The standard bearer 3 Series engine for almost 30 years, the M20 became the heart and soul of the 325i.

Originally used in the New Six model BMW 3.0Si in 1971, the M20 engine became famous as the engine of the E21 and E30 3 Series. M20 engines were also used in the E12 5 Series models.

The M20 was used in the E21 320 and 323 models, the E30 325 models and the E28 528e. The M20 engine was also used in some E34 5 Series 525 models, although the M50 was the engine of choice for the E34.

Sometimes called the "little six" or the "baby six" to distinguish it from the M30 "big six", the M20 is a belt-driven six-cylinder engine with a single overhead camshaft and 2 valves per cylinder. The M20 used Bosch K-Jetronic fuel injection.

E Power

While the M20 has a definite sporting side, one version of the M20 became famous as one of the most economical BMW engines ever built.

A specially equipped M20 engine powered the 325e and 528e models. The "e" refers to the Greek letter "eta" and stands for efficiency. The "e" M20 is a 2.7 liter engine, M20/27, and it was designed for fuel economy and reliability during the Seventies fuel crisis.

There were two distinct varieties of eta engines, those used in 1984-1987 models and the 1988 model:

Maintenance

The timing belt is the Achilles heel of the M20. Change it every 50k miles for long-life!

The M20 is an "interference" engine, since the valves open so far into the cylinders (they "interfere" in the piston space) that if the valve timing isn't perfect, the valves can be struck by a piston.

The "interference" design actually increases the engine's efficiency, because the wider the valves open, the more air/fuel you can get in and out of the cylinders.

But when the timing belt breaks on an interference engine, the pistons and valves can collide, causing extensive engine damage.