racer vs race car

racer and race car both are nouns.

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

As nouns, racer and race car are synonyms defined as:

Other synonyms of racer include racing car.

racer (noun)	race car (noun)
an animal that races	a fast car that competes in races
slender fast-moving North American snakes
a fast car that competes in races
someone who drives racing cars at high speeds