Lead was first used as an anti-knock agent in 1926. The greater breakthrough in catalytic cracking came when the initially developed fixed-bed catalytic process was replaced by the fluid-bed catalytic process. The fluid-bed process allowed for excellent control of temperature and reaction, which permitted better yields of petrol from the refineries. The introduction of the catalytic cracking process and catalytic reforming in the 1940's was significant for the manufacture of high-octane petrol components. The 1950's saw the start of the increase of the compression ratio in engines, requiring higher octane fuels. Octane ratings, lead levels, and vapour pressure increased. Minor improvements continued to be made to petrol formulations to improve yields and octane until the 1970s - when unleaded fuels were introduced to protect the exhaust catalysts that were being introduced for environmental reasons. From the 1970's until 1990 petrols were slowly changed as lead began to be phased out, lead levels plummeted and octane ratings initially decreased, and then remained 2 to 7 numbers lower.
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