If I had two cars and they both had engines, I would look at the
two engines (part) and compare them to one another - which is a
better engine? Then maybe I would do that with the two steering
wheels, seat liners, transmissions etc. This would be a part to
part comparison.
If I had two cars and I compared them to one another OVERALL by
looking at general functioning or general consumer's guidelines of
each one, this would be a specific comparison or a comparison.