BMW M78
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1.5 L (1,490 cc)
1.9 L (1,911 cc)
2.0 L (1,971 cc)
58 mm (2.28 in)
65 mm (2.56 in)
66 mm (2.60 in)
| BMW M78 | |
|---|---|
 | |
| Overview | |
| Production | 1933–1950 |
| Layout | |
| Configuration | Straight-six engine |
| Displacement | 1.2 L (1,182 cc) 1.5 L (1,490 cc) 1.9 L (1,911 cc) 2.0 L (1,971 cc) |
| Cylinder bore | 56 mm (2.20 in) 58 mm (2.28 in) 65 mm (2.56 in) 66 mm (2.60 in) |
| Piston stroke | 80 mm (3.15 in) 94 mm (3.70 in) 96 mm (3.78 in) |
| Cylinder block material | Iron |
| Cylinder head material | reverse-flow, iron |
| Valvetrain | OHV |
| Valvetrain drive system | chain |
| Combustion | |
| Operating principle | spark ignition (Otto) |
| Fuel system | 2 × horizontal draught carburettor |
| Fuel type | Petrol |
| Chronology | |
| Predecessor | None |
| Successor | BMW M337 |
The BMW M78 is a petrol overhead valve straight-six engine which was produced from 1933 to 1950.[1] It is the first straight-6 automobile engine produced by BMW, an engine layout which has been a key feature of the brand for many years since.[2]
The M78 was launched in the 1933 BMW 303.[3] In 1936, the higher performance BMW M328 straight-six engine began to be produced alongside the M78. In 1952, the BMW M337 engine was introduced as the replacement to the M78. Compared with the M78, the M337 has a revised cylinder head, a new inlet manifold and a reinforced crankshaft with bigger, more modern bearings.[4]: 88 [5]
The M78 was designed by Rudolf Schleicher[3]: 25 [6] as a medium power straight-6 engine.[7] Initial versions of the M78 had the same bore and stroke as the BMW 3/20's four-cylinder engine.[8] The M78 has two main cast pieces, the iron cylinder block, and the reverse-flow cylinder head, also made from iron. The oilpan, and the cylinder head cover are made from pressed steel. In addition to that, the M78 has a pressed-steel cover for the chain that drives the in-block camshaft. Both the crankshaft, and the camshaft have four bearings. The camshaft also drives the shaft for the oil pump and the ignition distributor. The engine has overhead valves that are actuated through pushrods and rocker arms. The water pump is flange-mounted to the cylinder head, and powered by a belt that is driven by a pulley that is flanged to the crankshaft. The belt also powers the alternator. The spark plugs are installed almost horizontally, below the intake manifold. BMW installed two horizontal draught Solex 26 mm carburettors next to the engine block and flange-mounted them to the intake manifold that sits atop them, i.e. the carburettors "hang down" from the intake manifold.[7]