Ive recently moved from the houston area where they use the 87 "methonal" gas which has a lower octane than real 87, I now live where there is just the old 87 will this throw off my tune. Somebody told me it would cause detination, but wouldnt that throw my check engine light on?
if its true that you used to run lower than 87 (say 85 which ive never heard of this) now your using 87 you will have no problems if you go the other way (from 93 to 87) then yes you could have problems.
In the parts of the houston area ethenol has replaced the normal octane booster, consiquently it can drop the octane ( it also makes the gas more soluable which has nothing ot do with whats being talked about)
Why take the chance? Just fill it up with 89 or 91 and forget about it. You'd have to drive a half million miles before the difference in price would add up to the cost of a new Mod motor!