BMW M20

BMW M20 engine

Manufacturer	BMW
Production	1976–1994
Predecessor	None
Successor	BMW M50
Configuration	Straight-6

The M20 is an inline-6 piston engine by BMW.

Initially designated M60, the 12-valve, belt driven SOHC design was introduced in the 1977 BMW 520/6 and 320/6 as an entirely new design. With displacements ranging from 2.0 to 2.7 liters, it was the "little brother" to the larger BMW M30 engine. It had 91 mm (3.6 in) bore-spacing instead of 100 mm (3.9 in) of the M30. It was intended to replace the larger displacement 4-cylinder motors and was born out of BMW's conviction that a small six had more development potential than a large four (i.e. 2 liters+)

Powering the E21 and E30 3-Series, as well as E12, E28 and E34 5 Series cars, it was produced for nearly two decades, with the last examples powering the E30 325i touring built until April 1993. By that time, the newer twin-cam M50 engines with 4 valves per cylinder had already been used in the E36 and E34 for a couple of years.

Three different head castings were used over the engine's production run. The earliest was #1264200 aka the "200". These were used in all e21 320/6 and 323i and e12 520/6 engines and later in the e28 and e30 eta engines (eta denoting the 'efficiency' version of the engine, with a lower engine redline amongst other focused differences aimed at increasing fuel economy). The next version was #1277731 aka the "731". This head was the same as the 200 but featured larger intake ports. The final version was #1705885 or "885" introduced in the 325i. Ports were further enlarged, valves were larger and the combustion chamber was redesigned to improve flow and thermodynamic efficiency.

As the BMW M21, it became a diesel engine that was also available with a turbocharger.

Overview

Models

Engine	Displacement	Power	Torque	Redline	Year
M20B20	2.0 L (1990 cc/121 in³)	92 kW (123 hp) @ 5800	165 N·m (121 lb·ft) @ 4000		1981
		92 kW (123 hp) @ 5800	170 N·m (125 lb·ft) @ 4000		1981
		95 kW (129 hp) @ 6000	174 N·m (128 lb·ft) @ 4000	6200	1985
		95 kW (129 hp) @ 6000	164 N·m (120 lb·ft) @ 4300	6200	1986
M20B23	2.3 L (2316 cc/141 in³)	102 kW (143 hp) @ 5300	205 N·m (151 lb·ft) @ 4000	6500	1982
		110 kW (150 hp) @ 6000	205 N·m (151 lb·ft) @ 4000	6500	1983
M20B25	2.5 L (2494 cc/152 in³)	126 kW (169 hp) @ 5800	226 N·m (166 lb·ft) @ 4000	6700	1985
		120 kW (161 hp) @ 5800	215 N·m (158 lb·ft) @ 4000	6700	1985
		125 kW (169 hp) @ 5800	222 N·m (163 lb·ft) @ 4300	6700	1987
M20B27	2.7 L (2693 cc/164 in³)	92 kW (123 hp) @ 4250	240 N·m (177 lb·ft) @ 3250	4800	1983
		95 kW (127 hp) @ 4250	240 N·m (177 lb·ft) @ 3250	4800	1986
		90 kW (120 hp) @ 4250	230 N·m (169 lb·ft) @ 3250	4800	1985
		95 kW (127 hp) @ 4800	230 N·m (169 lb·ft) @ 3200	5200	1987

M20B20

The M20B20 is a 2.0 L M20 engine. Entering production in March 1977 in the E12 520 and later in the E21 320/6, it has the distinction of having the longest production run in the M20 series with the last ones rolling off the line in early 1993.

Initially, the engine had a bore of 80 mm (3.1 in) and a stroke of 66 mm (2.6 in). Fueling was handled by a Solex four-barrel carburetor. These early versions all used the "200" head. When the E30 debuted in 1982, the M20B20 was fitted with Bosch L-Jetronic fuel injection and the revised "731" cylinder head featuring larger intake ports. In 1987, the M20B20 was again revised with the addition of Bosch Motronic engine management and a catalytic converter. Later models also received larger valves.

Applications:

• 1977–1981 E12 520/6
• 1979–1983 E21 320/6
• 1982–1984 E28 520
• 1982–1988 E28 520i
• 1983–1992 E30 320i
• 1988–1990 E34 520i

M20B23

This motor was only used in 3-series models and was not sold in North America. The 323i began as an E21 model in 1977 and was phased out in 1985 when it was replaced by the M20B25.

