I have an 85 z28 with carb. Should I be running 90 octane non-ethanol in it?

 

Is your motor stock, is it modified (how so), are you getting knocking?
Most healthy stock motors are fine with 87 octane... 93 octane will give you nothing unless you need a more detonation resistant fuel.
Now running non-ethanol fuel may have some benefits but it has more to do with the lack of ethanol than the octane rating.

 

93 with ethanol will be more resistant to detonation. 90 without will burn more efficiently. On a factory 85 z28 probably wont make any difference

 

Completely stock. I am thinking that 90 octane ethanol free is best for it?

 

Completely stock 87 would probably be best.

 

If it's the LG4 (VIN code H) then 87 would be best.

 

To clear up my title. The 93 octane has ethanol and the 90 octane does not

 

still 87

 

Doesn't matter. You don't need the octane rating if it's the LG4. It'll run the same and use less if you put in 87, and it'll cost less per unit, besides.

Now, if it's NOT the LG4, then the discussion might need to be revisited.