Seldom-seen ‘six’
Of the many configurations used for internal-combustion engines over the years, the flat-six is among the least common.
The layout, which consists of three cylinders mounted horizontally on either side of the crankshaft, is notable for its smooth running and low centre of gravity, but can also mean that the engine is too wide for some applications.
Only a few car manufacturers have attempted it, and of those just two have persevered with it for extended periods.
Here is a near-comprehensive list of cars with flat-six engines, arranged in chronological order.
1904 Wilson-Pilcher
The British Wilson-Pilcher company – named after founder Walter Gordon Wilson (also famous for designing a successful preselector gearbox) and aviation pioneer Percy Pilcher – is generally believed to be the first manufacturer to have made a car with a flat-six engine available to the public.
Its first models were flat-fours, but in 1904 Wilson-Pilcher produced one with a 4.1-litre flat-six, which was essentially 150% of the existing 2.7-litre ‘four’.
In the same year, Wilson-Pilcher was taken over by Armstrong Whitworth, which later evolved into Armstrong Siddeley.
1948 Tucker 48
Two world wars passed between the discontinuation of the Wilson-Pilcher and the arrival of the first reasonably well-known flat-six automobile.
The only model ever produced by the short-lived Tucker company was powered by a Franklin aero engine modified for automotive use, the changes including a switch from air to water cooling.
The 5.5-litre unit, which we believe to be the highest-capacity flat-six ever fitted to a car, was mounted behind the rear axle of the innovative 48, whose name was only just short of the number of cars produced before the company collapsed spectacularly.
1960 Chevrolet Corvair
The first flat-six to remain on the market for a full decade was fitted to what might to this day be Chevrolet’s most controversial car.
Gradually increasing in capacity over the years from 2.3 to 2.7 litres, the engine was both rear-mounted (as in the Tucker) and air-cooled, a system Chevrolet had tried with disastrous results in the early 1920s.
In the case of the Corvair, it was uncontroversial, unlike the swing-axle rear suspension whose behaviour was condemned in Ralph Nader’s book Unsafe At Any Speed.
From the 1963 model year, the Corvair became available with a turbocharged version of the same engine, initially producing 150 horsepower by the measuring system of the time, though confusingly enough the name Turbo-Air was used for the naturally aspirated units.
1961 Chevrolet Corvair 95
The minimal height of the flat-six allowed Chevrolet to follow Volkswagen’s lead with the Type 2 and produce rear-engined commercial vehicles with acceptably low load floors.
They were known as 95 after the length (in inches) of their wheelbases, which was over a foot shorter than that of the Corvair saloon they were based on.
The range included two pick-up trucks which were almost identical, except that the Loadside’s cargo bay was accessed through a panel in the rear and the Rampside’s through one on the right.
Neither was particularly popular, though the Rampside (pictured) remained on the market for longer.
1961 Chevrolet Greenbrier
The first of two Greenbrier models (unrelated to the slightly later Chevelle-based estate) was also part of the Corvair 95 range, but was not a pick-up.
Instead, it was a van, intended – depending on the exact model – either entirely for commercial use or for carrying up to nine passengers.
In its 1965 brochure, Chevrolet was still emphasising the benefits of the Greenbrier’s engine location (good traction, plenty of space up front) and air-cooling (no radiator or hoses to worry about), but the model was already on its way out.
It was replaced by the Chevy-Van, which had a water-cooled, in-line engine mounted between the front seats.
1964 Chevrolet Corvair
The Chevrolet Corvair was substantially redesigned for the 1965 model year, going on sale in this form in late 1964, and was described by one American reviewer as ‘the most beautiful car to appear in this country since before World War Two’.
Along with the change in appearance, there was a new rear-suspension system whose arrival predated the publication of the book in which Ralph Nader complained about how bad the original one was.
The flat-six engine was retained, always in 2.7-litre form in this generation, and once again there was a turbocharged version.
The Corvair was replaced in 1970 by the far more conventional Vega, and since then there have been no more flat-six Chevrolets, or any Chevrolets at all, until the eighth-generation Corvette, with the engine placed behind the passenger compartment.
