The Coordinating Research Council, Inc. has studied the octane requirement increase (ORI) of cars and light duty trucks since 1971. This paper investigates ORI trends and influencing effects for over 600 1980 through 1988 model-year vehicles. An analysis of these vehicles shows them to have an average ORI of 3.8 (R+M)/2 measured with refinery-like fuels, and suggests the average ORI decreased from 1980 through 1986, followed by a possible, but statistically uncertain, increase starting in 1987 or 1988. A number of factors have contributed to lower ORI, including higher compression ratios, use of aluminum cylinder heads, multiport fuel injection, and three-way catalyst systems.SPARK KNOCK IS THE NOISE produced by the auto-ignition of the air-fuel mixture in the end gases of the spark-ignition engine. The engine's octane number requirement (ONR) is defined in terms of the highest octane quality reference fuel that produces borderline spark knock under a defined set of operating conditions. In most of today's engines, the ONR increases rapidly when the engine is new, but the ONR eventually reaches a stabilized value at higher mileage (about 15,000 miles in modern engines). This increase in ONR is referred to as octane requirement increase (ORI).The literature suggests that ORI can be attributed to two major causes. The first is the influence on ORI of combustion-chamber heat-transfer and gas temperatures, as affected by combustion chamber deposits and engine design characteristics. It is suspected to have the largest effect on ORI. The second cause is the mechanical increase in compression ratio caused by the accumulation of deposits in the combustion chamber. This is thought to have a lesser effect on ORI, particularly for engines operated on unleaded gasoline.The Coordinating Research Council, Inc. (CRC) has cooperatively investigated octane number requirements of vehicles since 1946, and has reported on octane requirement increase since 1971.The objective of this paper is to conduct a systematic analysis of CRC octane requirement increase data on 1980 through 1988 model-year vehicles in order to establish trends and investigate factors influencing octane requirement increase.