The BMW M78 is a petrol overhead valve straight-six engine which was produced from 1933 to 1950. 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.
The M78 was launched in the 1933 BMW 303. 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.
Design The M78 was designed by Rudolf Schleicher as a medium power straight-6 engine. Initial versions of the M78 had the same bore and stroke as the BMW 3/20's four-cylinder engine. 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.
Models Model Displacement Power Year303 version 1933-1934315 version 1934-1937315/1 version 319 version 1935-1937319/1 version 326 version 1936-1950320 version 1937-1938327 version 1937-1953
303 version This first version of the M78 had a bore of , a stroke of and a compression ratio of 5.6:1. It produces and .
Applications: 1933-1934 303
315 version A development of the 303 version, where the bore was increased from and the stroke was increased from . The compression ratio remained at 5.6:1. This engine produces at 4000 rpm.
Applications: 1934-1937 315
315/1 version An upgraded version of the 315 engine with the compression ratio increased to 6.8:1 and using three Solex carburetors. This version produces at 4000 rpm.
Applications 1934-1937 315/1
319 version The 315 engine was enlarged to for the 319. This was a result of increasing the bore to and the stroke to . This engine produces at 3750 rpm.
Applications: 1935-1937 319 1937 329
319/1 version The 315/1 engine was enlarged to for the 319/1. This was a result of increasing the bore to and the stroke to . This engine produces at 4000 rpm.
Applications: 1935-1937 319/1
326 version The 319 engine was enlarged to for the 326 by increasing the bore by one millimetre to . Twin Solex carburetors were used. The compression ratio was raised to 6.0:1. This engine produces at 3750 rpm.
Applications: 1936-1946 326 1938-1950 321
320 version The 326 engine was detuned (by using a single carburetor) for the 320 and 321. This engine produces at 3750 rpm.
Applications: 1937-1938 320 1938-1941 321
327 version 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 at 4500 rpm. The engine retains its 66 by 96 mm bore and stroke, resulting in a displacement of . The engine has a BMEP of at 3750/min, equivalent to a max torque of at 3750/min.
Applications: 1937-1955 327 1949-1953 340
See also BMW List of BMW engines
References