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Calculating an Engines Compression Ratio
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The compression ratio has a tremendous impact on the performance characteristics of an engine.
A street engine must be able to live with the dismal octane rating of current unleaded gasolines.
On the other hand, a gas-burning race motor needs a compression ratio that extracts the maximum
power from high octane racing fuel without inviting destructive detonation.
An engine that runs on alcohol runs little risk of encountering detonation, but it needs the highest
possible compression ratio to make up for lower heat value in alcohol fuels. Thus, whether you are
blueprinting an engine for the boulevard or for the Daytona 500, calculating the compression ratio
is a vital step.
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The actual arithmetic required to compute compression ratio is relatively easy. The real work is
coming up with the numbers to plug into the compression equation. Compression ratio is defined as
the ratio between the volume above the piston at BDC and the volume above it at TDC.
Since combustion chambers, piston domes, and valve notches are very irregular shapes, the only
way to determine their exact volume is by measuring how much liquid they hold or displace. Once
these preliminary measurements have been made, you will be able to compute the compression
ratio in just a few minutes on a pocket calculator.
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Combustion Chamber Volume
Combustion chamber volume is an
important element in the compression
ratio equation. Seal the edges of the
chambers with a light coat of white
grease. Don’t forget to install a spark
plug.
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Measuring the combustion chamber volume is popularly called “cc’ing the chambers.” This refers to
the fact that chamber volume is measured in cubic centimeters (cc). You will need a 100.00cc glass
burette, a stand, and a 6 x 6-inch piece of 0.250-inch-thick Plexiglas for this exercise. (See the tool
chapter for tips on buying burettes.) You will also need a colored liquid for the actual measurement.
Solvent tinted with a few drops of machinist’s dye or automatic transmission fluid works well, since
these mixtures will not rust metal surfaces. If you are building an engine that must meet factory
specifications for compression, then you should use the same measuring fluid that the tech
inspectors use. The combustion chamber volume of NHRA Stock and Super Stock cars, for
example, is usually measured with rubbing alcohol that has been dyed with food coloring. If you are
running close to the edge of legality, a slight difference in chamber volume measurements may
leave you with an engine that is declared illegal. This is why you should strive to use the same
equipment and procedures as the tech inspectors.
You must also learn how to read a burette. The surface tension of the liquid in the burette tube
causes it to form a cup that is called a meniscus. Take your readings at the bottom of the meniscus
to ensure accuracy and consistency.
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The Plexiglas square is used to seal the combustion chamber. Drill a 0.250-inch-diameter hole near
one edge of this plate, and chamfer the hole with a countersink. Insert the valves in the chamber
and seal them with a light coating of white grease on the valve seats. Install a spark plug with the
same heat range as the plugs you intend to use. Prop the head up on the workbench so that the
deck surface is facing you. The head should be slightly high on the spark plug side of the chamber.
Spread a light coat of grease around the chamber, and then press the Plexiglas plate on the deck
surface with a slight twisting motion. The hole in the Plexiglas should be at the upper edge of the
combustion chamber, and the chamfer should be facing upward.
As you push the Plexiglas down on the deck surface, the grease will form a seal between the plate
and head. If the grease oozes into the chamber, it will affect the accuracy of your measurements;
so remove the plate, clean out the grease, and try again, using less grease on the deck surface
this time. On some cylinder heads, the edges of the valves may hold the plate away from the head
surface. If you encounter this situation, grind a small groove in the plate where it hits the valve
head. Fill this groove with grease and squeegee off the excess grease with the edge of a plastic
credit card.
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Press a Plexiglas plate firmly against
the grease. The grease should form
a continuous seal without oozing into
the chamber. Make sure the filling
hole in the plate is at the highest
point in the chamber. If the edges of
the valves hit the plate, grind small
clearance notches in the Plexiglas
and fill them with grease.
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Once the chamber has been sealed, you are ready to measure the volume. Fill the burette with
fluid, and then open the valve until the meniscus is exactly at the zero mark. Check for air bubbles
in the burette tube and valve. Position the tip of the burette over the hole in the Plexiglas plate,
open the valve, and begin filling the combustion chamber. Look for leaks in the ports, around the
spark plug, and between the plate and deck surface—these will all upset the accuracy of your
measurements. If the cylinder head chambers are larger than 100.00cc, you must close the burette
valve when the meniscus reaches the 100.00cc mark, and then refill and “zero” the burette again
before continuing to fill the chamber.
