ardor vs fire in the belly

ardor

noun
  • Spirit; enthusiasm; passion. 

  • Intense heat. 

  • Great warmth of feeling; fervor; passion. 

fire in the belly

noun
  • The emotional stamina and vigor, passion, or inner drive to achieve something, to take action, etc. 

How often have the words ardor and fire in the belly occurred in a corpus of books? (source: Google Ngram Viewer )