View source for Flat-twelve engine

{{Short description|Horizontally-opposed 12-cylinder piston engine}}
{{Use dmy dates|date=June 2019}}
[[File:Ferrari Testarossa Motor.jpg|thumb|upright=1.2|1984-1991 [[Ferrari Testarossa]] mid-mounted engine]]
A '''flat-twelve engine''' (also known as a '''horizontally opposed-twelve''') is a twelve-cylinder [[piston engine]] with six cylinders on each side of a central [[crankshaft]].

Flat-twelve engines are less common than [[V12 engine]]s, but they have been used in various racing cars during the 1960s and 1970s, and in mid-engined Ferrari road cars from 1973 to 1996.

== Design ==
{{see also|Flat engine}}

Unlike most flat-twin, flat-four, and flat-six engines, flat-twelve engines typically use the crankshaft configuration of a 180° [[V engine]]. Instead of the [[boxer engine]] design of each piston having its own crankpin, flat-twelve engines have each pair of pistons sharing a [[crankpin]], and thus are flat, but not true boxers.

Compared with a V12 engine, a flat-twelve has a lower [[center of gravity]], but because a flat-twelve is wider they are rarely used in front-engined cars.

==Racing cars==
=== Formula One ===
The first known flat-twelve engine was built by Porsche in 1947 for the abandoned [[Cisitalia Grand Prix]] racing car. The engine, known as the Porsche Typ 360, was supercharged and had a displacement of {{convert|1.5|L|cuin|0|abbr=on}}.<ref>{{cite web |title=Cisitalia Grand Prix (Porsche type 360) |url=https://www.stuttcars.com/porsche-models/360/ |website=www.stuttcars.com |access-date=6 October 2019 |language=en}}</ref> One engine was built and the car conducted top speed testing, but it never competed in any races.<ref name="stevemckelvie">{{cite web |url=https://stevemckelvie.files.wordpress.com/2013/01/cisitalia-build-story-by-martin-schroeder.pdf |title=Cisitalia 360 Grand Prix Car |website=stevemckelvie.files.wordpress.com |url-status=dead |archive-url=https://web.archive.org/web/20150227010337/https://stevemckelvie.files.wordpress.com/2013/01/cisitalia-build-story-by-martin-schroeder.pdf |archive-date= 27 February 2015}}</ref>

In 1964–65, the [[Ferrari 158#Ferrari 1512|Ferrari 512 F1]] competed in several Formula One races.<ref>{{cite web |title=1964 - 1965 Ferrari 1512 F1 - Images, Specifications and Information |url=http://www.ultimatecarpage.com/car/139/Ferrari-1512-F1.html |website=www.ultimatecarpage.com |access-date=5 October 2019}}</ref> The 512 F1 was powered by a {{convert|1.5|L|cuin|0|abbr=on}} flat-twelve engine and raced alongside the V8-engined Ferrari 158 upon which it was based. Ferrari returned to using flat-twelve engines in 1970, when the [[Ferrari 312B]] switched from the V12 engine used by its predecessor. Its successor, the [[Ferrari 312T]], was introduced in 1975 and won the Formula One constructors championships from 1975 to 1979.<ref>{{cite web |title=Niki Lauda |url=https://www.formula1.com/en/championship/drivers/hall-of-fame/Niki_Lauda.html |website=www.formula1.com |access-date=5 October 2019 |language=en}}</ref><ref>{{cite web |title=Jody Scheckter |url=https://www.formula1.com/en/championship/drivers/hall-of-fame/Jody_Scheckter.html |website=www.formula1.com |access-date=5 October 2019 |language=en}}</ref> The success of the Ferrari 312T led other Formula One teams to build flat-twelve engines, such as the 1979 [[Alfa Romeo 177]].

In 1990, the [[Subaru 1235]] flat-twelve engine was built for Subaru's unsuccessful attempt to compete in Formula One as an engine manufacturer.