As the measuring fluid fills the chamber, take care to prevent air pockets from forming. When the
chamber is almost full, turn down the flow out of the burette so that the liquid just drips into the
chamber. You may need to move the head from side-to-side or tap on the Plexiglas to dislodge any
air bubbles that have collected under the sealing plate. When the fluid level just reaches the bottom
of the hole in the Plexiglas plate, close the burette valve. Read the volume that was required to fill
the chamber, remembering to take the reading at the bottom of the meniscus. Write down this
figure for later use in the compression computations.
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Carefully fill the chamber with colored
solvent or alcohol from a burette or
graduated cylinder. Check for leaks
around the valves and the spark plug.
Tilt the head to prevent air bubbles
from forming as the liquid fills the
chamber.
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If you are building a legal engine for class racing, you must check the volume of all the remaining
combustion chambers to make sure there are no chambers that will push the compression ratio
over the allowable limit! Even if a tech inspector will never examine the engine, it is a good idea to
check the volume of several other chambers. Always check at least a pair of chambers in each
cylinder head to prevent basing your compression ratio calculations on wrong numbers.
Congratulations! You have just cc’d your first cylinder head.
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Valve Relief Volume
The volume of the valve notches in the piston, like the size of the combustion chambers, will have
an effect on the compression ratio. If the piston tops are perfectly flat—without valve reliefs and
without a dome—then the volume of the cylinder can be easily computed from the bore and stroke
dimensions. If, however, the piston top is notched, dished, or domed, the volume of these features
must be measured before the compression ratio can be accurately determined.
This section is concerned with the simplest and most common type of piston: a flattop with one or
more valve reliefs. (If the engine is equipped with domed or dished pistons, you will have to use the
technique described in the following section.) You will need a chunk of modeling clay and a burette
to measure the volume of the valve notches.
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To measure the volume of the valve notches
on a flat-top piston, fill one relief with clay.
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Carefully remove the clay mold and drop it
into a half-filled burette. The change in
the fluid level equals the volume of the
valve relief.
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First soften the clay by squeezing it with your fingers until it becomes pliable. Fill one of the valve
reliefs with the soft clay, taking care to push the clay in to the corners of the notch. Then skim the
clay with a metal rule to level it with the piston top. The next step is to carefully remove the clay from
the notch without compressing or squeezing it. A sharp penknife or small screwdriver is helpful
when coaxing the clay out of the valve notch.
To measure the volume of this clay impression of the valve notch, fill a burette tube or graduated
cylinder to a convenient point on the scale— 50.00cc, for example. Then drop the clay into the
liquid and note how far the liquid level in the burette rises. If the fluid rises to the 52.00cc mark the
volume of the valve notch is 2.00cc.
Most production pistons have a number of identical valve notches on the tops. If a single valve
relief in a four-notch Chevrolet flattop piston, for example, has a volume of 2.00cc, then the total
valve relief volume would be 8.00cc. If the piston has notches of unequal size—typically, there may
be a large intake relief and a smaller exhaust relief—then each notch must be measured separately
and the volumes added to come up with a total valve relief capacity.
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Previous | Next
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This has been a sample page from
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The Step-By-Step Guide to Engine Blueprinting
by Rick Voegelin
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Practical Methods for Racing and Rebuilding
How to buy machine shop work
Selecting and preparing parts
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This completely revised and updated version containing an
additional 32 pages is simply the best book you can buy on
engine preparation for street or racing! Rick Voegelin's highly
acclaimed combination of savvy writing and wrenching skills puts
this best-seller in a class by itself. All important preparation
techniques are clearly illustrated and explained in this easy-to-
read text. Engine Blueprinting shows the reader how to use
precision measuring tools, calculate compression ratios, degree a
camshaft, and much more! Loaded with helpful advice, this book
should be in every enthusiast's tool box.
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Click below to view sample
pages from each chapter.
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"Rick Voegelin's book, The Step-by-Step Guide to Engine
Blueprinting, is an excellent source of performance-oriented
engine building information for the beginner and the seasoned
veteran alike. This digest should be in every enthusiast's greasy
mitts."-- Steve Magnante, HOT ROD
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Chap. 1 - Engine Blueprinting
Chap. 2 - Cylinder Block
Chap. 3 - Crankshaft
Chap. 4 - Connecting Rods
Chap. 5 - Pistons
Chap. 6 - Cylinder Heads
Chap. 7 - Camshaft
Chap. 8 - Compression Ratio
Chap. 9 - Balancing
Chap. 10 - Assembly Tips
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Softbound
8-3/8 x 10-7/8
160 pages
400 b/w photos
Item #SA21
Price: $18.95
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Click here to buy now!
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How to Rebuild the Small-Block Ford
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Price:
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Price:
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