Here is our compression ratio calculator for 2-stroke or 4-stroke engines.
By entering your crankshaft stroke, bore (piston diameter), and combustion chamber volume (which you can measure with a syringe), you can obtain your volumetric ratio / compression ratio.
Since 2-stroke engines only start compressing when the exhaust port is closed, it is preferable to use the so-called "Japanese ratio."
To do this, instead of entering your crankshaft stroke, enter the distance between the top of your exhaust port (the moment when the piston fully closes it) and TDC! It is important to understand that, unlike 4-stroke engines, a 2-stroke engine requires a different compression ratio depending on whether its exhaust timing is large or small! In general, A liquid-cooled engine with a 175° exhaust timing will require a standard ratio of around 12.5:1
: 1 whereas an engine with a 200° exhaust timing will require a standard ratio closer to 15:1
Some guidelines:
When filling your combustion chamber with a syringe and oil to measure its volume with the piston at TDC, fill up to the top of the spark plug thread! Then Subtract 1.5 ml for a short-reach spark plug.
Subtract 2.3 ml for a long-reach spark plug.
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