

Title: Calculating Compression Ratio
Question: I am interested in increasing the performance of my street rod engine. The engine that I am going to use in my street rod will be bored out and rebuilt with new pistons, rings, valves, and cylinder heads. I understand that the compression ratio of the engine might be changed. What would be a good procedure to calculate the actual compression ratio during the rebuilding process?
Answer: Often when an engine is used for higher performance, various components and dimension are changed within the engine. For example, if an engine is bored out, this changes the compression ratio. If an engine uses high performance pistons, and bigger valves, the compression ratio will also change. Remember that the compression ratio is defined as the total displacement above the piston at bottom dead center (BDC) as compared to the total displacement above the piston at top dead center (TDC). The following procedure can be used to calculate the final compression ratio. Keep in mind, that depending upon exactly what is done with the engine, the procedure may change slightly.
1. First, calculate the displacement of the piston from bottom dead center (BDC) to top dead
center (TDC). Use the following formula to calculate cubic inches:
(Cubic inch piston displacement = .785 X Bore squared X Stroke)
Cylinder bore______________. (See photo below)
Engine stroke _____________.
Displacement of piston from BDC to TDC in cubic inches = ___________.
Question: I am interested in increasing the performance of my street rod engine. The engine that I am going to use in my street rod will be bored out and rebuilt with new pistons, rings, valves, and cylinder heads. I understand that the compression ratio of the engine might be changed. What would be a good procedure to calculate the actual compression ratio during the rebuilding process?
Answer: Often when an engine is used for higher performance, various components and dimension are changed within the engine. For example, if an engine is bored out, this changes the compression ratio. If an engine uses high performance pistons, and bigger valves, the compression ratio will also change. Remember that the compression ratio is defined as the total displacement above the piston at bottom dead center (BDC) as compared to the total displacement above the piston at top dead center (TDC). The following procedure can be used to calculate the final compression ratio. Keep in mind, that depending upon exactly what is done with the engine, the procedure may change slightly.
1. First, calculate the displacement of the piston from bottom dead center (BDC) to top dead
center (TDC). Use the following formula to calculate cubic inches:
(Cubic inch piston displacement = .785 X Bore squared X Stroke)
Cylinder bore______________. (See photo below)
Engine stroke _____________.
Displacement of piston from BDC to TDC in cubic inches = ___________.
2. Next calculate the displacement designed into the shape of the cylinder head combustion chamber area.
To get the exact displacement, (with both valves closed), fill the cylinder head chamber with water using
a syringe (see photo below). This syringe measures in cubic centimeters. To change cubic centimeters
to cubic inches, divide the cubic centimeters displacement measured with the syringe by 16.387 to get
cubic inches. (1 cubic inch = 16.387 cubic centimaters)
To get the exact displacement, (with both valves closed), fill the cylinder head chamber with water using
a syringe (see photo below). This syringe measures in cubic centimeters. To change cubic centimeters
to cubic inches, divide the cubic centimeters displacement measured with the syringe by 16.387 to get
cubic inches. (1 cubic inch = 16.387 cubic centimaters)
Displacement of cylinder head chamber in cubic centimeters__________.
Displacement of cylinder head chamber in cubic inches = ___________.
3. Next, calculate the displacement that the thickness of the head gasket makes.
Measure the thickness of the cylinder head gasket_________.
Measure the diameter of the cylinder head gasket hole______.
Using the formulaDisplacement = .785 X Bore squared X head gasket thickness, calculate the
displacement made fro the head gasket.
Displacement from thickness of head gasket in cubic inches = _________.
4. Now calculate the exact volume that is displaced by the recessed areas on top of the piston
(so the valves don’t hit the piston when they open). See photo below. Do this by filling the displaced
area with water and measuring the amount with the syringe. Then change the cubic centimeters to cubic
inches (remember 1 cubic inche = 16.387 cubic centimeters)
Displacement of recessed area on top of piston in cubic centimeters_______.
Displacement of recessed area on top of piston in cubic inches = __________.
Displacement of cylinder head chamber in cubic inches = ___________.
3. Next, calculate the displacement that the thickness of the head gasket makes.
Measure the thickness of the cylinder head gasket_________.
Measure the diameter of the cylinder head gasket hole______.
Using the formulaDisplacement = .785 X Bore squared X head gasket thickness, calculate the
displacement made fro the head gasket.
Displacement from thickness of head gasket in cubic inches = _________.
4. Now calculate the exact volume that is displaced by the recessed areas on top of the piston
(so the valves don’t hit the piston when they open). See photo below. Do this by filling the displaced
area with water and measuring the amount with the syringe. Then change the cubic centimeters to cubic
inches (remember 1 cubic inche = 16.387 cubic centimeters)
Displacement of recessed area on top of piston in cubic centimeters_______.
Displacement of recessed area on top of piston in cubic inches = __________.
5. If there are more areas of displacement not mentioned above, do similar calculations to determine
additional displacement areas.
6. Next, calculate the total displacement above the piston at BDC by adding the following:
Displacement of piston from BDC to TDC in cubic inches___________.
Displacement of cylinder head chamber in cubic inches___________.
Displacement from thickness of head gasket in cubic inches_________.
Displacement of recessed area on top of piston in cubic inches________.
Other areas of displacement in cubic inches_____________________.
TOTAL DISPLACEMENT AT BDC = ______________________.
7. Now calculate the total displacement at TDC by adding each of the following:
Displacement of cylinder head chamber in cubic inches__________.
Displacement of thickness of head gasket in cubic inches_________.
Displacement of recessed area on top of piston in cibic inches_____.
TOTAL DISPLACEMENT AT TDC = ___________________.
4. To get the compression ratio, divide the displacement at BDC by the displacement at TDC as shown.
BDC Displacement / TDC Displacement =
Compression ratio = __________ to 1
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additional displacement areas.
6. Next, calculate the total displacement above the piston at BDC by adding the following:
Displacement of piston from BDC to TDC in cubic inches___________.
Displacement of cylinder head chamber in cubic inches___________.
Displacement from thickness of head gasket in cubic inches_________.
Displacement of recessed area on top of piston in cubic inches________.
Other areas of displacement in cubic inches_____________________.
TOTAL DISPLACEMENT AT BDC = ______________________.
7. Now calculate the total displacement at TDC by adding each of the following:
Displacement of cylinder head chamber in cubic inches__________.
Displacement of thickness of head gasket in cubic inches_________.
Displacement of recessed area on top of piston in cibic inches_____.
TOTAL DISPLACEMENT AT TDC = ___________________.
4. To get the compression ratio, divide the displacement at BDC by the displacement at TDC as shown.
BDC Displacement / TDC Displacement =
Compression ratio = __________ to 1
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