M20

The M20 is an straight-6 piston engine of BMW, whose 12-valve, belt-driven SOHC design in three parts, featuring a main Block with a Sump at the bottom and a Head at the top.

M20B25 Engine with battery-in-boot

History

The M20 was introduced in the 1977 BMW 520/6 and 320/6. With displacements ranging from 2.0 to 2.7 liters, it was the "little brother" to the larger BMW M30 engine, and was then known as the M60. 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 litres+)

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 sometime in 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.

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

Versions

Models

Engine	Displacement	Power	Torque	Redline	Year
M20B20	2.0 L (1990 cc/121 in³)	92 kW (123 hp) @ 5800	165 N·m (121 ft·lbf) @ 4000		1981
		92 kW (123 hp) @ 5800	170 N·m (125 ft·lbf) @ 4000		1981
		95 kW (127 hp) @ 6000	174 N·m (128 ft·lbf) @ 4000	6200	1985
		95 kW (129 hp) @ 6000	164 N·m (120 ft·lbf) @ 4300	6200	1986
M20B23	2.3 L (2316 cc/141 in³)	102 kW (143 hp) @ 5300	205 N·m (151 ft·lbf) @ 4000	6500	1982
		110 kW (139-150 hp) @ 6000	205 N·m (151 ft·lbf) @ 4000	6500	1983
M20B25	2.5 L (2494 cc/152 in³)	126 kW (168 hp) @ 5800	226 N·m (166 ft·lbf) @ 4000	6700	1985
		120 kW (170 hp) @ 5800	215 N·m (158 ft·lbf) @ 4000	6700	1985
		125 kW (168 hp) @ 5800	222 N·m (163 ft·lbf) @ 4300	6700	1987
M20B27	2.7 L (2693 cc/164 in³)	92 kW (121 hp) @ 4250	240 N·m (177 ft·lbf) @ 3250	4500	1983
		95 kW (127 hp) @ 4250	240 N·m (177 ft·lbf) @ 3250	4500	1986
		90 kW (120 hp) @ 4250	230 N·m (169 ft·lbf) @ 3250	4500	1985
		95 kW (127 hp) @ 4800	230 N·m (169 ft·lbf) @ 3200	4500	1986

Components

The engine in all E30s is a three-part design, featuring a main Block with a Sump at the bottom and a Head at the top.

Head

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 = "Power With 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.

Rocker Cover

Despite the three styles of Head, the rocker cover is interchangeable across all M20 engines. It is positioned on 8 studs and fixed with M10 nuts. It has one hole for the Oil Cap, and one port for the breather hose.

When removing and replacing the rocker cover, it is worth replacing the rocker cover gasket.

Breather Hose

To balance the pressure inside the head, a rubber hose runs from the top of the rocker cover to the throttle body.

Common Problems

Not Starting

For all non-start situations, learn more about troucbleshooting your engine.

Servicing

Main article: Basic M20 Servicing

All engines need their fluids and filters changed at regular intervals, but sometimes more technical things are required to keep things running smoothly.

Learn more about adjusting the valves.

When changing your cambelt, it is recommended that you change your water pump too. Make sure you fit the correct water pump for your engine.

Common Upgrades

Stroker Engines

The M20 block is capable of much larger capacities and power than BMW made use of. If you'd like to rebuild your M20 engine into a larger capacity, learn more about the most common stroker permutations.

The most common option is a 2.8 stroker, which can be built using standard components. Learn more about building a 2.8..

One of the Zoners, Daimlerman, took the time to document his own construction. Learn more about Daimlerman's 2.7

Engine Management

Upgrading the engine management system will give you better control over the running of the engine. A simple chip can yield better performance or economy for specific situations. Upgrading the entire system to the latest version of Motronic can also deliver results. However, if you want complete control over every aspect of your engine, you should consider converting to Megasquirt.

Big Bore Throttle Bodies (BBTB)

The throttle body is the air flap that directly governs how much air the engine is allowed to suck in, and is controlled by the accelerator. Fitting a bigger throttle body will yield a lot better results than any other bolt-on modification. Learn more about Big Bore Throttle Bodies.

See Also

Engines M21 M30