The BMW M78 is an overhead valve straight-six petrol 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 The BMW 303 was a small family saloon produced by BMW in 1933 and 1934. It was the first BMW motor car with a six-cylinder engine and the first BMW motor car with the "kidney grille" associated with the brand. The platform developed for the 30 ...

. In 1936, the higher performance

BMW M328 The BMW M328 is an overhead valve straight-six petrol engine which was produced from 1936 to 1940. It was a high-performance development of the BMW M78 engine that was produced alongside the M78. Compared with the M78, the M328 has an aluminium ...

straight-six engine began to be produced alongside the M78. In 1952, the

BMW M337 The BMW M337 is a straight-6 OHV petrol engine installed at a slanted angle which was produced in three variants (M337/1, M337/2 and M337/3) from 1952-1958. Built to power BMW's first new car after World War II, the M337 engine was a replacement ...

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 Bore or Bores often refer to: *Boredom * Drill Relating to holes * Boring (manufacturing), a machining process that enlarges a hole ** Bore (engine), the diameter of a cylinder in a piston engine or a steam locomotive ** Bore (wind instruments), ...

and

stroke A stroke is a medical condition in which poor blood flow to the brain causes cell death. There are two main types of stroke: ischemic, due to lack of blood flow, and hemorrhagic, due to bleeding. Both cause parts of the brain to stop functionin ...

as the

BMW 3/20 The BMW 3/20 PS was the first BMW automobile designed entirely by BMW. It was manufactured from 1932 to 1934, replacing the 3/15 model that was initially an Austin 7 manufactured under licence from the Austin Motor Company. The engine used in t ...

's four-cylinder engine. The M78 has two main cast pieces, the iron cylinder block, and the

reverse-flow cylinder head In engine technology, a reverse-flow or non-crossflow cylinder head is one that locates the intake and exhaust ports on the same side of the engine. The gases can be thought to enter the cylinder head and then change direction to exit the head. T ...

, 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 An overhead valve (OHV) engine, sometimes called a ''pushrod engine'', is a piston engine whose valves are located in the cylinder head above the combustion chamber. This contrasts with earlier flathead engines, where the valves were located bel ...

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

303 version

This first version of the M78 had a bore of , a stroke of and a

compression ratio The compression ratio is the ratio between the volume of the cylinder and combustion chamber in an internal combustion engine at their maximum and minimum values. A fundamental specification for such engines, it is measured two ways: the stati ...

of 5.6:1. It produces and . Applications: * 1933-1934

303 __NOTOC__ Year 303 ( CCCIII) was a common year starting on Friday (link will display the full calendar) of the Julian calendar. It was known in the Roman Empire as the Year of the Consulship of Diocletian and Maximian (or, less frequently, y ...

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 __NOTOC__ Year 315 ( CCCXV) was a common year starting on Saturday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Constantinus and Licinianus (or, less frequently, year ...

315/1 version

An upgraded version of the 315 engine with the compression ratio increased to 6.8:1 and using three

Solex Solex may refer to: * Solex (musician), Dutch musician * Solex Carburetor, a French manufacturer of carburetors and the powered bicycle VéloSoleX * Solex College, a former private for-profit college in Chicago, Illinois * Solex Unit, a fictiona ...

carburetor A carburetor (also spelled carburettor) is a device used by an internal combustion engine to control and mix air and fuel entering the engine. The primary method of adding fuel to the intake air is through the venturi tube in the main meteri ...

s. 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 {{M1 year in topic __NOTOC__ Year 326 ( CCCXXVI) was a common year starting on Saturday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Constantinus and Constantinus (or, ...

* 1938-1950

321 Year 321 ( CCCXXI) was a common year starting on Sunday (link will display the full calendar) of the Julian calendar. In the Roman Empire, it was known as the Year of the Consulship of Crispus and Constantinus (or, less frequently, year 1074 ' ...

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 __NOTOC__ Year 320 (Roman numerals, CCCXX) was a leap year starting on Friday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Constantinus and Constantinus (or, less freq ...

* 1938-1941

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 The mean effective pressure (MEP) is a quantity relating to the operation of a reciprocating engine and is a measure of an engine's capacity to do work that is independent of engine displacement.Heywood, J. B., "Internal Combustion Engine Fundam ...

of at 3750/min, equivalent to a max torque of at 3750/min. Applications: * 1937-1955 327 * 1949-1953

340 Year 340 (Roman numerals, CCCXL) was a leap year starting on Tuesday (link will display the full calendar) of the Julian calendar. At the time, it was known as the Year of the Consulship of Acindynus and Valerius (or, less frequently, year 1093 ...

References

{{BMW_automotive_petrol_engines:_1920s_to_1940s M78 Straight-six engines Gasoline engines by model