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{{Infobox automobile engine
|name          = BMW M78
|image         = BMW M78 engine in BMW-Museum in Munich, Bayern.JPG
|production    = 1933–1950
|configuration = [[Straight-six engine]]
|valvetrain    = [[Overhead valve engine|OHV]]
|block         = Iron
|timing        = chain
|fuelsystem    = 2&nbsp;× horizontal draught carburettor
|head          = reverse-flow, iron
|displacement  = {{convert|1182|cc|L|1|order=flip|abbr=on}}<br/>{{convert|1490|cc|L|1|order=flip|abbr=on}}<br/>{{convert|1911|cc|L|1|order=flip|abbr=on}}<br/>{{convert|1971|cc|L|1|order=flip|abbr=on}}
| bore = {{convert|56|mm|in|2|abbr=on}}<br/>{{convert|58|mm|in|2|abbr=on}}<br/>{{convert|65|mm|in|2|abbr=on}}<br/>{{convert|66|mm|in|2|abbr=on}}
| stroke = {{convert|80|mm|in|2|abbr=on}}<br/>{{convert|94|mm|in|2|abbr=on}}<br/>{{convert|96|mm|in|2|abbr=on}}
|operating principle = [[spark ignition]] (Otto)
|fueltype      = [[Gasoline|Petrol]]
|predecessor   = None
|successor     = [[BMW M337]]
}}

[[File:MHV BMW 303 1934 01.jpg|thumbnail|right|[[BMW 303]] (1933-1934)]]
The '''BMW M78''' is a [[Gasoline|petrol]] overhead valve [[straight-six engine]] which was produced from 1933 to 1950.<ref name="carfolio303">{{cite web|url=http://www.carfolio.com/specifications/models/car/?car=96008 |title=1933 BMW 303 Saloon technical specifications |publisher=Carfolio.com |access-date=2012-06-03}}</ref> 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.<ref>{{cite web|last=Murphy |first=Tom |url=http://wardsauto.com/news-amp-analysis/inline-6-helped-forge-bmw-brand |title=Inline-6 Helped Forge BMW Brand |website=www.wardsauto.com |date=19 November 2007 |access-date=3 June 2012}}</ref>

The M78 was launched in the 1933 [[BMW 303]].<ref name="noakes ultimate history"/> 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.<ref name="Norbye">{{cite book |last=Norbye |first=Jan P. |title=BMW - Bavaria's Driving Machines |year=1984 |publisher=Publications International |location=Skokie, IL |isbn=0-517-42464-9 |url-access=registration |url=https://archive.org/details/bmwbavariasdrivi00norb }}</ref>{{refpage|page=88}}<ref name="DeutscherSportverlag">{{cite book|last1=Oswald|first1=Werner|title=Alle BMW-Automobile 1928–1978|date=1980|publisher=Motorbuch Verlag|location=Stuttgart|isbn=3-87943-584-7|language=de}}</ref>

== Design ==
The M78 was designed by Rudolf Schleicher<ref name="noakes ultimate history">{{cite book|last1=Noakes|first1=Andrew|title=The Ultimate History of BMW|date=2008|publisher=Parragon Books|isbn=978-1-4075-3512-8}}</ref>{{refpage|page=25}}<ref>{{cite journal|last1=Zeichner|first1=Walter|title=Vor 70 Jahren - BMW 303, der erste Sechszylinderwagen|journal=BMW Mobile Tradition Live|volume=July 2003|pages=8–11|url=https://bmw-grouparchiv.de/research/detail/index.xhtml?id=3736667|access-date=20 August 2017|language=de}}</ref> as a medium power straight-6 engine.<ref name="Kremser_208" /> Initial versions of the M78 had the same [[bore (engine)|bore]] and [[stroke (engine)|stroke]] as the [[BMW 3/20]]'s four-cylinder engine.<ref>{{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}}</ref> 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<!--sic--> Solex 26&nbsp;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.<ref name="Kremser_208">{{Citation |last=Kremser |first=Hans |year=1942 |title=Der Aufbau schnellaufender Verbrennungskraftmaschinen für Kraftfahrzeuge und Triebwagen |series=Die Verbrennungskraftmaschine |editor=Hans List |volume=11 |publisher=Springer |location=Wien |isbn=978-3-7091-9755-4 |language=de |pages=208}}</ref>

