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Displacement and Compression Ratio

Most standard production machines will have these figures clearly stated in the owner's manual. However, if you are fitting non standard parts and tuning an engine this page shows you how to quickly and easily calculate engine displacement and compression ratio.

Required Information

All you need to know before getting started is:

  • Engine bore in cm

  • Engine stroke in cm

  • Volume of combustion chamber in cc

 

Other units such as mm can be used but a correction factor at the end will be required. When using mm for bore and stroke answer will be given in cubic millimetres. This can be divided by 1000 to get capacity in cubic centimetres (cc). 

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Notation

A = cross sectional area of the cylinder bore in cm²

CR = compression ratio

CV = combustion chamber volume in cc

D = displacement in cc

r = radius (half the bore, not forgetting any oversizes) in cm

S = engine stroke in cm

Barrel and head sketch New.jpg
Measure the volume of the combustion chamber

The easiest way to do this is by setting the engine with the piston at TDC and filling the combustion chamber, through the spark plug hole, with thin oil or similar until the top of the fluid is just level with the bottom thread of the plug hole. Use a burette or syringe to fill the chamber and make a note of the amount of fluid used. 

On most engines it is necessary to lean the engine over so that the plug hole is vertical to ensure the combustion chamber is completely full with no air pockets. 

Disp.JPG
Calculating engine displacement

The displacement of a cylinder is found by multiplying the cross sectional area of the cylinder by the height or, in this case, the stroke. 

The cross sectional area and, in turn, the displacement is found using the equation shown here. Don't forget to include any over size if the engine has been re-bored.  In the case of a multi cylinder engine simply multiply this answer by the number of cylinders.

CR.JPG
Calculating compression ratio

Using the engine displacement and the combustion chamber volume the compression ratio can be easily calculated. Simply add the displacement to the combustion chamber volume and then divide the answer by the combustion chamber volume. 

Feeling baffled?

If you have read all of the above and are left feeling confused, or would rather be out on your bike than sitting indoors with a pen and paper, we’ve created a spreadsheet do all the hard work. Simply input the information from your bike and get the answers calculated for you. 

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