The straight-6 (also inline-6, I-6, or I6) is an internal combustion engine with six cylinders aligned in a single row. The name slant-6 is sometimes used when the cylinders are at an angle from the vertical.
Straight-6 engines have perfect primary and secondary balance and require no balance shaft.
Usually a straight-6 was used for engine displacements between about 2.5 and 4.0 L. Sometimes this configuration is used to make smaller engines which tend to be powerful and very smooth running, but also rather expensive to manufacture and physically longer than alternative layouts. The smallest production straight-6 was found in the Benelli 750 sei motorcycle, displacing 747.7 cc. The largest are used in tractor-trailer combinations and some low speed diesels in cargo and passenger ships. These engines have a displacement of 1000 L or more.
Straight-6 engines were historically more common than V6s, mainly because the length of such engines was not such a concern in rear wheel drive vehicles but also because V6s (unlike the crossplane V8) were somewhat difficult to make smooth-running. The widespread use of front-wheel-drive and transverse ("east-west") engine configurations in smaller cars saw that the shorter engine length of the V6 became highly desirable, and these days most six-cylinder engines are made in the V configuration.
Straight-6 engines in Europe
Many manufacturers build cars equipped with straight-6 engines. Manufacturers BMW and Volvo both produce multiple models with straight-6s. BMWs are rear-wheel drive, but Volvo builds cars equipped with front-wheel drive and a transverse straight-6.
Audi has never offered straight-6 engines; until the mid-1990s, however, straight-5 gasoline engines were available for most models, some of them turbocharged.
Although Mercedes-Benz used to build many straight-6s both as gas and diesel engines, it has recently abandoned the layout and now only engineers V6 engines (they have retained the straight-6 layout for medium duty diesel applications such as the MBE 906).
BMW, on the other-hand, is one of the few remaining manufacturers to persist with the straight-6 configuration, making petrol and turbo-diesel engines ranging from 2.0 to 3.2 L in displacement (as of 2005). In 2006, they announced that the upcoming 335i model of their E92 3-series coupé will have a 3.0L twin-turbo straight-6, showing the company's continued dedication to the straight-6 engine configuration.
Opel has also used a straight-6 engine in the 1970s until the early 1990s, ranging between 2.5 and 4.0 L. They powered Opel's top of the line models, including the Monza, the Omega and the Commodore.
In 1959, Saab had an experimental car with two transverse straight-3 engines bolted together — the Saab Monster.
Straight-6 engines in Britain
The straight-6 was the archetypal British engine for sports and luxury cars for many years. Rolls-Royce used straight-6 engines until changes in their design made the shorter V8 layout more suitable.
Jaguar used them, from 1949 until the mid 1990s in form of the legendary twin-camshaft Jaguar XK6 engine, until, at Ford's insistence, they adopted a V8. Aston Martin used a straight-6 for many years, as did Austin-Healey in their Austin-Healey 3000. MG also used a straight-6 in their MGC.
Bristol produced a straight-6 until 1961, based on BMW plans, that was also used in many small manufacturers' cars.
The compact Triumph straight-6 powered their high-end saloon and sports cars from the mid 1950s to the mid 1970s.
British sports car company TVR has designed its own straight-6, known as the Speed Six, and now uses it exclusively in all of their models.
Land Rover used a 2.6L straight-6 from 1967 in certain series Land Rover models.
Straight-6 engines in the United States
Engines of this type were popular before World War II in mid-range cars. Most manufacturers started building straight-6 engines when cars grew too large for the straight-4.
After the war, larger cars required larger engines, and the straight-6 became the base engine model used on economy cars only. The vast majority of American cars during this period had V8s.
The Chrysler Corporation had noteworthy slant-6 engines, used in the Plymouth Valiant and Dodge Dart A-body models of the 1960s and 1970s.
