Standard engine in 2WD models was a 92-horsepower, 2.5-liter 4-cylinder. A 2.8-liter V6, rated 125 horsepower, went into 4WD models and was an option for 2WD trucks. A 5-speed manual gearbox was standard with the 4-cylinder and 2.8-liter V6; 4-speed automatic optional for the 4-cylinder. Pickups might instead have an optional 160-horsepower, 4.3-liter V6, offered only with automatic transmission. On-demand, part-time Insta-Trac 4WD has shift-on-the-fly capability between 2WD and 4WD High. Automatic-locking front hubs were standard with the Insta-Trac system. Previously an option, the 4.3-liter V6 became standard on all 4WD models for 1991, and could be ordered with manual shift for the first time. Four-cylinder engines gained 11 horsepower during the 1991 model year, courtesy of a new intake manifold and cylinder head, and revised exhaust system.