80/87, 100/130 and 115/145 grades of avgas.

However, just a few decades later we discovered how to "crack" long-chain hydrocarbons into shorter ones. Cracking diesel produced (mostly) gasoline--and with a much higher fraction of octane than gasoline distilled directly from crude oil. This allowed creating a 100-octane grade of avgas without any lead, but some old engines actually need a little lead for other reasons, so 100LL was born.

In short, you can take any grade of unleaded gasoline and boost it to any higher grade just by adding enough lead. Start with a higher grade of unleaded, though, and you won't have to add as much.

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    $\begingroup$ @JohnK If you store gasoline in a vented container, the lighter alkanes evaporate more easily, so the octane rating actually goes up over time. However, a cold engine actually needs those lighter alkanes during startup, leading to complaints about "stale" gasoline, and it's worse with unleaded which has fewer light alkanes to start with. Use a sealed container, and both will be stable for millions of years. $\endgroup$ – StephenS Apr 3 '19 at 15:47
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    $\begingroup$ Octane rating doesn't measure actual octane content, it measures susceptibility to detonaton, AKA "knock". So ethanol, which contains no octane at all, nevertheless has an octane rating of 113. $\endgroup$ – jamesqf Apr 3 '19 at 17:05
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    $\begingroup$ @jamesqf Exactly what "octane rating" means depends on where you are and what you care about, and it's not critical to the overall point anyway, so I simplified. Feel free to edit if you think it has a material affect on accuracy or usefulness. $\endgroup$ – StephenS Apr 3 '19 at 21:30
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    $\begingroup$ @Sean Older cylinders rely on a microscopic coating of lead to protect the valve seats and guides from wear; newer ones are made from harder alloys that don't need it. Cars had the same problem, but they don't live as long as planes do. $\endgroup$ – StephenS Apr 4 '19 at 0:28
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    $\begingroup$ Everybody has gone to valve seats made from stellite so the lead isn't necessary as a lubricant any more. The TEL would form a oxide with some exotic name I can't remember, and this would lubricate the interface of the valve with the seat as it closes and has to slide a bit as two conical surfaces self-align. I run mogas in my Lyc O-290 but try to run a tank of 100LL from time to time for the benefit of the valves. Otherwise the engine does NOT like 100LL and the lower plugs start to foul after only about 25 hrs, even with obsessive leaning. The odd side effect of mogas is a lot more soot. $\endgroup$ – John K Apr 4 '19 at 0:32

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