BMW M20

Early M20 engine with K-Jetronic

The first models to use the M20 were the E12 520/6 and the E21 320/6, which used the 1,991 cc (121 cu in) M20B20.[12][13] This engine uses a bore of 80 mm (3.15 in) and a stroke of 66 mm (2.60 in).[12] The M20B20 initially used a Solex 4A1 four-barrel carburetor,[14] had a compression ratio of 9.2:1 and a redline of 6,400 rpm.

The M20B20 first became fuel-injected in 1981,[15] with Bosch K-Jetronic used.[16] 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 finally dropped and replaced with M20.[17]

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.[18][17]

The M20B20 was not sold in North America.

Applications:

• 1976–1981 E12 520/6 (carburettor)
• 1977–1982 E21 320/6 (carburettor)
• 1981–1982 E28 520i (K-Jetronic)[19]
• 1982–1984 E28 520i (L-Jetronic)[20]
• 1982–1984 E30 320i (L-Jetronic)
• 1984–1987 E28 520i (LE-Jetronic)[21]
• 1984–1987 E30 320i (LE-Jetronic)
• 1986–1987 E28 520i (Motronic)[22]
• 1987–1992 E30 320i (Motronic)
• 1988–1990 E34 520i (Motronic)

Six months after the M20 was launched with the carburetted M20B20, the larger, fuel-injected M20B23 was introduced for March 1978. The initial version of the M20B23 uses the same head (known as "200") and block as the M20B20 but a longer 76.8 mm (3.02 in) stroke crank. The bore is 80 mm (3.15 in). Fuel injection was K-Jetronic, the compression ratio is 9.5:1 and the redline is 6,400 rpm.[14]

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 M20B20; this resulted in a power output of 139 bhp (104 kW).

In 1984, the fuel-injection, exhaust and camshaft were upgraded to LE-Jetronic and power increased to 112 kW (150 bhp) with a redline of 6,500 rpm.[23]

The M20B23 was not sold in North America.

Applications:

• 1977–1982 E21 323i (K-Jetronic)[24]
• 1982–1984 E30 323i (L-Jetronic)
• 1984–1987 E30 323i (LE-Jetronic)

In 1985, the M20B25 replaced the M20B23. The M20B25 has a capacity of 2,494 cc (152.2 cu in) and initially produced 126 kW (171 PS; 169 bhp) (without a catalytic converter).[25] It has an upgraded cylinder head (known as "885"), a bore of 84 mm (3.31 in), a stroke of 75 mm (2.95 in), a compression ratio of 9.4:1, a redline of 6,500 rpm and uses Bosch Motronic 1.1 engine management.[26][17]

In 1987, a catalyzed model with Motronic 1.3 engine management was introduced.[27][17] The compression ratio was reduced to 8.8:1 but thanks to the more sophisticated electronics power remained nearly as before, at 125 kW (170 PS; 168 bhp). The uncatalyzed engine was kept in production for Southern Europe and other markets where unleaded petrol was not regularly available.

Applications:

• 1985–1993 E30 325i
• 1989–1990 E34 525i
• 1988–1991 Z1

The M20B27 was designed for efficiency (thus the e for the Greek letter eta in 325e) and low-rev torque.[28] 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 75 to 81 mm (2.95 to 3.19 in), resulting in a capacity of 2,693 cc (164.3 cu in).[26] As per the M20B25, the bore is 84 mm (3.31 in). 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. A compression ratio of 9.0:1 was used on the US 528e, and 11:1 on Euro 525e and 325e cars - this was reduced to 10.3:1 in 1985.[29] The initial M20B27 engines produces 92 kW (123 bhp) and 240 N⋅m (177 lb⋅ft) at 3,250 rpm for models without a catalytic converter.[26] Models with a catalytic converter produce 90 kW (121 bhp) and 230 N⋅m (170 lb⋅ft).

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. 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.[30] The first car to use the M20B27 was the US market 528e in 1982.

In late 1987, the fuel injection was upgraded to Motronic 1.3 on the US market plastic bumper 325e 'Super Eta',[31][32] 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.[33] Power output increased to 95 kW (127 bhp) 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 145 kW (194 bhp) and the second revision, often referred to as "Evo2" or on the VIN plate as "HP2" produced 155 kW (208 bhp).

Applications:

• 1982–1987 E30 325e, 325e
• 1982–1988 E28 525e (called 528e in North America)
• 1989–1992 E30 325iS (only available in South Africa)