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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
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 may 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.)
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
This diagram shows all the volumes you need to know to calculate an engine’s true compression ratio
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 cylinder 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 is at top dead
center.
Combustion Chamber Volume
Combustion chamber volumes for stock heads and aftermarket heads are typically available from
the manufacturer. If you can’t 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.

cc/16.4 = ci

Compressed Head Gasket Volume
Compressed head gasket volume is simply the volume of the cylinder hole in the head gasket —
think of it as a very shallow cylinder. So, its volume is computed the same way you compute cylinder
volume:

B x B x Gasket Thickness x 0.7854 = Compressed Head Gasket Volume

In this case, the gasket’s compressed thickness is .038 inches, so . . .

4.000” x 4.000”x .038” x 0.7854 = 0.4775232 ci
Piston/Deck Height Volume
Piston/Deck height volume is the small volume at the top of the cylinder that is not swept by the
piston. Measure piston/deck height with a dial indicator. Bring the piston to top dead center (TDC)
and measure the distance from the top of the piston to the deck of the block. This is normally
somewhere between .008 and .025 inch. If the block deck has been machined, say .010 inch, then
deck height will be smaller.
Once again, this volume is a shallow cylinder. Compute its volume by plugging the piston/deck
height measurement (D) into the cylinder volume formula:

B x B x D x 0.7854 = Piston/Deck Height Volume

In our example, this measurement was .015 inch, so we plug in that value to compute piston/deck
height volume in cubic inches.

4.000” x 4.000” x .015” x 0.7854 = 0.188496 ci
Previous


This has been a sample page from

How to Rebuild the Small Block Ford How to Rebuild the Small Block Ford
by George Reid
One of the best reference books available for rebuilding the
Ford 221, 260, 289, 302, Boss 302, 351W, 351C, 351M and 400
Over the years, the small-block Ford has remained one of the most
popular and widely used engines on the planet. From the earliest
Fairlanes and Mustangs to the latest Mustangs and light trucks, the
Ford small-block has powered them all. With the amount of aftermarket
support and rebuildable cores out there, you don’t have to worry about
spending an arm and a leg for a quality rebuild – especially if you do
the teardown and assembly yourself. This all-new color edition of How
to Rebuild the Small-Block Ford guides you step by step through a
rebuild, including: planning your rebuild, disassembly and inspection,
choosing the right parts, machine work, assembling your engine, and
first firing and break-in. The Workbench format also gives you helpful
hints and tips on performance upgrades, including cams, heads,
ignition, induction, and more. It also points out problem areas to watch
for, professional builder tips, jobs that need special care or special
tools, and more. Whether you're a first-time engine builder or a
seasoned professional, this is the essential guide to rebuilding your
small-block Ford.
Chap. 1 - Before You Begin
Chap. 2 - Engine Disassembly
Chap. 3 - Selecting Parts
Chap. 4 - Machine Shop
Chap. 5 - Engine Assembly
Chap. 6 - Break-In Tuning
Chap. 7 - Buyer's Guide
Chap. 8 - Engine Math
This is one of the best reference books available for rebuilding the small
block Ford and something that any enthusiast will love!
How to Rebuild
the Small Block Ford
by George Reid
Condition: NEW
8-1/2 x 11"
Softbound
144 pages
495 Color Photos

Item: SA102
Price: $22.95
Click here to buy now!

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How to Build Big-Inch Ford Small Blocks
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 building of a small block Ford
stroker, paying special attention to the effect that
increasing the bore and stroke have on the engine as a
whole. 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
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How to Build Big-Inch Ford Small Blocks
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This book addresses high-performance V-8 engines
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Emphasis throughout is a budget approach to building
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High Performance Engines The Ford Small Block
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