BMW M20

The M20 is BMW’s “little six” — a compact single-overhead-cam inline-six built from
1977 to 1993, and the engine that made a six-cylinder 3 Series possible. It is also the engine
of the eta experiment, one of the more interesting things BMW did in the 1980s, and one our
archive can document from the brochures themselves.

What the M20 is

A cast-iron-block inline-six with an aluminium cylinder head and a belt-driven single
overhead camshaft, built in 2.0, 2.3, 2.5 and 2.7-litre forms. It was designed to be short
enough to fit where a four-cylinder engine had gone.

The 1987 3 Series Convertible
brochure describes the 325i version exactly: “Under the hood is a 2.5 liter, 6-cylinder
engine with 168 hp and maximum torque of 164 ft./lb, with Digital Motor Electronics for maximum
engine efficiency.” The same brochure’s technical data gives the construction: “Water-cooled
2.5 ltr., 6-cylinder 4-stroke inline engine, longitudinally mounted and inclined, light-alloy
cylinder head, overhead camshaft with 7 bearings, crankshaft running in 7 bearings with 12
counterweights and torsional vibration damper.”

The eta engine, in BMW’s own words

The most unusual M20 is the 2.7-litre “eta” of the 325e and 528e, and the
1985 3 Series brochure explains what BMW was
attempting more clearly than most later accounts do.

The brochure sets out the problem as throttling losses — an engine at part throttle
“breathing through a stuffy nose”, in its phrase. The solution: “BMW engineers increase
breathing efficiency in the load and rpm ranges most used by BMW drivers. At the same time,
they decrease the final drive ratio, thereby lowering the Eta’s engine’s rotational speeds
(rpm). An idle control is added to reduce idling speeds. Then, to retain the Eta’s acceleration
ability despite the lower rpm level, engine torque (or power) at low and medium engine speeds
is increased.”

That is a coherent engineering argument, made in a sales brochure, in 1985: a large-capacity
six tuned for torque at low revs and geared long, rather than a small engine worked hard. The
brochure credits “Digital Motor Electronics” — “this ingenious computerized engine
management system helps the Eta-six extract maximum performance from minimum fuel” — for
making it work.

The name is the Greek letter eta, the symbol used in engineering for efficiency. It is not
an abbreviation of anything.

Where it appears in our records

Two specification pages, and that is the whole of what United States
government records give us for an engine that ran sixteen years. Fuel economy data begins in
the mid-1980s and rated power only from 1998, five years after the M20 finished. The 320i,
323i, 325e, 325i and 528e are all outside it.

The brochures fill part of that gap, and they are the reason this page can quote a
horsepower figure at all: 168 hp for the 1987 325i, printed by BMW at the time.

What we do not publish

No reliability rating. The NHTSA complaint database records faults against a
vehicle, not an engine, and for cars this old the surviving record reflects what people
were still complaining about decades later rather than how the engine was built. The specific
weakness these engines are known for — the timing belt, which is an interval item rather
than a defect — is a maintenance fact, not a reliability statistic, and we are not going
to dress it up as one.

Common questions

Which BMWs have the M20?

The E21 320i and 323i, the E30 320i, 323i, 325e, 325es, 325i, 325is and 325ix, the E28 528e,
and the early E34 525i. In our United States specification records, which start in 1989, only
the 525i appears.

How much horsepower does the M20 have?

It depends on the version. For the 2.5-litre in the 1987 325i, the brochure in this archive
says 168 hp and 164 lb-ft. The 2.7-litre eta made less power and more low-end torque by design,
and the smaller versions less again.

What is a BMW eta engine?

The 2.7-litre M20 fitted to the 325e and 528e, tuned for efficiency: more torque low down, a
lower rev limit and longer gearing. BMW’s own explanation is quoted above from the
1985 3 Series brochure.

Is the M20 related to the M30?

They are contemporaries and both single-cam inline-sixes, but they are separate designs. The
M30 is the larger “Big Six” of the 5, 6 and 7 Series; the M20 is the
smaller unit built to fit the 3 Series engine bay.

What replaced the M20?

The twin-cam M50, from 1990 in the E34 and 1991 in the E36.

Sources

Quotations from the 1985 3 Series and
1987 3 Series Convertible brochures in
this archive. Model years and fuel economy from EPA fuel economy data.