car horn vs motor horn

car horn and motor horn both are nouns.

Word	Noun	Adjective	Verb	Adverb
car horn	Yes	No	No	No
motor horn	Yes	No	No	No

As nouns, car horn and motor horn are synonyms defined as:

car horn and motor horn: a device on an automobile for making a warning noise

Other synonyms of car horn include automobile horn, hooter, horn.

car horn (noun)	motor horn (noun)
a device on an automobile for making a warning noise	a device on an automobile for making a warning noise