6.20 Can mothballs increase octane?

The legend of mothballs as an octane enhancer arose well before WWII when naphthalene was used as the active ingredient. Today, the majority of
mothballs use para-dichlorobenzene in place of naphthalene, so choose carefully if you wish to experiment :-). There have been some concerns about the toxicity of para-dichlorobenzene, and naphthalene mothballs have again become popular. In the 1920s, typical gasoline octane ratings were 40-60 [11], and during the 1930s and 40s, the ratings increased by approximately 20 units as alkyl leads and improved refining processes became widespread [12].

Naphthalene has a blending motor octane number of 90 [52], so the addition of a significant amount of mothballs could increase the octane, and they were soluble in gasoline. The amount usually required to appreciably increase the octane also had some adverse effects. The most obvious was due to the high melting point ( 80C ), when the fuel evaporated the naphthalene would precipitate out, blocking jets and filters. With modern gasolines, naphthalene is more likely to reduce the octane rating, and the amount required for low octane fuels will also create operational and emissions problems.