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What is displacement?

Displacement is the volume of air "displaced" by the pistons during a single engine cycle.  In terms of a single cylinder, displacement is the volume of the space traveled by the piston.  The two factors that make up engine displacement are bore and stroke.

To calculate the displacement of an engine, first calculate the displacement of a single cylinder.  The formula for the volume of a cylinder: 

V = hpi.gif (843 bytes)r2

where h is the height of the cylinder and r is the radius of the cylinder (halfway across).  If given the bore (diameter), multiplying it by (dividing by 2) will yield r.  h is expressed as stroke thereby giving the formula for engine displacement:

Displacement = Stroke * pi.gif (843 bytes) * (Bore / 2)2 * [# of cylinders]

Note that when Stroke and Bore are expressed in inches, displacement is solved in units of cubic in (ci).  If Stroke and Bore are expressed in mm, then calculated displacement (in cubic mm) must be divided by 1000 to be expressed in cubic cm (cc), which is the same as milliliters (ml).  Also note that engine displacement is normally expressed in liters (l) by dividing the displacement in cc once again by 1000.

For Subaru's 2.5L H4 released in the U.S. 2000 models, the Bore is 3.92" and the Stroke is 3.11".  Therefore:

Displacement = 3.11 * pi.gif (843 bytes) * (3.92 / 2)2 * 4

Keep in mind that the Bore and Stroke numbers are not exact.  They are rounded, so the calculated displacement will not be exact.  In this case, displacement is calculated as 150.14 ci., but the published displacement is actually 2458 cc (2.5 l), or 149.996 ci.

Displacement Calculator (in)

Type in your values Bore, Stroke, and Cylinders, then press =.