We share the formula to calculate displacement of an engine when we know the bore and stroke, in simple terms…
The most common terms which we use in our regular motorcycling talks is the displacement or the cubic capacity of an engine; probably more than all the other aspects combined. But not everyone will know how is the exact displacement calculated when we know the bore and stroke of an engine. In this story we will tell you how to do it, in very simple language…
We must know the following terms..
An engine comprises of a cylinder which is nothing but a metal with a well defined big hole in it.
- Bore – Bore is the diameter of this hole and the piston moves up and down inside it.
- Stroke – Stroke is the depth of this hole.
Formula to Calculate Displacement of an Engine
Displacement = (π/4) x Bore x Bore x Stroke x Number of cylinders
π, called as ‘pi’ is a mathematical constant and is equal to 3.14
Let us take some examples
- Bore – 56.0 mm = 5.6 cm
- Stroke – 60.7 mm = 6.07 cm
- Cylinder – 1
Calculation – (3.14/4) x 5.6 x 5.6 x 6.07 x 1 = 149.428 cm3
Kawasaki Ninja H2
- Bore – 7.6 cm
- Stroke – 5.5 cm
- No of cylinders – 4
Displacement = (3.14/4) x 7.6 x 7.6 x 5.5 x 4 = 997.515 cc
KTM 390 Duke
- Bore – 8.9 cm
- Stroke – 6.0 cm
- Cylinders – 1
Displacement = (3.14/4) x 8.9 x 8.9 x 6.0 x 1 = 373.079 cc
Interestingly, this was highlighted when Harley announced the exact cubic capacity of its upcoming 338 cc motorcycle. An analysis predicted its Benelli relation. The smallest Harley may use the crankshaft of the 300cc Benelli engine (stroke = 45.2 mm) and cylinder of the 500cc engine (bore = 69 mm). Here is the calculation then..
(3.14/4) x 6.9 x 6.9 x 4.52 x 2 = 337.8 cc
As you can see this way we arrive at the exact same displacement what Harley has revealed. You can read more similarities here.
We have tried to keep the calculation and details as simple as possible and the examples with various engine sizes. If you want to read more such informative articles, you can head to this page.