BMW M78


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 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.

Design

The M78 has an iron block and cylinder head. It was designed by Rudolf Schleicher.
Initial versions of the M78 had the same bore and stroke as the four-cylinder engine from the BMW 3/20. Later on, the bore spacing was increased to allow for an increase in bore and to provide for more crankshaft bearings.
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.

Models

303 version

This first version of the M78 had a bore of, a stroke of and a compression ratio of 5.6:1. It produced and.
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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 produced at 4000 rpm.
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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.
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The 315 engine was enlarged to for the 319. This was a result of increasing the bore to and the stroke to. This engine produced at 3750 rpm.
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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 produced at 4000 rpm.
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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 produced at 3750 rpm.
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The 326 engine was detuned for the 320 and 321. This engine produces at 3750 rpm.
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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.
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