A four-stroke straight-four always has at least one cylinder on its power stroke, unlike three-cylinder or two-cylinder engines where power delivery gaps occur. The layout uses a single cylinder head, making it less complex and less expensive to manufacture than a V4 or flat-four engine.
Petrol straight-four engines in modern production cars typically displace between 1.3 and 2.5 litres, though the range extends from as little as 356 cc in the 1963–1967 Honda T360 kei truck to 3.0 litres in the Porsche 944 and 968. Diesel variants reach larger displacements, including a 3.2-litre Mitsubishi unit used in the Pajero and 5-litre engines common in European and Asian trucks rated between 7.5 and 18 tonnes.
Straight-four engines achieve perfect primary balance because pistons move in pairs — one pair always rises as the other descends. However, they suffer from a secondary imbalance: the acceleration of pistons through the upper half of the crankshaft rotation is faster than through the lower half, because connecting rods are finite in length. This produces an up-and-down vibration at twice crankshaft speed. The problem intensifies at higher engine speeds because the effect grows quadratically with rpm, making it particularly pronounced in large-displacement or high-revving engines.
A balance shaft system, consisting of two shafts with eccentric weights rotating in opposite directions at twice crankshaft speed, counteracts this secondary imbalance. The system was invented in 1911 and patented by Mitsubishi Motors in the 1970s, introduced in the Mitsubishi Astron engine as the "Silent Shaft" system, and has since been licensed to numerous manufacturers.
The most influential early racing straight-four was the Peugeot engine that won the 1913 Indianapolis 500, designed by Ernest Henry with double overhead camshafts and four valves per cylinder. This arrangement became the standard template for racing inline-fours. The Miller engine, directly inspired by the Peugeot design, dominated American racing through the 1920s and early 1930s, and its successor, the Offenhauser engine, remained competitive from 1933 until 1981. The Offenhauser achieved five consecutive Indianapolis 500 victories between 1971 and 1976.
In European Grand Prix racing, 1.5-litre supercharged inline-fours powered voiturette-class cars such as the Maserati 4CL and various English Racing Automobiles models in the pre-war period. These carried over into the early postwar era and provided the technical foundation of what became Formula One, before the straight-eight Alfettas established dominance in the first championship seasons.
Aurelio Lampredi's Ferrari straight-four began as a 2.0-litre Formula 2 unit for the Ferrari 500, evolved to 2.5 litres for the Ferrari 625 in Formula One, and reached 3.4 litres in the Ferrari 860 Monza sports car. The Coventry Climax FPF started as a 1.5-litre Formula 2 engine, grew to 2.0 litres for Formula One in 1958, and in 2,495 cc form won the World Championship with Cooper's chassis in 1959 and 1960.
The 1980s Formula One turbo era produced the most extreme straight-four in racing history: the BMW M12/13. Built on a production cast-iron block, it powered Brabham, Arrows, and Benetton and won the 1983 World Championship. The 1986 qualifying version was reported to generate approximately 1,300 hp at 5.5 bar of boost pressure.
Belgian manufacturer FN Herstal produced the first inline-four motorcycle in 1905, with the engine mounted upright and the crankshaft running longitudinally. Other early adopters included Henderson, Ace, Cleveland, and Indian in the United States, as well as Nimbus in Denmark and Windhoff in Germany.
The configuration became the dominant format for performance street motorcycles following Honda's introduction of the SOHC CB750 in 1969. Today all major Japanese manufacturers offer inline-four motorcycle models, alongside MV Agusta and BMW. The 2009 Yamaha R1 introduced a crossplane crankshaft to an inline-four motorcycle engine, firing at uneven intervals to improve secondary balance at high rpm and provide more controllable torque delivery at racing speeds.
In the Moto2 class, Honda supplied a 600 cc inline-four based on the CBR600RR from the class's introduction in 2010, producing approximately 150 hp. From 2019 this was replaced by a Triumph 765 cc triple engine.