BMW M78
title: "BMW M78" type: doc version: 1 created: 2026-02-28 author: "Wikipedia contributors" status: active scope: public tags: ["bmw-engines", "straight-six-engines", "gasoline-engines-by-model"] topic_path: "general/bmw-engines" source: "https://en.wikipedia.org/wiki/BMW_M78" license: "CC BY-SA 4.0" wikipedia_page_id: 0 wikipedia_revision_id: 0
::data[format=table title="Infobox automobile engine"]
| Field | Value |
|---|---|
| name | BMW M78 |
| image | BMW M78 engine in BMW-Museum in Munich, Bayern.JPG |
| production | 1933–1950 |
| configuration | Straight-six engine |
| valvetrain | OHV |
| block | Iron |
| timing | chain |
| fuelsystem | 2 × horizontal draught carburettor |
| head | reverse-flow, iron |
| displacement | 1182 cc |
| 1490 cc | |
| 1911 cc | |
| 1971 cc | |
| bore | 56 mm |
| 58 mm | |
| 65 mm | |
| 66 mm | |
| stroke | 80 mm |
| 94 mm | |
| 96 mm | |
| operating principle | spark ignition (Otto) |
| fueltype | Petrol |
| predecessor | None |
| successor | BMW M337 |
| :: |
|name = BMW M78 |image = BMW M78 engine in BMW-Museum in Munich, Bayern.JPG |production = 1933–1950 |configuration = Straight-six engine |valvetrain = OHV |block = Iron |timing = chain |fuelsystem = 2 × horizontal draught carburettor |head = reverse-flow, iron |displacement = 1182 cc 1490 cc 1911 cc 1971 cc | bore = 56 mm 58 mm 65 mm 66 mm | stroke = 80 mm 94 mm 96 mm |operating principle = spark ignition (Otto) |fueltype = Petrol |predecessor = None |successor = BMW M337
::figure[src="https://upload.wikimedia.org/wikipedia/commons/e/ea/MHV_BMW_303_1934_01.jpg" caption="[[BMW 303]] (1933-1934)"] ::
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.{{Citation | author-link = :de:Werner Oswald (Automobilhistoriker) | last = Oswald | first = Werner | year =1979 | title =Deutsche Autos 1920-1945 – Alle deutschen Personenwagen der damaligen Zeit | publisher = Motorbuch-Verlag | location = Stuttgart | language = de | edition= 3rd | isbn = 978-3-87943-519-7 | pages = 53, 60}} 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
::data[format=table]
| Model | Displacement | Power | Year | {{Citation |
|---|---|---|---|---|
| 303 version | 1182 cc | 22 kW | 1933-1934 | |
| 315 version | 1490 cc | 25 kW | 1934-1937 | |
| 315/1 version | 40 PS | |||
| 319 version | 1911 cc | 33 kW | 1935-1937 | |
| 319/1 version | 55 PS | |||
| 326 version | 1971 cc | 50 PS | 1936-1950 | |
| 320 version | 45 PS | 1937-1938 | ||
| 327 version | 55 PS | 1937-1953 | ||
| author-link = :de:Werner Oswald (Automobilhistoriker) | last = Oswald | first = Werner | year =1979 | title =Deutsche Autos 1920-1945 – Alle deutschen Personenwagen der damaligen Zeit |
| :: |
303 version
This first version of the M78 had a bore of 56 mm, a stroke of 80 mm and a compression ratio of 5.6:1.
Applications:
- 1933-1934 303
315 version
A development of the 303 version, where the bore was increased from 56 to and the stroke was increased from 80 to.
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 40 PS at 4000 rpm.
Applications
- 1934-1937 315/1
319 version
The 315 engine was enlarged to 1911 cc for the 319. This was a result of increasing the bore to 65 mm and the stroke to 96 mm. This engine produces 45 PS at 3750 rpm.
Applications:
319/1 version
The 315/1 engine was enlarged to 1911 cc for the 319/1. This was a result of increasing the bore to 65 mm and the stroke to 96 mm. This engine produces 55 PS at 4000 rpm.
Applications:
- 1935-1937 319/1
326 version
The 319 engine was enlarged to 1971 cc for the 326 by increasing the bore by one millimetre to 66 mm. Twin 26 mm Solex carburetors were used. The compression ratio was raised to 6.0:1.
Applications:
320 version
The 326 engine was detuned (by using a single carburetor) for the 320 and 321. This engine produces 45 PS at 3750 rpm.
Applications:
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 55 PS at 4500 rpm. The engine retains its 66 by 96 mm bore and stroke, resulting in a displacement of 1971 cm3. The engine has a BMEP of 6.7 kp/cm2 at 3750/min, equivalent to a max torque of 104 N·m at 3750/min.
Applications:
References
References
- "1933 BMW 303 Saloon technical specifications". Carfolio.com.
- Murphy, Tom. (19 November 2007). "Inline-6 Helped Forge BMW Brand".
- Norbye, Jan P.. (1984). "BMW - Bavaria's Driving Machines". Publications International.
- (1980). "Alle BMW-Automobile 1928–1978". Motorbuch Verlag.
- (2008). "The Ultimate History of BMW". Parragon Books.
- "Vor 70 Jahren - BMW 303, der erste Sechszylinderwagen". BMW Mobile Tradition Live.
- Kremser, Hans. (1942). "Der Aufbau schnellaufender Verbrennungskraftmaschinen für Kraftfahrzeuge und Triebwagen". Springer.
- (July 2025). "BMW M78 Automobilmotor 1,2 Liter".
- "1935 BMW 319 Saloon technical specifications".
- "1937 BMW 320 Saloon technical specifications".
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