1964 Porsche 911
Ferdinand Porsche favoured engines with horizontally opposed cylinders, using them in the Volkswagen Beetle and the Porsche 356 (the first production car bearing his own name), but the company he founded did not get round to creating a flat-six until long after his death in 1951.
Originally known as the 901, the 911 was revealed to the public for the first time in 1963, but did not go on the market until the following year.
Like all its successors, it had a rear-mounted, flat-six engine which initially measured 2 litres and produced 129bhp, though larger capacities and higher power outputs soon became available.
1965 Yenko Stinger
The Stingers, of which more than 100 were built, were second-generation Corvairs produced by Don Yenko of Pennsylvania.
Although they did not retain their original badging, the Corvairs were supplied directly by Chevrolet, and updated by Yenko to make them competitive in Sports Car Club of America circuit racing.
Changes included a dual-reservoir brake master cylinder and a variety of upgrades to the flat-six engine, the most radical of which is believed to have raised the power output to around 240 horsepower.
Yenko would perform similar work on later Chevrolets, but none of those had flat-six engines.
1969 Porsche 914
Developed in association with Volkswagen, the 914 was Porsche’s first mid-engined road car.
Power was always supplied by a horizontally opposed engine, either a VW ‘four’ or the ‘six’ which had by now become very familiar through the success of the Porsche 911, though it was detuned for this application.
The flat-six versions, known as 914/6, were far less popular than the four-cylinder ones, and were soon discontinued, while their less-powerful siblings remained on the market until 1976.
1973 Porsche 911
Although other opinions have been expressed, Porsche itself has said that the second generation of the 911 began when the G-series arrived in 1973.
911s of all kinds built over the next decade and a half are grouped together, and although there were many developments in that time, the flat-six engine remained a constant.
The first turbocharged Porsche 911, known either as the 911 turbo or the 930 depending on where it was sold, quickly became famous after its launch in 1974 both for its startling straight-line performance and for what has been described as ‘challenging’ behaviour through corners.
1986 Porsche 959
For most of the 1970s and 1980s, Porsche was the only manufacturer building cars with flat-six engines, and almost all of them, after the discontinuation of the 914/6, were 911s.
The limited-production 959, whose rear-mounted, 2.9-litre ‘six’ was twin-turbocharged and drove all four wheels, was the only exception.
Its primary purpose was to act as a homologation special, allowing Porsche to use modified versions in motorsport.
Today, though, the 959's reputation rests on its phenomenal performance as a standard road car, the most notable figure being a top speed of just under 200mph.
1987 Subaru XT
Also known as the Alcyone or Vortex, the wedge-shaped XT was Subaru's first two-door coupé.
At its launch in 1985, it was available only with a 1.8-litre engine of the flat-four layout Subaru is best known for.
The car was reworked in 1987, and the changes included the introduction of Subaru's first flat-six, which was very much larger than the ‘four’ at 2.7 litres.
If you wanted a high-performance Subaru XT, this was the engine you needed, but since a disappointingly low number of people appeared to want an XT of any sort, the model was abandoned in 1991.
1989 Porsche 911
A quarter of a century after it was revealed to the public, the Porsche 911 went into its third generation – codenamed 964 and often referred to as such outside the company – in 1989.
Around 85% of the components were new, and coil springs replaced torsion bars in the suspension, but the flat-six wasn't going anywhere.
In most cases, it had a capacity of 3.6 litres, nearly double that of the first engine fitted to a production 911.
Early turbocharged units measured 3.3 litres, and in the case of some naturally aspirated, high-performance models the capacity was 3.8 litres.
1991 Subaru SVX
Subaru’s XT gave way to the SVX, though there wasn’t much connection between them apart from the fact that they were both two-door coupés.
The SVX was larger and had rounder styling, and was significantly more powerful.
The only engine available was a 3.3-litre flat-six – the largest fitted to any production Subaru in the 20th century – with an output well in excess of 200bhp.
Performance was blunted by the use of an automatic gearbox (it’s said that Subaru didn’t have a manual that could handle the 3.3’s torque), but since the SVX was marketed to some extent as a luxury vehicle, this perhaps wasn’t considered a major problem.