When cars began to get smaller again in the 1970s, the trend was towards the greater compactness enabled by the V6 layout, and straight-6 engines became rare in American cars except for trucks and vans. Jeeps were an exception to the rule, getting the AMC Straight-6 engine as the base engine option in 1972, and getting a high-performance 4.0 L option in 1987. Usage of the AMC 4.0 has been declining in Jeep vehicles since the 2002 replacement of the Jeep Cherokee with the Liberty, which features the Chrysler 3.7 L V6 instead. It has declined further since the 2005 introduction of the third generation Jeep Grand Cherokee, which also uses the 3.7 L V6. The last application of the 4.0 was in the 2006 Jeep Wrangler; for 2007 the engine has been replaced with a 3.8 L V6.
Ford used a straight-6 in baseline Mustangs. They were also found in many F150's up until 1997 when they were replaced with a V6.
In 2001 General Motors introduced a new family of straight engines, the Atlas, for use in the newly-introduced Chevrolet TrailBlazer/GMC Envoy. The straight-6 was chosen for development because of the desirable operating characteristics of its self balanced design.
As far as passenger vehicles are concerned, straight-6 engines might be making a comeback in some larger vehicle types such as trucks and SUVs. Examples include the 5.9 L Cummins Turbo Diesel engine used in Dodge Rams and GM's 4.2L "Vortec 4200".
Straight-6 engines in Asia
The Japanese have used the straight-6 with great success since the 1960s in a wide range of applications, from passenger vehicles, to sports cars, to SUV's. Both Datsun and Toyota were among the first in this trend, though Prince Motors (later acquired by Datsun) and others offered straight-6s in that time too.
Toyota started with their M-series engine and later the F, FZ, G, and JZ engines, and Datsun started with their H-series and later the L and RB (used in the Nissan Skyline up until the 'R34' model). engines. Honda built the Honda CBX motorcycle from 1978 till 1981. In 1990's Toyota offered representatives of all 5 families in their vehicles: the G in the Altezza (and others); the M and its replacement, the JZ, in the Toyota Supra (and others); and the F and its replacement, the FZ, in the Land Cruiser. In the 2000's, Toyota's still offers the FZ-series, G-series and the JZ-series engines.
In Korea, GM Daewoo's Magnus (sold abroad as the Chevrolet Evanda, Chevrolet Epica or Suzuki Verona) comes with a Daewoo-designed straight-6.
Straight-6 engines in Australia
Historically, all three manufacturers in Australia used straight-6s. Chrysler had built the Slant 6 in Australia and the unique to Australia Hemi straight-6. These engines were used in the Chrysler Valiant and the Valiant Charger producing up to 320hp. Chrysler no longer owns any factories in Australia.
Holden built 161, 186 and 202in^3 straight-6s from 1968-1984. They were used in the Kingswood, Torana and in the early Commodores. Modern Commodores use V6s.
Ford Australia has been producing straight-6s since 1960 and is the only manufacturer in Australia to still build them. Ford has built 144, 200, 240 and 250in^3 engines, with the 240 being called the 3.9l or 4.0l and the 200 being called the 3.3l. They have been used since 1960 in the Falcon, 1970-1982 in the Cortina and from 2004 in the Ford Territory. The current straight-6 engines in the Falcon and Territory are called the Barra.
The high-performance division of Ford Australia, Ford Performance Vehicles, produce vehicles equipped with the 4.0 litre DOHC 24-valve turbocharged straight-6 with variable cam timing, which produces 270 kW (362 hp) @ 5250 rpm and 550Nm (406 ft·lbf) @ 2000 - 4250 rpm — the highest level of torque in any Australian production car to date.
Diesel straight-6 engines
The straight-6 in diesel form with a much larger displacement is commonly used for various industrial applications. These range from various types of heavy equipment to power generation to transit buses or coaches. As with everyday passenger vehicles, the smooth running characteristics of the straight-6 engine is what makes it desirable for industrial use. In addition, a straight-6 engine is mechanically simpler than a V6 or V8. It has only one cylinder head and half as many camshafts as a V engine.