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Calculating Compression Ratio

      U = Uncompressed Distance
      C = Compressed Distance
Ratio = (U divided by C) : 1
Uncompressed Value =
Compressed Value =
Ratio = : 1

Measuring Values U & C

  • With the piston at top-dead-center, measure the distance form the top of the cylinder to the top of the piston. Do this by placing a level or other straight edge across the mouth of the cylinder (with the head off,) then using a tape measure or ruler measure the distance from the edge to the top of the piston. This gives the "compressed" value C.
  • The "uncompressed" value U can be obtained by two ways. The first is to just add the value of the engine's stroke to the compressed value. The second way is to measure the distance from the top of the cylinder to the top of the piston at bottom-dead-center.
  • For regular engine heads (not a flat-head,) add an extra 1 inch to both values Uncompressed and Compressed. This will compensate for the extra volume of the recessed head .


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