Technician a says that a ratio is a comparison between two values. technician b says that in a compression ratio, the main values are the cylinder volumes at the two different piston positions, bdc and tdc. who is right

Both technicians are right.

The compression ratio is the ratio ...
  (cylinder volume at BDC) / (cylinder volume at TDC)

Technician A might need to add that the comparison is one in which one value is divided by another (as opposed to comparison by subtraction).

Technician B might need to add that the volumes are divided one by the other. Knowing only two values doesn't tell you their ratio. Additional math must be performed.

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