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One way of boosting octane would be to add aromatic hydrocarbons. For example, technical xylene (natural mix of ortho-, meta- and para-isomers plus ethylbenzene) has a RON of about 118. It is somewhat toxic, but not nearly so toxic as tetraethyllead. You have to add a fair amount of it to the fuel, but it is inexpensive - in some countries it may be cheaper than avgas, so in principle you could run your engine on neat xylene. During WWII, benzene and toluene were routinely used as fuel for highly supercharged piston engines; you can use them as well, but they are more volatile, more toxic and have a lower octane rating (though still above 100).
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