Vehicle Emissions Reduction

Leaded gasoline has other effects on the engine. As engine technology advanced during the era of leaded gas, designers used lead to serve as a lubricant between exhaust valves and their seats, enabling them to use a lower grade metal on the valve seat itself.

The use of leaded fuel with these low-grade valve seats shielded them from excessive wear (known as "valve seat recession"), which can occur at high speeds in engines without hardened valve seats. Valve seat recession can result in hard starting, engine stalling and reduced power output. Retaining this protective function is the reason that the US EPA limited gasoline lead content to 0.1 g/gallon, rather than banning its use entirely in 1985. Sweden found that only 0.07 g/liter was needed to lubricate the valves; at this level, they also found that there was no need for scavengers.

Real world experience has shown that the actual incidence of valve seat recession is small even in vulnerable vehicles, however. Only vehicles which travel consistently at very high loads and speeds appear to be at all susceptible. And even for these vehicles, additives other than lead have been shown to protect valve seats. Sweden and Austria have found that sodium and potassium based additives are adequate substitute lubricants. Sweden also found no evidence that these additives introduce any additional health concerns, either with regard to regulated emissions or with biological tests. Thus, there is presently little technical argument for retaining any lead in gasoline if the refining capacity exists to provide the required octane in some other manner.