The BMW M78 is a petrol overhead valve straight-six engine which was produced from 1933 to 1950.

[3] In 1936, the higher performance BMW M328 straight-six engine began to be produced alongside 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.

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.

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.

[11] Applications: An upgraded version of the 315 engine with the compression ratio increased to 6.8:1[4]: 39–40 [12] and using three Solex carburetors.

[4]: 44 Applications: The 319 engine was enlarged to 2.0 L (1,971 cc) for the 326 by increasing the bore by one millimetre to 66 mm (2.6 in).

[15] Applications: The 327 used a version of the 326 engine with a new cylinder head, yielding a 6.3:1 compression ratio and a slight power increase to 40 kW (55 PS) at 4500 rpm.