Calculation of compression ratio
To begin with, one must distinguish between compression ratio and compression pressure.
The compression pressure is the value you get when you test comp with a meter in the spark plug hole.
Compression ratio is a value such as 10: 1.
This says that the combustion chamber plus the volume of the cylinder head is one tenth compared to the total volume of the cylinder.
To calculate the compression ratio, you must know the following.
Cylinder volume / Top gasket volume / Combustion chamber volume.
We start by calculating cylinder volume.
You take the cylinder diameter squared multiplied by pi (3.14) multiplied by the stroke, finally divided by 1000.
A little simpler explained so do as the following example.
In this example, we imagine that you have a B20 engine drilled to 92mm with standard stroke (80mm) and cylinder head gasket (8.24cc)
The top has a measured volume of 46cc
Cyl / engine volume
92 x 92 x 3.14 x 80/1000 = 2126.16cc (= total engine volume)
2126.16 / 4 = 531.54cc (= cylinder volume)
Volume of the top gasket.
93.5 x 93.5 x 3.14 x 1.2 x 0.25 / 1000 = 8.24cc
The volume of the cylinder head.
This is a little more difficult because here you have to measure the volume.
You need a measuring glass or a large syringe that is graduated with ml.
Use diesel or similar to fill your measuring glass.
Do not forget to read how much liquid you have in the measuring glass.
You also need a piece of plexiglass with two small holes in it, one for filling and one for aeration.
Apply some grease to the sealing surface of the valves and around the combustion chamber.
Put on the plexiglass sheet and fill the combustion chamber so it is completely full.
Read how much you have left in your measuring glass, the difference = the volume of the combustion chamber.
In the example case, it was 46ml
Now we have all the values we need to calculate the compression ratio.
531.54cc = cylinder volume
8.24cc = volume of cylinder head sacking
46cc = volume of the combustion chamber
Add the top gasket and the cylinder head volume 8.24 + 46 = 54.24cc
Then add the cylinder volume 531.54 with 54.24 = 585.78
Then divide 585.78 by 54.24 = 10.799 = about 10.8
8.24 + 46 = 54.24
54.24 + 531.54 = 585.78
585.78 / 54.24 = 10.799 = about 10.8: 1
You have now calculated that you have a compression ratio of 10.8 to 1 (10.8: 1)
If it is really careful, you should also expect that often the pistons do not turn evenly with the block surface (deck surface)
Then you have to measure how far below (or above) the piston turns, and take this into account
The volume you calculated should be added to the volume of the cylinder head and gasket.
To begin with, one must distinguish between compression ratio and compression pressure.
The compression pressure is the value you get when you test comp with a meter in the spark plug hole.
Compression ratio is a value such as 10: 1.
This says that the combustion chamber plus the volume of the cylinder head is one tenth compared to the total volume of the cylinder.
To calculate the compression ratio, you must know the following.
Cylinder volume / Top gasket volume / Combustion chamber volume.
We start by calculating cylinder volume.
You take the cylinder diameter squared multiplied by pi (3.14) multiplied by the stroke, finally divided by 1000.
A little simpler explained so do as the following example.
In this example, we imagine that you have a B20 engine drilled to 92mm with standard stroke (80mm) and cylinder head gasket (8.24cc)
The top has a measured volume of 46cc
Cyl / engine volume
92 x 92 x 3.14 x 80/1000 = 2126.16cc (= total engine volume)
2126.16 / 4 = 531.54cc (= cylinder volume)
Volume of the top gasket.
93.5 x 93.5 x 3.14 x 1.2 x 0.25 / 1000 = 8.24cc
The volume of the cylinder head.
This is a little more difficult because here you have to measure the volume.
You need a measuring glass or a large syringe that is graduated with ml.
Use diesel or similar to fill your measuring glass.
Do not forget to read how much liquid you have in the measuring glass.
You also need a piece of plexiglass with two small holes in it, one for filling and one for aeration.
Apply some grease to the sealing surface of the valves and around the combustion chamber.
Put on the plexiglass sheet and fill the combustion chamber so it is completely full.
Read how much you have left in your measuring glass, the difference = the volume of the combustion chamber.
In the example case, it was 46ml
Now we have all the values we need to calculate the compression ratio.
531.54cc = cylinder volume
8.24cc = volume of cylinder head sacking
46cc = volume of the combustion chamber
Add the top gasket and the cylinder head volume 8.24 + 46 = 54.24cc
Then add the cylinder volume 531.54 with 54.24 = 585.78
Then divide 585.78 by 54.24 = 10.799 = about 10.8
8.24 + 46 = 54.24
54.24 + 531.54 = 585.78
585.78 / 54.24 = 10.799 = about 10.8: 1
You have now calculated that you have a compression ratio of 10.8 to 1 (10.8: 1)
If it is really careful, you should also expect that often the pistons do not turn evenly with the block surface (deck surface)
Then you have to measure how far below (or above) the piston turns, and take this into account
The volume you calculated should be added to the volume of the cylinder head and gasket.