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The straight-6, inline 6 (I-6), or slant-6 is an internal combustion engine with six cylinders aligned in one row.

Straight-6 engines have perfect primary and secondary balance and require no balancing shafts.

Usually a straight-6 was used for engine sizes between about 2.5 and 4 litres. 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.

Straight-six engines were historically more common than V6's, mainly because the length of such engines was not such a concern in rear-drive vehicles but also because V6's (unlike the 90-degree V8) were somewhat difficult to make smooth-running. The widespread use of front-wheel-drive and "east-west" engine configurations 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.

The front wheel drive Suzuki Verona and rear wheel drive Ford Falcon remain some of the very few straight sixes still in use in contemporary passenger vehicles, along with some Mercedes and BMW rear wheel drive luxury cars. Mercedes has recently given up the staight six, and now only engineers V6 engines. BMW on the other hand is one of the few remaining manufacturers to persist with the I-6 configuration, making excellent petrol and turbo-diesel engines ranging from 2.0 to 3.0 litres in displacement (as of 2004). Toyota also uses large displacement I-6 engines in their Prado off-road vehicles.

As far as passenger vehicles are concerned, inline six engines might be making a comeback in some larger vehicle types such as trucks and SUVs. In 2001 General Motors introduced an I-6 Vortec engine. The I-6 was chosen for development because of the desirable operating characteristics of its self balanced design. Jeeps have also had the 4.0L I-6 as an engine option since 1987, and going further back in history American Motors Corporation had noteworthy slant six engines.

Also the inline 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. As with everyday passenger vehicles, the smooth running characteristics of the I-6 engine is what makes it desirable for industrial use.

See also: straight engine