==Models==
{| class=wikitable
|-
! Model !! Displacement !! Power !! Year
|-
|[[#M78 (303 version)|303 version]]|| {{convert|1182|cc|cuin|1|abbr=on}} || {{convert|22|kW| PS|0|abbr=on}} || 1933-1934
|-
|[[#M78 (315 version)|315 version]]|| rowspan=2|{{convert|1490|cc|cuin|1|abbr=on}} || {{convert|25|kW| PS|0|abbr=on}} || rowspan=2|1934-1937
|-
|[[#M78 (315/1 version)|315/1 version]]|| {{convert|40|PS| kW|0|order=flip|abbr=on}}
|-
|[[#M78 (319 version)|319 version]]|| rowspan=2|{{convert|1911|cc|cuin|1|abbr=on}} || {{convert|33|kW| PS|0|abbr=on}} || rowspan=2|1935-1937
|-
|[[#M78 (319/1 version)|319/1 version]]|| {{convert|55|PS| kW|0|order=flip|abbr=on}}
|-
|[[#M78 (326 version)|326 version]]|| rowspan=3|{{convert|1971|cc|cuin|1|abbr=on}} || {{convert|50|PS| kW|0|order=flip|abbr=on}} || 1936-1950
|-
|[[#M78 (320 version)|320 version]]|| {{convert|45|PS| kW|0|order=flip|abbr=on}} || 1937-1938
|-
|[[#M78 (327 version)|327 version]]|| {{convert|55|PS| kW|0|order=flip|abbr=on}} || 1937-1953
|-
!colspan="4"|<ref>{{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 = 60…63}}</ref>
|}

=== 303 version ===
This first version of the M78 had a bore of {{convert|56|mm|in|2|abbr=on}}, a stroke of {{convert|80|mm|in|2|abbr=on}} and a [[compression ratio]] of 5.6:1.<ref name=Norbye />{{refpage|pages=33-36}} It produces {{convert|22|kW| PS|0|abbr=on}} and {{convert|68|Nm|lbft|0|abbr=on}}.<ref>{{cite web|title=BMW M78 Automobilmotor 1,2 Liter|url=https://bmw-grouparchiv.de/research/detail/index.xhtml?id=3939054|website=www.bmw-grouparchiv.de|access-date=20 August 2017|language=de}}</ref><ref name="carfolio303" />

Applications:
* 1933-1934 [[BMW 303|303]]

=== 315 version ===
A development of the 303 version, where the bore was increased from {{convert|56|to|58|mm|in|2|abbr=on}} and the stroke was increased from {{convert|80|to|94|mm|in|2|abbr=on}}.<ref name=Norbye />{{refpage|pages=33-36,39-40}} The compression ratio remained at 5.6:1.<ref name=Norbye />{{refpage|pages=39-40}}<ref name="carfolio315" /> This engine produces {{convert|34|PS| kW|0|order=flip|abbr=on}} at 4000 rpm.<ref name="carfolio315">{{cite web|url=http://www.carfolio.com/specifications/models/car/?car=96052 |title=1934 BMW 315 Saloon technical specifications |website=www.carfolio.com |access-date=3 June 2012}}</ref>

Applications:
* 1934-1937 [[BMW 315#315|315]]

=== 315/1 version ===
An upgraded version of the 315 engine with the compression ratio increased to 6.8:1<ref name=Norbye />{{refpage|pages=39-40}}<ref name="carfolio315_1">{{cite web|url=http://www.carfolio.com/specifications/models/car/?car=96063 |title=1934 BMW 315/1 Sport technical specifications |website=www.carfolio.com |access-date=3 June 2012}}</ref> and using three [[Solex Carburetor|Solex]] [[carburetor]]s.<ref name=Norbye />{{refpage|pages=39-40}}<ref name="carfolio315_1" /> This version produces {{convert|40|PS| kW|0|order=flip|abbr=on}} at 4000&nbsp;rpm.<ref name="carfolio315_1" />

