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