BMW M20

The BMW M20 is a SOHC straight-six petrol engine which was produced from 1977 to 1993. It was introduced eight years after the larger BMW M30 straight-six engine, which remained in production alongside the M20.

The first cars to use the M20 were the E12 5 Series and the E21 3 Series. The initial M20 model had a displacement of , with later versions having displacements of up to .

The M20 began to be phased out following the introduction of the M50 engine in 1990. The final M20 engines were fitted to the E30 3 Series wagon (estate) and convertible model built in April 1993.

The M20 was the basis for the BMW M21 diesel engine. It is also loosely related to the BMW M70 V12 petrol engine.

History

By the 1970s, BMW felt the need for a six-cylinder engine smaller than the BMW M30, to use in the 3 Series and 5 Series. The resulting M20 had a displacement of , BMW's smallest straight-six engine of its day. BMW presented the M20 engine at the 1977 IAA as a 90 kW 2.0-litre Solex 4A1 carburetted version, and as a 105 kW 2.3-litre K-Jetronic multi-point injected version. Later versions had displacements up to . The M20 was used in the E12 5 Series, E21 3 Series, E28 5 Series, E30 3 Series and E34 5 Series.

Early versions of the M20 were sometimes referred to as the "M60", although the ''M60'' designation has since been re-used for a V8 engine produced from 1992 to 1996.

Design

As per the M30, the M20 has an iron block, aluminium head and a SOHC valvetrain with 2 valves per cylinder. It has a traditional rocker arm design and no hydraulic tappets. The major differences to the M30 are:
* A timing belt rather than a timing chain
* Bore spacing of , rather than
* Slant angle of 20 degrees, compared with 30 degrees for the M30.

Models

M20B20

The first models to use the M20 were the E12 520/6 and the E21 320/6, which used a version known as the ''M20B20VE'' or ''M60/2''. This engine uses a bore of and a stroke of . A Solex 4A1 four-barrel carburetor was used in the M20B20VE ("VE" is for ''vergaser''- "carburettor" in German), and it has a compression ratio of 9.2:1 and a redline of 6,400 rpm.

The M20 first became fuel-injected in 1981, with Bosch K-Jetronic used in a version called the ''M20B20KE''. The compression ratio was raised to 9.9:1.

In September 1982 (coinciding with the release of the E30 3 Series), the fuel injection was updated to LE-Jetronic with a redline of 6,200 rpm. Other upgrades included a larger port (known as "731") cylinder head, a lighter block and new manifolds. The "M60" designation was dropped and this version was known as the ''M20B20LE''.

In 1987, the M20B20 was again revised with the addition of Bosch Motronic engine management, a catalytic converter and a compression ratio of 8.8:1.

The M20B20 was not sold in North America.

Applications:
* 1977–1981 E12 5 Series 520/6 (carburettor)
* 1977–1982 E21 3 Series 320/6 (carburettor)
* 1981–1982 E28 5 Series 520i (K-Jetronic)
* 1982–1984 E28 5 Series 520i (L-Jetronic)
* 1982–1984 E30 3 Series 320i (L-Jetronic)
* 1984–1987 E28 5 Series 520i (LE-Jetronic)
* 1984–1987 E30 3 Series 320i (LE-Jetronic)
* 1986–1987 E28 5 Series 520i (Motronic)
* 1987–1992 E30 3 Series 320i (Motronic)
* 1988–1990 E34 5 Series 520i (Motronic)

M20B23

In March 1978 (six months after the M20 was launched), a fuel-injected and larger displacement version known as the ''M20B23KE'' (or ''M60/5'') was introduced. This version uses the same head (known as "200") and block as the 2.0 litre version but a longer stroke crank. The bore is and it has a capacity of . Fuel injection was K-Jetronic, the compression ratio is 9.5:1, the power output is and the redline is 6,400 rpm.

The 1982 version used LE-Jetronic, the 731 cylinder head, a compression ratio of 9.8:1 and the other upgrades as the per the 2.0 litre version. This version is called the ''M20B23LE'' and has a power output of .

In September 1983, the M20B23LE's fuel-injection, exhaust and camshaft were upgraded and power increased to with a redline of 6,500 rpm. The version continued to be available in certain markets with strict emissions regulations (such as Switzerland) until replaced by the 325i.

The M20B23 versions were not sold in North America.

Applications:
* 1977–1982 E21 3 Series 323i (K-Jetronic)
* 1982–1984 E30 3 Series 323i (L-Jetronic)
* 1984–1987 E30 3 Series 323i (LE-Jetronic)

M20B25

In 1985, the M20B25 replaced the M20B23. The M20B25 has a capacity of and initially produced (without a catalytic converter). It has an upgraded cylinder head (known as "885"), a bore of , a stroke of , a compression ratio of 9.4:1, a redline of 6,500 rpm and uses Bosch Motronic 1.1 engine management.

In 1987, a catalyzed model with Motronic 1.3 engine management was introduced. The compression ratio was reduced to 8.8:1 but thanks to the more sophisticated electronics power remained nearly as before, at . The uncatalyzed engine was kept in production for Southern Europe and other markets where unleaded petrol was not regularly available.

Applications:
* 1985–1993 E30 3 Series 325i
* 1989–1990 E34 5 Series 525i
* 1988–1991 Z1

M20B27

The M20B27 was designed for efficiency (thus the ''e'' for the Greek letter ''eta'' in ''325e'') and low-rev torque. This is an unusual design strategy for a BMW straight-six engine, which are usually designed for power at high RPM. Compared with the M20B25, the stroke is increased from , resulting in a capacity of . Since many markets tax cars based on engine displacement, the eta's larger displacement meant that it was not suitable for all markets. It was expressly developed with the American market in mind. As per the M20B25, the bore is . To reduce friction and improve efficiency, the M20B27 changes include using the '200' version of the head (which has smaller ports), a different camshaft, four camshaft journals and softer valve springs. Due to these changes the rev limit on the M20B27 was reduced to 4,800 rpm. The initial version, called the ''M20B27ME'', produces and at 3,250 rpm for models without a catalytic converter. Models with a catalytic converter produce and .

In the United States, BMW's corporate average fuel economy was at risk of not meeting requirements by 1984, primarily due to higher sales of their bigger, more expensive cars in the early 1980s. The first car to use the M20B27 was the US market 528e in 1982. The compression ratio of the U.S. M20B27ME version was 9.0:1, compared with for cars sold elsewhere 11.0:1.

In 1985, the ''M20B27ME.E'' version was introduced, increasing power output to despite a lower compression ratio of 10.3:1.

In late 1987, the fuel injection was upgraded to Motronic 1.3 on the US market plastic bumper 325e and 528e 'Super Eta', the cylinder head changed to the "885" version, the compression ratio was reduced to 8.5:1 and the redline increased to 5,200 rpm. Power output increased to at 4,800 rpm.

In the early 1990s BMW South Africa used components from the Alpina C3 2.7 to produce an E30 specifically for Stannic Group N production car racing. The first iteration of this engine used in the E30 325iS produced and the second revision, often referred to as "Evo2" or on the VIN plate as "HP2" produced .

Applications:
* 1982–1987 E30 3 Series 325e, 325e
* 1982–1988 E28 5 Series 525e (called 528e in North America)
* 1989–1992 E30 3 Series 325iS (only available in South Africa)

See also

* BMW
* List of BMW engines

References

{{DEFAULTSORT:Bmw M20
M20
Straight-six engines
Gasoline engines by model