Studies with cars and with full scale engines in the laboratory have provided additional information on the parameters which affect ORI in today's cars. Tests conducted with cars showed that ORI with unleaded gasolines is about two units higher than with leaded gasolines and that coolant system degradation had little or no effect. Studies conducted in the laboratory showed that oil consumption rate, oil multi-grade range, fuel detergent additives, and coolant temperature all affected ORI to some degree. Oil ash level and a manganese antiknock additive had no effect. Combustion chamber deposits studies showed that a good correlation exists between ORI and the ratio of carbon to metals in the deposits. This correlation suggests that the carbon to metals ratio should be minimized to reduce ORI. Additional studies are needed to confirm these results and to further define the parameters which must be controlled to minimize ORI.
