MOTHBALLS: HIGH FLYERS OR FLY-BY-NIGHT?
Back in the day—1965—were we doing anything by adding mothballs to the gas tank in my new Plymouth 383?
Ken Lipman Via email
The legend about mothballs improving internalcombustion engine performance dates back to the 1920s. In those days, the real causes of spark knock were poorly understood, there was no uniform gasoline quality standard, and the octane scale for rating gasoline quality as well as the means to test gasoline’s antiknock resistance had not yet been invented. In terms of octane as we understand it today, 1920s motorcar gasolines were around 40 to 60 octane! By the 1930s and 1940s, tetraethyl lead and improved refining processes combined with a true understanding of spark knock, pressure waves, and autoignition lead to the establishment of the octane scale, and street gasoline octane rose into the 60- to 80-octane range.
Back in those days “traditional” mothballs consisted of about 99.9 percent Naphthalene (C10H8), a polycyclic aromatic hydrocarbon. Its blending octane number in modern terms is in the 90- to 92-octane range. When used with that old-school, low-quality gasoline, adding Naphthalene in significant amounts was found to reduce spark knock (in today’s terms, perform as an octane enhancer), and proved to be a real power-adder on the very low-compression car engines of that day.
The most commonly cited gas/Naphthalene blend for street use was said to be 1 mothball for every 4 or 5 gallons of gas. Best results were achieved by premixing in 1-gallon containers and—to remove impurities and any residual solids—straining the brew into the gas tank through funnel-shaped paper filters typically used when mixing automotive paint.
With the development of truly high-octane automotive gasolines in the late 1950s, Naphthalene was no longer needed and even proved counterproductive. With a much higher melting point than gasoline, Naphthalene tends to precipitate out when gasoline starts to evaporate, clogging up jets or fuel injectors, causing the engine to carbon-up, and detrimentally affecting many rubber seals. It definitely wasn’t needed by the 1960s when 100-plus octane gas became widely available. Although Naphthalene may slightly increase octane on today’s 87-octane unleaded regular gas, it will decrease the octane of commonly available 92-octane (or higher) unleaded premium fuels. Incidentally, modern gasoline, consisting as it does of various hydrocarbon chains, may itself contain a little Naphthalene, but no more than 1 percent.
Beware: Today many mothballs are no longer made using Naphthalene. Instead, modern mothballs and so-called “moth crystals” often consist of 1,4-dichlorobenzene (sometimes labeled on the package as paradichlorobenzene, p-dichlorobenzene, pDCB, or PDB). Under combustion in the cylinders, 1,4-dichlorobenzene undergoes a chemical reaction, one byproduct of which is hydrochloric acid!
Warning: Both Naphthalene and 1,4-dichlorobenzene are hazardous to humans in large amounts (you don’t even need to swallow them), can affect blood chemistry, and may cause cancer.
If you want a real octane boost, consider true high-octane gas (as made by Rockett Fuels and other racing-gas specialists). In a pinch, there’s always Toluene. It has an R+M/2 octane rating of around 114.
[ Mothballs made from Naphthalene helped raise octane with crummy pre–World War II automotive fuels, but provides little of any benefit with today’s gasolines. Besides toxicity, downsides include clogged jets and injectors, engine carbon deposits, and degradation of rubber seals.