tighten up vs limit

tighten up is not a noun while limit is a noun.

Word	Noun	Adjective	Verb	Adverb
tighten up	No	No	Yes	No
limit	Yes	No	Yes	No

As verbs, limit is a hypernym of tighten up; that is, limit is a word with a broader meaning than tighten up:

Other hypernyms of tighten up include bound, confine, restrict, throttle, trammel.

tighten up (noun)	limit (noun)
	the greatest possible degree of something
	the boundary of a specific area
	as far as something can go
	the greatest amount of something that is possible or allowed
	the mathematical value toward which a function goes as the independent variable approaches infinity
	final or latest limiting point

tighten up (verb)	limit (verb)
severely restrict in scope or extent	restrict or confine within limits
	place limits on (extent or amount or access)
	decide upon or fix definitely