Maximum Compression Ratio

With all of the above posts, You have been given good counsel, play it safe unless you have the time & $$$ to blow it up and learn the hard way. Even just saying iron vs
aluminum doesn't cut it because of different chambers/spark plug positions/depths etc. etc. etc.. You have to consider if it's not a racing for the money project, You want
it to live long enough to enjoy, even if it does eventually give up(which can happen even if You do everything right), and with longevity comes carbon buildup in the chambers
etc. the older an engine gets, the more octane needy/sensitive it gets....keep that in mind.