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Modifying Small Block Ford Cylinder Heads
Modifying Small
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Engine Math
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, carburetors, 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 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 5.0L. (Note: One liter is equal to about 61 cubic inches.)
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:

•    Combustion Chamber Volume (C)
•    Compressed Head Gasket Volume (G)
•    Piston/Deck height (D)
•    Piston Dish Volume (P) or Dome Volume (-P)
•    Cylinder Volume (V)
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
Combustion Chamber Volume
Combustion chamber volumes for stock heads and aftermarket heads are typically available from
the manufacturer. If you cant find the info or if you’ve modified the combustion chambers, you’ll
have to measure the volumes (using a plastic deck plate, burettes, and a graduated cylinder) or
have your local machine shop do it for you.

Converting cc’s to ci’s

Combustion chamber volume, dome volume, and dish volume are generally measured in cc’s, not
cubic inches. To convert cc’s to cubic inches, divide the measurement in cc’s by 16.4.
This diagram shows all the volumes you need to know to
calculate an engine’s true compression ratio: Cylinder
volume (V), piston dome (-P) or dish volume (P),
piston/deck height (D), compressed gasket volume (G),
and the combustion chamber volume (C). The
compression ratio is the volume of the piston and
combustion chamber (V + P + D + G +C) when the
piston is at bottom dead center, compared to the volume
of the combustion chamber (P + D + G +C) when the
piston as at top dead center.
This diagram shows all the volumes you need to 
know to calculate an engines compression 
ratio
Previous


This has been a sample page from

How to Build Big Inch Ford Small Blocks How To Build Big-Inch Ford Small Blocks
by George Reid
Have you been dreaming about a little extra displacement for
your Ford? By increasing the bore and stroke of your current
engine, you can add those cubic inches without the hassle of
switching to a big block. George Reid thoroughly explains the
concept of building a stroker, paying special attention to the
effect that increasing the bore and stroke have on the engine
as a whole. With this information, you’ll be better able to tailor
your heads, cam, intake manifold, carburetor, and exhaust
system to get the most out of the extra cubes. Also included
is a complete guide to factory head and block castings, as
well as aftermarket block and head guides, so you can
choose exactly the right parts for your project. This book is
the definitive guide for building a big-inch Ford small block,
complete with four engine buildups ranging from 331 to 408
cubic inches. Read the sample pages to learn more!

Temporarily Out of Stock - More On their way!

Click below to view sample
pages from each chapter
Chap. 1 - Ford Small Block
Chap. 2 - Stroked Engines
Chap. 3 - 289 and 302 Stroker
Chap. 4 - 351W Stroker Kits
Chap. 5 - 351C Stroker Kits
Chap. 6 - Stroker Vehicles
Chap. 7 - Engine Math
Sftbd.
8-1/2 x 11
128 pages
300 black & white photos
Item #SA85P
Price: $22.95
Click here to buy now!


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