The cubic inch is a unit of measurement for volume in the Imperial units and United States customary units systems. It is the volume of a cube with each of its three dimensions (length, width, and depth) being one inch long.
The cubic inch and the cubic foot are still used as units of volume in the United States, although the common SI units of volume, the liter, milliliter, and cubic meter, are also used, especially in manufacturing and high technology.
Engine displacement is the volume swept by all the pistons inside the cylinders of a reciprocating engine in a single movement from top dead centre (TDC) to bottom dead centre (BDC). It is commonly specified in cubic centimetres (cc), litres (l), or (mainly in North America) cubic inches (CID). Engine displacement does not include the total volume of the combustion chamber.
A metric engine is an American expression which refers to an internal combustion engine, often for automobiles, whose underlying engineering design is based on a metric system of units, particularly SI.
As American industry converted from traditional units to SI in the late 20th century, the automotive industry responded by transitioning its auto and engine designs to be "metric" rather than "English".