Engine Math/ Quick Facts
 
 
  When you’re building an engine, it’s nice to be armed with the facts necessary to do it
successfully. Much of engine building is about math — machining dimensions, compression
and rod ratios, bore sizes, stroke, journal diameters, carburetor and port sizes, dynamic
balancing, and all the rest of it. Without math, you cannot successfully build an engine. What
follows are quick facts that will help you in your Ford engine building.
 
 
  Cubic-Inch Displacement
Cubic-inch displacement is simply the volume displaced by the cylinders of your engine. So, if
we calculate the volume of one cylinder, and multiply that figure times the number of cylinders,
we have the engine’s displacement.
 
 
The formula for a cylinder’s volume is:

Pi x r2 x S = Volume of one cylinder.
 
   
  Where Pi is a mathematical constant equal to 3.14159; R is the radius of the cylinder, and S is
the stroke. If you think back to your high school geometry, you’ll remember that a circle’s
radius is half the diameter. In this case, the diameter is equal to the bore (B), so 1/2B=r. Plug
that in, and our formula becomes:

Pi x (1/2B)2 x S = Volume of one cylinder
 
  We can simplify this further by plugging in the numerical value for Pi, then doing some basic
algebra that doesn‘t necessarily need to be covered here — but trust us: the equation before
is equal to this equation:

B x B x S x 0.7854 = Volume of one cylinder
 
 
  To determine the engine’s displacement, factor in the number of cylinders (N):

B x B x S x 0.7854 x N = Engine displacement
 
 
  So, let’s use this to figure out the displacement of a Ford engine that has a 4-inch Bore and a
3-inch Stroke:

4.000” x 4.000” x 3.00” x 0.7854 x 8 = 301.59 ci

Ford rounded 301.59 up to 302 ci, or 4.9L.

(Note: One liter is equal to about 61 cubic inches.)
 
When the piston is at BDC, the total volume is all of these volumes added together. When the
piston is at TDC, the total volume is all of these EXCEPT the Cylinder Volume (V). So … true
compression ratio is this:

V + D + G + C + P
D + G + C + P

 

Calculating Compression Ratio
An engine’s compression ratio is the ratio between two volumes: The volume of the cylinder
and combustion chamber when the piston is at BDC, and the volume of the combustion
chamber when the piston is at TDC. But there’s more to consider than just cylinder volume and
head cc’s. To get the engine’s TRUE compression ratio, you need to know these volumes:
 

 

When the piston is at BDC, the total volume is all of these volumes added together. When the
piston is at TDC, the total volume is all of these EXCEPT the Cylinder Volume (V). So … true
compression ratio is this:

V + D + G + C + P
D + G + C + P
 

 

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