Lead free definition
Until 1973 all new vehicles used gasoline that had tetraethyl lead or some other lead compound as an additive to increase octane rating and reduce knock or detonation . In 1973, almost all new cars required lead-free gasoline (unleaded gas ). The exhaust valves in older cars were lubricated by the lead, so they were redesigned to accommodate the new gasoline . To offset the detonation problem, the compression ratio was also reduced. Lead-free gasoline was introduced to remove the lead in the exhaust fumes as a health hazard.