Applications
* 1934-1937 [[BMW 315#315/1|315/1]]

=== 319 version ===
The 315 engine was enlarged to {{convert|1911|cc|cuin|1|abbr=on}} for the 319.<ref name="carfolio319">{{cite web|url=http://www.carfolio.com/specifications/models/car/?car=101618 |title=1935 BMW 319 Saloon technical specifications |website=www.carfolio.com |access-date=3 June 2012}}</ref> This was a result of increasing the bore to {{convert|65|mm|in|2|abbr=on}} and the stroke to {{convert|96|mm|in|2|abbr=on}}. This engine produces {{convert|45| PS|kW|0|abbr=on|order=flip}} at 3750 rpm.<ref name=Norbye />{{refpage|page=44}}

Applications:
* 1935-1937 [[BMW 315#319|319]]
* 1937 [[BMW 315#329|329]]

=== 319/1 version ===
The 315/1 engine was enlarged to {{convert|1911|cc|L|1|abbr=on|disp=flip}} for the 319/1.<ref name="carfolio319" /> This was a result of increasing the bore to {{convert|65|mm|in|2|abbr=on}} and the stroke to {{convert|96|mm|in|2|abbr=on}}. This engine produces {{convert|55| PS|kW|0|abbr=on|order=flip}} at 4000 rpm.<ref name=Norbye />{{refpage|page=44}}

Applications:
* 1935-1937 [[BMW 315#319/1|319/1]]

=== 326 version ===
The 319 engine was enlarged to {{convert|1971|cc|L|1|abbr=on|disp=flip}} for the 326 by increasing the bore by one millimetre to {{convert|66|mm|in|abbr=on}}. Twin {{convert|26|mm|abbr=on}} Solex carburetors were used. The compression ratio was raised to 6.0:1.<ref name=Norbye />{{refpage|page=46}} This engine produces {{convert|50|PS|kW|0|abbr=on|order=flip}} at 3750 rpm.<ref name=Norbye />{{refpage|page=46}}<ref name="usautoparts.net">{{cite web|url=http://www.usautoparts.net/bmw/models/vintage/326.htm |title=326 Cabrio and Limousine |website=www.usautoparts.net |url-status=dead |archive-url=https://web.archive.org/web/20080212152427/http://www.usautoparts.net/bmw/models/vintage/326.htm |archive-date=12 February 2008 }}</ref>

Applications:
* 1936-1946 [[BMW 326|326]]
* 1938-1950 [[BMW 321|321]]

=== 320 version ===
The 326 engine was detuned (by using a single carburetor) for the 320 and 321. This engine produces {{convert|45|PS| kW|0|order=flip|abbr=on}} at 3750 rpm.<ref>{{cite web|url=http://www.carfolio.com/specifications/models/car/?car=101750 |title=1937 BMW 320 Saloon technical specifications |website=www.carfolio.com |access-date=3 June 2012}}</ref>

Applications:
* 1937-1938 [[BMW 320|320]]
* 1938-1941 [[BMW 321|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 {{convert|55|PS|kW|abbr=on|order=flip}} at 4500&nbsp;rpm.<ref name=Norbye />{{refpage|page=68}} The engine retains its 66 by 96&nbsp;mm bore and stroke, resulting in a displacement of {{cvt|1971|cm3|in3|1}}. The engine has a [[Mean effective pressure|BMEP]] of {{cvt|6.7|kp/cm2|MPa lb/in2|2}} at 3750/min, equivalent to a max torque of {{cvt|104|N·m|kpm lbft|1}} at 3750/min.<ref name="Kremser_208" />

Applications:
* 1937-1955 [[BMW 327|327]]
* 1949-1953 [[BMW 340|340]]

== See also ==
* [[BMW]]
* [[List of BMW engines]]

==References==
{{reflist|30em}}

{{BMW_automotive_petrol_engines:_1920s_to_1940s}}

[[Category:BMW engines|M78]]
[[Category:Straight-six engines]]
[[Category:Gasoline engines by model]]