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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.

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