Production period:	1987-1994
Manufacturer:	BMW
How it works:	Otto
Engine Type:	R4
Valve control:	OHC
displacement:	1596-1795 cm 3
Fuel supply:	manifold injection
Engine charging:	none
Power:	73-85 kW
Previous model:	M10
Successor:	M43

The BMW M40 is an inline-four SOHC piston engine which replaced the M10 and was produced from 1987–1995.

The M40 is a water-cooled four-cylinder gasoline engine with a crankcase made of gray cast iron . The cylinder spacing is 91 mm. 

The engine has a cast crankshaft with eight counterweights.  The cross-flow cylinder head consists of chill casting. The overhead, belt driven five bearings camshaft of the engine is made of chilled cast iron manufactured and operated per cylinder via drag levers each an inclined by 14 ° inlet and an outlet; the intake valve discs are 42 mm larger than the 36 mm exhaust valves. They have a hydraulic valve clearance compensation. Furthermore, the camshaft of the BMW M40, although made of shell-molded, is susceptible to "shrinkage", which is caused by the direct sliding of the drag levers on the camshaft. Mixture preparation is carried out by means of intake manifold injection ,  which is controlled by the Bosch Motronic M 1.3.  It contains the Lamba probe for the regulated three-way catalyst . Note: The four-valve version is the BMW M42 . 

Displacement ranges from 1.6 L (1596 cc; 97.4 cu in) to 1.8 L(1796 cc; 109.6 cu in). It began to be replaced by the M43 SOHC engine in 1991, however from 1989 the M40 was produced alongside the higher performance M42 DOHC four-cylinder engine.

Compared with its M10 predecessor, the M40 uses a belt-driven camshaft, and hydraulic tappets. Like the M10, the M40 uses an iron block and an aluminium head.

Fuel injection for the E30 versions is Bosch Motronic 1.3, and the E36 versions use Motronic 1.7.

M40B16

The M40B16 is a 1.6 L (1,596 cc) version of the M40, which has a bore of 84 mm (3.3 in) and a stroke of 72 mm (2.8 in). It produces 102 hp (76 kW) and 105 lb·ft (142 N·m) of torque. The E30 version uses Bosch Motronic 1.3 fuel injection, while the E36 version uses Motronic 1.7.

Applications:

• 1988–1994 E30 316i
• 1990–1994 E36 316i
• 1992–1993 Bertone Freeclimber 2

M40B18

The M40B18 is a 1.8 L (1,796 cc) version of the M40, which has a bore of 84 mm (3.3 in) and a stroke of 81 mm (3.2 in). It produces 113 hp (84 kW) and 120 lb·ft (160 N·m) of torque. Like the M40B16, the fuel injection system is Bosch Motronic 1.3 for the E30 versions and Motronic 1.7 for the E36.

Applications:

• 1987–1994 E30 318i
• 1988–1994 E34 518i
• 1992–1993 E36 318i