=== Sports car racing ===
{{Listen|filename=Porsche 917 40th Anniversary.ogg|title=Porsche 917 flat-12|description=Eleven 917s pull away at Goodwood Festival of Speed 2009}}
In 1969, the [[Porsche 917]] sports prototype racing car introduced an air-cooled flat-twelve engine. This flat-twelve engine was based on the previous flat-eight engine, but it used a V12 crankshaft configuration instead of the [[boxer engine|boxer]] configuration used by the flat-eight. The domination of the Porsche 917 probably influenced Ferrari, because they switched from V12 engines to flat-twelve engines (using a boxer configuration) for the [[Ferrari 312 PB]], which competed from 1971 to 1973. Alfa Romeo also used flat-twelve engines in the 1973-1976 [[Alfa Romeo Tipo 33#Alfa Romeo 33TT12|Alfa Romeo 33TT12]] and [[Alfa Romeo Tipo 33#Alfa Romeo 33SC12|Alfa Romeo 33SC12]] sports prototype racing cars.

For the 1991 sports-prototype racing category, Mercedes-Benz switched from a twin-turbo V8 engine to a naturally aspirated flat-twelve engine for in the [[Mercedes-Benz C291]] racing car. This engine employed a cylinder-head design with exhaust ports where the intake ports would normally be (on top of the engine, pointing upwards). The intake ports are between the intake and exhaust camshafts, just above the spark-plugs, pointing at an outward angle from the vertical. This was done to allow the engine to be installed lower in the chassis. The C291 was unsuccessful and Mercedes withdrew from sports-prototype racing after the 1991 season.

== Production cars ==
From 1973 to 1996, Ferrari used flat-twelve engines in various production models: the 1973-1976 [[Ferrari Berlinetta Boxer#365 GT/4 BB|Ferrari 365 GT/4 BB]], the 1976-1981 [[Ferrari Berlinetta Boxer#512 BB|Ferrari 512 BB]], the 1981-1984 [[Ferrari Berlinetta Boxer#512 BBi|Ferrari 512 BBi]], the 1984-1991 [[Ferrari Testarossa#Testarossa|Ferrari Testarossa]], the 1991-1994 [[Ferrari Testarossa#512 TR|Ferrari 512 TR]] and the 1994-1996 [[Ferrari Testarossa#F512 M|Ferrari F512 M]].

== Other uses ==
During World War II, the British [[Covenanter tank|Covenanter]] tank was powered by a [[Henry Meadows|Meadows]] D.A.V flat-12, 340&nbsp;hp (250&nbsp;kW) and the [[Churchill tank|Churchill]] by a 350&nbsp;hp (261&nbsp;kW) Bedford engine flat 12.<ref name="Fletcher, Churchill Tank, engine" >{{Cite book
  |title=Churchill Tank
  |edition=Reprint of original WW2 operating manuals
  |last=Fletcher |first=David
  |author-link=David Fletcher (military historian)
  |year=1983
  |orig-year=1944
  |publisher=[[HMSO]] / [[The Tank Museum]]
  |isbn=978-0-11-290404-5
  |ref={{harvid|Fletcher|Churchill Tank}}
  |pages=vii, 9(3), 29(19)
}}</ref> Another military vehicle usage was the [[Panhard EBR]] armored cars during the 1950s.<ref>{{cite book |last1=Ogorkiewicz |first1=R.M. |title=AFV Weapons Profile 039 Panhard Armoured Cars |publisher=Profile Publications}}</ref>

During the 1940s, the [[Franklin Engine Company]] in the United States produced a flat-twelve aircraft engine called the O-805-2.<ref>{{cite web |title=Aircooled Motors (Franklin) O-805-2, Horizontally-Opposed 12 Engine |url=https://airandspace.si.edu/collection-objects/aircooled-motors-franklin-o-805-2-horizontally-opposed-12-engine |website=www.airandspace.si.edu |access-date=8 October 2019 |language=en |date=20 March 2016 |archive-date=8 October 2019 |archive-url=https://web.archive.org/web/20191008081442/https://airandspace.si.edu/collection-objects/aircooled-motors-franklin-o-805-2-horizontally-opposed-12-engine |url-status=dead }}</ref>

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

{{Piston engine configurations}}

[[Category:Flat engines|12]]
[[Category:Piston engine configurations]]