1994 Porsche 911
The 993 generation of the Porsche 911 marked a noticeable design change, with the once prominent headlights now settling into the front wings and the bumpers now following the shape of the body, rather than standing proud of it.
The flat-six – still, anachronistically for the 1990s, air-cooled – was of course carried over from the 964, and was now available only in 3.6- or 3.8-litre form.
A high-performance derivative introduced in 1995 had two small turbochargers and four-wheel drive, a combination which until then Porsche had used only for the 959.
Very popular in its day and highly esteemed now, the 993 was a crucial car for Porsche, whose long-running attempts to make a success of sports cars with front-mounted, water-cooled, non-flat-six engines were finally abandoned in the mid 1990s.
1996 Porsche 911 GT1
Although the iconic numbers are included in its name, and there was a slight resemblance to the 993, the GT1 varied dramatically from all previous Porsche 911s.
Yes, there was a flat-six – a 3.2-litre, twin-turbo – but it was water-cooled, and mounted ahead of the rear wheels rather than behind them.
The GT1 was really a competition car, but Porsche had to build a few road-legal examples to keep the rule-makers happy.
The engine of the Strassenversion (German for ‘street version’) was slightly detuned compared with the racers, but it still produced around 540bhp, which was enough to allow a 0-62mph time of just 3.6 secs.
1996 Porsche Boxster
The original Boxster was the first Porsche (excluding the limited-production 550 Spyder) designed specifically as a convertible, and the first (excluding the even more limited-production 911 GT1) with a mid-mounted, water-cooled engine.
Comparisons with the 911 GT1 end there, because the Boxster was intended to be reasonably, rather than dramatically, fast, with an emphasis on handling and driving enjoyment rather than straight-line performance.
Accordingly, the flat-six at first had a very modest capacity of 2.5 litres, though this was later raised to 2.7 litres, and a 3.2 was also added to the range in due course.
1997 Porsche 911
An urgent need for cost reduction led Porsche to allocate some components to both the Boxster and the new 911, which were also developed alongside each other by the same teams.
The Boxster was obviously completely new, because it had no predecessor, but so was the 996-generation 911.
It was related to earlier models only in its basic shape and layout, and was the first in the series (the GT1 again excepted) with a water-cooled engine.
Despite their shared development, the gap between the 911 and Boxster was as wide in this generation as it would ever be – no Boxster of the period ever had a flat-six larger than 3.2 litres, but the engines in the 911 measured either 3.4 or 3.6 litres, the higher capacity being used for both naturally aspirated and turbocharged units.
1999 Subaru Legacy and Outback
Having previously fitted flat-sixes only to two-door coupés, Subaru brought the layout into the mainstream at the turn of the century.
The first two generations of Legacy were powered only by flat-fours, but a 3-litre ‘six’ became available in the third, and was the largest engine in the range.
It wasn’t the most powerful, being exceeded in this respect by the turbocharged, 2-litre ‘four’, but the ‘six’ was intended for drivers who preferred their progress to be relaxed rather than frantic.
The same engine was also used in the second Subaru Outback, a more off-road-specific version of the third Legacy available only with an estate body.
2003 Subaru Legacy and Outback
For the fourth Legacy and third Outback, Subaru upgraded its 3-litre flat-six.
It was still less powerful than the smaller, turbocharged, four-cylinder motors used in the Legacy, but its output was increased.
The racier engines were unsuitable for, and not used in, the Outback, so in this case the ‘six’ was more or less the equal most powerful engine along with the 2.5-litre turbo.
2004 Porsche 911
The 997-generation Porsche 911 was a development of the 996, with changes including the abandonment of the earlier car’s ‘fried-egg’ headlights in favour of less controversial oval ones.
The most common capacity for the flat-six was now 3.6 litres, but some versions had a 3.8, while a 4.0 (which, despite its size, could rev well beyond 8000rpm) was used in a rare version of the GT3.
The GT2 was fitted with a twin-turbocharged 3.6 which produced 523bhp, and was uprated to 612bhp for the GT2 RS in 2010.