It initially used the same head and block as the carbureted 2.0 liter but used a longer 76.8 mm (3.0 in) stroke crank. It also featured Bosch K-Jetronic Injection. So equipped the E21 323i produced 143 hp (107 kW). The E30 version featured the revised "731" head with larger intake ports and used L-Jetronic instead of K-Jet. Early E30 323i enginess made 139 hp (104 kW) but later versions got updated cams and power rose to 150 hp (110 kW). The early E30 323i, had more torque and was quicker^{[citation needed]}.

Applications:

• 1977–1982 E21 323i
• 1982–1985 E30 323i

M20B25

The German version of the M20B25 produced 170 hp (130 kW) with a catalytic converter and 171 hp (128 kW) without. It featured the newly designed "885" head featuring an open, higher-volume combustion chamber design. The new engine had an 84 mm (3.3 in) bore, and 75 mm (3 in) stroke. The redesigned pistons and combustion chambers made better power, resisted detonation better and had higher thermodynamic efficiency. It was equipped with Bosch Motronic 1.1/1.3 Adaptive fuel injection. The new 12 valve engineering gave M20B25 the name 'verschieden', meaning 'different' in German. The engine is often referred to as 'the differing one' due to the anomalous valve system. With a minor modification that adjusts the idle speed of a cold engine, the 325i fulfills Euro 2 emission requirements in Germany, resulting in a lower tax compared to Euro 1 (as of 2005).

Applications:

• 1986–1994 E30 3 Series
• 1989–1990 E34 5 Series
• 1988–1991 E30 Z1

M20B27

The 2.7 L M20B27 was designed for efficiency (thus the e for the Greek letter eta in 325e). It had an 84 mm (3.3 in) bore and a longer 81 mm (3.2 in) stroke for a total displacement of 2693 cc. The eta engine used the same "200" head casting as the carbureted 2.0 liter and K-jetronic 2.3 as featured in the E21 but only utilizes four camshaft bearings for reduced internal friction. The heads still have seven journals cast into them though and can be drilled to oil a seven bearing cam.

Output was 121 hp (90 kW) at 4250 rpm for all models produced through model year 1987 and 127 hp (95 kW) at 4800 rpm for the final year of production in 1988. Peak torque is 170 lb·ft (230 N·m) at about 3250 rpm for all years of the eta. BMW built this engine with fuel economy and low-rev drivability as the top priority. In production it was coupled to a 2.93:1 differential for the E28 5 series and E30 3 series models starting model year 1986. The earlier 325e used a 2.79:1 ratio. The tall final drive gearing was required to keep the engine in its best operating range at various road speeds. These features did achieve exceptional efficiency, but at the expense of the sparkling performance typically associated with the marque.

Many eta engines used Bosch Motronic engine management systems that were calibrated for maximum fuel economy. The result were very conservative spark advance and fuel delivery curves. In addition, the Motronic had a built-in rev limiter that engaged at about 4750 rpm. While this was hardly sporting, due to the very low overlap and short duration of the cam and the long, narrow intake tubing, the motor would not have made much power over 5,000 rpm anyway. The low redline was an added safety margin to compensate for the four bearing cam and soft valve springs.

In cars produced from September 1987, the eta engine got a substantial refresh from the factory. This included the more recent Bosch Motronic version 1.1/1.3 with adaptive idle control and revised circuitry, the "885" head casting with the larger valves, ports and revised combustion chambers; new pistons to fit the 2.7 L stroke with the 2.5 L head; a special one year only intake manifold with even narrower runners that opened up at the flanges to match the larger ports of the new head; a dual exhaust system; a special eta-tuned 7-bearing cam; and a 5300 rpm rev limit. A new 6000 rpm tachometer was also used.

Applications:

• 1982–1987 E30 325/325e/325es
• 1982–1987 E28 525e (or 528e in the US)

See also

BMW M70 - Dual-bank M20

External links

• BMW E30 Timing Belt replacement procedure Most important maintenance item on BMW M20 motor
• Pictorial for Timing Belt and Water pump Replacement Timing Belt and Water pump replacement with lots of pictures for the M20.