race car vs motorcar

race car and motorcar both are nouns.

Word	Noun	Adjective	Verb	Adverb
race car	Yes	No	No	No
motorcar	Yes	No	No	No

As nouns, motorcar is a hypernym of race car; that is, motorcar is a word with a broader meaning than race car:

race car: a fast car that competes in races
motorcar: a motor vehicle with four wheels; usually propelled by an internal combustion engine

Other hypernyms of race car include auto, automobile, car, machine.

race car (noun)	motorcar (noun)
a fast car that competes in races	a motor vehicle with four wheels; usually propelled by an internal combustion engine