2005 Porsche Boxster and Cayman
It was important that the Boxster had to be slower than the flagship Porsche 911, otherwise there would be one less reason for people to spend more money on the latter.
Nevertheless, capacities and power outputs of the flat-six in the smaller car kept increasing during the second generation.
A 3.4-litre version was introduced, and the amount of available power, which had been only around 200bhp back in 1996, had risen to more than 300bhp by the time this version was discontinued in 2012.
The first Cayman was simply the second Boxster in coupé form, and became the favourite Porsche of drivers who felt it had both a stiffer bodyshell than the Boxster and a more sensibly located engine than the 911.
2005 Subaru Tribeca
The Subaru Tribeca was a Legacy-based five- or seven-seat SUV, only ever fitted with the marque’s flat-six engine.
To begin with, this was the second version of the 3-litre unit used in the Legacy and Outback.
As part of a 2007 update, that engine was replaced by a 3.6-litre version – the largest in Subaru’s history – which produced a little more power at slightly fewer revs.
A mildly restyled Tribeca known as the 9-6 never went into production, but if this had happened it would have been the only Saab ever sold with a flat-six.
2009 Subaru Legacy and Outback
Shortly after making its debut in the Tribeca, the 3.6 replaced the 3.0 in the Subaru Legacy and Outback ranges.
It occupied about the same place in the Legacy line-up, relative to the turbos, as the 3.0 had done in the previous generation.
However, due to a policy revision, it became by far the most powerful unit available in the Outback.
2011 Porsche 911
For four years after the launch of the Porsche 911 codenamed 991, the engine line-up was much the same as it had been for the 997.
The situation changed considerably in 2015, when a 3-litre twin-turbo became standard on the mainstream versions.
This was the smallest-capacity engine fitted to a 911 since the 1980s, and the first with forced induction to be made available at the bottom of the range.
In dramatic contrast, a 3.8-litre, twin-turbo flat-six gave the GT2 RS a power output of 691bhp, the highest in 911 history.
2012 Porsche Boxster and Cayman
For their third and second generations respectively, the Boxster and Cayman were mostly available with flat-sixes in the by now familiar capacities of 2.7 and 3.4 litres.
The exception was a 3.8, as fitted to the Carrera S, which occupied the second position from the bottom of the 911 range.
In the ‘junior’ Porsches, its output of nearly 400bhp made it the strongest unit available.
It was used in only two models: the Cayman GT4 (pictured) and, with slightly reduced power, the Boxster Spyder.
2014 Subaru Legacy and Outback
The Tribeca, which had never sold in particularly large numbers, was discontinued after the 2014 model year, and so the 3.6-litre Subaru flat-six lost one of its homes.
Two more remained, however, since the Legacy and Outback both went into new generations at about the same time as their SUV relative was canned.
No major changes seem to have been regarded as necessary, and the engine continued much as before with an output of around 260bhp.
The end of the story was not far off, though – production of these generations came to an end in 2019, and no Legacy or Outback manufactured since then has ever been powered by a flat-six.
2019 Porsche 911
The 3-litre, turbocharged ‘six’ was carried over from the previous generation into the one launched in 2019, and was joined by a naturally aspirated 4-litre unit and a twin-turbo with the unfamiliar capacity of 3.7 litres.
The last of these was available with various outputs, the highest – as used in the turbo S – being 641bhp.
Latterly, a 3.6-litre engine has been part of a petrol-electric drivetrain called T-Hybrid, which is available on GTS models.
2019 Porsche Boxster and Cayman
Since 2016, the Boxster and Cayman have been referred to collectively as the 718 series, though the original names are still used to identify the roadster and the coupé respectively.
At first, the 718s were available only with a 2.5-litre, turbocharged flat-four, a sensible enough measure which nevertheless caused consternation among people who didn’t want Porsches to sound like Subaru Imprezas.
Order was restored in 2019 when the cars became available with a 4-litre, naturally aspirated flat-six, which was more powerful than the ‘four’.
As fitted to the Cayman GT4 RS, announced in November 2021, it produced 500PS (493bhp), or around two and a half times the output of the engine in the first Boxster.
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