Differences

• The major difference between the Ford 390 and the 302 is displacement. The 390 has 390 cubic inches of displacement, whereas the 302 displaces 302 cubic inches. Displacement refers to the volume covered by the all pistons from top to bottom dead center--it's actually a measurement of combustion chamber volume. With increased displacement, an engine generates more power. It also needs more fuel to run at its optimum range than an engine with less displacement needs to run at its optimum range.
  
  

Ford 390

• The Ford 390, an FE-series engine, was manufactured between 1961 through the early 1970s. The engine block and heads are considerably larger than the 302's. More closely related to other big-block Fords of the era such as the 427 and 428, the 390 was standard Ford issue for mid-sized to large cars and pickup trucks. The 390 was built to be a quiet, smooth-running engine with the power and torque to handle the needs of the large cars and pickups of the era.
  
  

Ford 302

• As a Ford "90 degree series" engine manufactured between 1968 to 1990, the 302 was originally manufactured to compete with similar Chevrolet engines of the day. Although used in other Fords including the Crown Victoria and the F-150, the 302 is most associated with the Mustang. Big power in a small package was its goal, completely opposite the mission of the 390. Later on in the 1980s and 1990s as fuel cost became an auto-industry issue, the 302 became the large, powerful engine for most Fords, as big blocks like the 390 were fuel-inefficient.