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 BMW models 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 touring 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 saw a need for a new I6 engine to bridge the displacement gap between their smaller M10 I4 and the M30 I6 engines. This new engine would solve the following business- and engineering-related issues:

The M30, despite being a turbine-smooth inline-6, was too large and too heavy to fit in the 3 Series engine bay. The M10, despite being a small and light inline-4, became too buzzy and unrefined at larger displacements; 2.0 L is typically an upper limit for inline-4 engines in terms of maintaining refinement. A 2.0 L variant of this new I6 would offer European buyers refinement and the turbine-like smoothness of an I6 while bypassing European displacement taxes. The new engine could be evolved to accommodate the tightening emissions standards of the 1970s and 1980s.

BMW debuted the M20 engine at the 1977 IAA in two forms: a 2.0 L variant with Solex 4A1 carburetor making 90 kW and a 2.3 L variant with Bosch K-Jetronic MPFI making 105 kW; both variants had a 80 mm bore.BMW AG (ed.): IAA Frankfurt/Main 15.9. bis 25.9.1977, p. 5, retrieved 17 September 2021 In 1982 BMW increased the bore to 84mm, ultimately leading to 2.5 L and 2.7 L variants. 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 The M20 shares the following design characteristics with the M10 and M30: Hans-Rüdiger Etzold: So wird’s gemacht (volume 68): BMW 5er Reihe 9/72 bis 7/81 (TYP E12), BMW 5er Reihe 7/81 bis 8/87 (TYP E28), Delius Klasing, Bielefeld, 1990, ISBN 978-3-7688-0666-4

The engine block is a closed deck design that is made out of cast iron and features siamesed cylinder bores The cylinder head is a cross-flow design made out of cast aluminum alloy The valvetrain layout is SOHC with 2 valves per cylinder. Rocker arms with manually adjustable eccentric pushers actuate the valves

The M20 differs from the M10 and M30 in the following ways: The camshaft is driven using timing belt instead of a timing chain to reduce noise and production costs The bore spacing is reduced from to to reduce engine length and weight The slant angle is reduced from 30 degrees to 20 degrees when mounting the engine in the engine bay, resulting in a more upright engine

Throughout the M20's production run, three different cylinder head casting variants were used:

1264200 (200): this variant was used for pre-1983 M20B20 and M20B23 engines, as well as in the pre-1988 M20B27 engine. This casting features very small intake ports with 40mm intake and 34 mm exhaust valves. 1277731 (731): this variant was used for post-1982 M20B20 and M20B23 engines. It keeps the same valve diameters as the 200 casting but enlarges the intake ports. 1705885 (885): this variant was used for the M20B25 and the 1988 M20B27 engines. This casting features the largest intake ports. The intake and exhaust valve diameters are both enlarged by 2 mm each over the 200 and 731 castings.

Variants VersionDisplacementPowerTorqueBoreStrokeCompression Ratio Years Notes M20B20 at 6,000 rpm at 4,000 rpm 9.2:1 1977-1982 Solex 4A1 variant at 5,800 rpm at 4,000 rpm 9.0:1 (catalyst) 9.8:1 (non-catalyst)1982-1984L-Jetronic fuel injection with "200" head at 4,000 rpm 1984-1987 LE-Jetronic fuel injection with "731" head at 6,000 rpm at 4,300 rpm 8.8:11986-1992 Motronic 1.1/1.3 engine management Compression ratio reduced to accommodate emissions controls and unleaded gasolineM20B23 at 5,300 rpm at 4,500 rpm 9.5:11977-1982 K-Jetronic fuel injection at 5,300 rpm at 4,000 rpm 9.0:1 (catalyst) 9.8:1 (non-catalyst)1982-1983L-Jetronic at 6,000 rpm 1983-1987 LE-JetronicM20B25 at 5,800 rpm at 4,000 rpm 9.4:11985-1990 For markets that did not require a catalytic converter at 5,800 rpm at 4,300 rpm 8.8:11987-1993 For markets that required a catalytic converterM20B27 at 4,250 rpm at 3,250 rpm 9.0:11982-1987 For markets that required a catalytic converter at 4,250 rpm at 3,250 rpm 11.0:11982-1985 For markets that did not require a catalytic converter Early high compression variant at 4,250 rpm at 3,250 rpm 10.2:11985-1987 For markets that did not require a catalytic converter Compression ratio reduced to mitigate knocking at 4,800 rpm at 3,200 rpm8.5:11988"Super" Eta engine Motronic 1.1

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 the United States, but was available in the E30 (320i) in Canada.

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) 1989–1992 Bertone Freeclimber (Freeclimber I)

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.

For the 1988 model year, the digital motor electronics was upgraded to Motronic 1.1 on the US market plastic bumper 325e and 528e 'Super Eta',Automobil Revue '83, p. 206 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) 1989–1992 Bertone Freeclimber (Freeclimber I)

See also

BMW List of BMW engines

References