hot rod vs hot-rod

hot rod and hot-rod both are nouns.

Word	Noun	Adjective	Verb	Adverb
hot rod	Yes	No	No	No
hot-rod	Yes	No	No	No

As nouns, hot rod and hot-rod are synonyms defined as:

hot rod and hot-rod: a car modified to increase its speed and acceleration

hot rod (noun)	hot-rod (noun)
a car modified to increase its speed and acceleration	a car modified to increase its speed and acceleration