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HELPFULL CALCULATIONS
Click on the list below to take you to the various charts and calculations on this page.
- Engine Displacement
- Stroke Length
- Cylinder Bore Diameter
- Formula For Milling Pistons
- Compression Ratio
- Total Combustion Chamber Volume For a Specific Compression Ratio
- Cylinder Head Deck Machining To Reduce Total Chamber Volume
Engine Displacement
Engine displacement = bore X bore X stroke X 0.7854 X number of cylinders
Example:
Cylinder bore diameter = 4.000"
Stroke length = 3.480"
Number of cylinders = 8
Engine displacement = bore X bore X 0.7854 X number of cylinders
Engine displacement = 4.000 X 4.000 X 3.480 X 0.7854 X 8
Engine displacement = 349.8586 cubic inches (round up to 350 cubic inches)
Stroke Length
Stroke Length = engine displacement / (bore X bore X 0.7854 X number of cylinders)
Example:
Engine Displacement = 350 cubic inches
Cylinder bore diameter = 4.000"
Number of cylinders = 8
Stroke Length = engine displacement / (bore X bore X 0.7854 X number of cylinders)
Stroke Length = 349.8486 / (4.000 X 4.000 X 0.7854 X 8)
Stroke Length = 3.480"
Cylinder Bore Diameter
Cylinder bore diameter = square root of [engine displacement/(stroke X 0.7854 X number of cylinders)]
Example 1:
Engine Displacement = 350 cubic inches
Stroke Length = 3.480"
Number of cylinders = 8
Cylinder bore diameter = square root of [engine displacement/(stroke X 0.7854 X number of cylinders)]
Cylinder bore diameter = √[349.8486 / (3.480 X 0.7854 X 8)]
Cylinder bore diameter = 4.000"
Example:
NASCAR® has a 358 cubic inch maximum engine size rule. If we use a 3.480" crank, what is biggest bore allowed?
Example 2:
Engine Displacement = 358 cubic inches
Stroke Length = 3.480"
Number of cylinders = 8
Cylinder bore diameter = square root of [engine displacement/(stroke X 0.7854 X number of cylinders)]
Cylinder bore diameter = √[358 / (3.480 X 0.7854 X 8)]
Cylinder bore diameter = 4.046"
Formula For Milling Pistons
(For 4032 material only)
Piston dome cc's to gram conversion: 1cc (volume) = 2.8 grams (weight)
This is a good way to remove excess dome without having to re-cc piston: Mill a small amount and re-weight piston until total reduction is reached.
Example 1:
A piston has 12.5cc effective dome volume. The desired effective dome volume is 10.5cc.
To remove 2.0cc, cut 5.6 grams (2 X 2.8) from the piston dome.
Compression Ratio
Compression ratio = (swept volume + total chamber volume) / total chamber volume
It is important that we understand two terms and their relationship to compression ratio: Swept Volume and Total Chamber Volume. Swept Volume is the area the piston travels through bottom dead center to top dead center. Total Chamber Volume is all the area above the piston at top dead center. This would include the area above the piston in the cylinder block, the area of the compressed head gasket, the combustion chamber, the valve pocket, and the dome of the piston. The compression ratio is the relationship of the swept volume to the total chamber volume.
To start, we need to know the Swept Volume of one cylinder. The size of one cylinder figured in cubic centimeters.
Swept volume (cc) = cylinder bore diameter (inches) X cylinder bore diameter (inches) X stroke (inches) X 12.8704
Example:
Engine Displacement = 350 cubic inches
Cylinder bore diameter = 4.000"
Number of cylinders = 8
Stroke Length = engine displacement / (bore X bore X 0.7854 X number of cylinders)
Stroke Length = 349.8486 / (4.000 X 4.000 X 0.7854 X 8)
Stroke Length = 3.480"
Cylinder head cc = 72.18 cc
Piston = flat top with two valve pockets that measure a total of 4 cc
Head gasket = 4.000" round and 0.038" thick when compressed
Deck clearance = The piston at top dead center is 0.010" below the surface of the deck
Gasket cc = bore X bore X compressed thickness X 12.8704
Gasket cc = 4.000 X 4.000 X 0.038 X 12.8704
Gasket cc = 7.83 cc
Deck clearance volume = bore X bore X deck clearance X 12.8704
Deck clearance volume = 4.000 X 4.000 X 0.010 12.8704
Deck clearance volume = 2.059 cc
Total chamber volume = 72.18 + 7.83 + 4 + 2.059
Total chamber volume = 86.07 cc
Now we are finally ready to calculate the compression ratio!
Example:
Swept volume = 716.62 cc
Total chamber volume = 86.07 cc
Compression ratio = (swept volume + total chamber volume) / total chamber volume
Compression ratio = (716.16 + 86.07) / 86.07
Compression ratio = 9.33:1
Total Combustion Chamber Volume For a Specific Compression Ratio
Cylinder head chamber volume = swept volume / (desired compression ratio - 1)
Example:
Swept volume = 716.62 cc
Desired compression ratio = 11:1
Cylinder head chamber volume = swept volume / (desired compression ratio - 1)
Cylinder head chamber volume = 716.62 / (11:1 - 1)
Cylinder head chamber volume = 71.66 cc
Cylinder Head Deck Machining To Reduce Total Chamber Volume
Cylinder head deck material removal = (current chamber volume - desired chamber volume) X deck material per cc
By experience, we have learned that a small block Chevy cylinder head will need 0.006" deck removed for each cc we want to reduce. An open chamber big block will take 0.005" per cc. These numbers will put us in the ballpark. Always check by "cc-ing" the cylinder head chamber volume for accuracy.
Example:
Current chamber volume = 86.07 cc
Current chamber volume = 71.66 cc
Deck material removal per cc = 0.006"
Deck material to remove = (current chamber volume - desired chamber volume) X deck material per cc
Deck material to remove = (86.07 - 71.66) X 0.006
Deck material to remove = 0.086"