Mitsubishi Astron engine

It employed a hemispherical cylinder head, chain-driven single overhead camshaft (SOHC) and eight valves (two per cylinder). United States passenger car versions had a small secondary intake valve referred to as the "Jet Valve". This valve induced swirl in the intake charge, enabling the use of leaner fuel/air mixtures for lower emissions. It was designed as a cartridge containing the valve spring and seat which simply screwed into a threaded hole in the head, similar to a spark plug but inside the cam cover.

A 4G94 engine in a Mitsubishi Lancer.

In 1975, the Astron 80 introduced a system dubbed "Silent Shaft"; the first use of twin balance shafts in a modern engine. It followed the designs of Frederick Lanchester, whose original patents Mitsubishi had obtained, and proved influential as later engines from Fiat/Lancia, Saab and Porsche all licenced this technology.

The 4D5 engine was a range of four cylinder diesel engines which were part of the "Astron" family, and introduced in 1980 in the then new fifth generation Galant. The first turbodiesel to be offered in a Japanese passenger car, it later proved popular in the emerging SUV and minivan markets where Mitsubishi was highly successful, until superseded by the 4M4 range in 1993. However, production of the 4D5 continued throughout the 1990s as a lower-cost option than the more modern powerplants.

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