Back to Cars
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 = hr^{2}
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 ½ (dividing by 2) will yield r. h is expressed as
stroke thereby giving the formula for engine
displacement:
Displacement = Stroke *
* (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 *
* (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 =.
