Tetra-ethyl lead

• it reduced premature ignition - detonation or "knocking" of the engine;

• the lead deposited on the valve seats reduced wear on them;

• and it poisoned the environment with lead

The production of harder metals for valves and valve-seats, a reduction in the compression ratio demanded and substitution with compounds in the fuel that reduce pre-detonation and a reduced tolerance for poisonous waste lead to cessation of its use in the Western countries in the latter part of the 20th century.