Im contemplating taking a 572 that made big power and dropping compression just enough to run pump fuel on the street and maybe using a timing retarder and separate tank with good fuel.
But my question is is I drop the compression just for pump gas only how much power loss would you think I would see?

Edit to add info
572 with small dish piston
Victor heads ported and the combustion chamber has been opened up to the big bore size unshrouding the intake valve a lot!
275/283
715/675
Indy intake dominator
Made 930/840 at 6400rpm

Looking at the pistons and the chamber it doesn't look like a lot of compression because a lot of material was removed in the chamber.
Haven't CCed the heads yet but I'm thinking it can't be much over pump gas compression now.

What is the ratio now?

I don't know but looking at that big combustion chamber and a dish in the piston i can't imagine more than 12:1. Like I said haven't checked the chamber yet so that's just a guess comparing stock chamber to this one.

And what fuel made 930Hp?

C12

I'll bet it doesn't need C12.

I dont think you would lose much dropping to 11:1.

From David Vizard book on how to make HP, "chart on page 36 of his "How to Build Horsepower, Volume 1" publication, going from 11 to 13:1 would increase torque/power by 2.4%. With David's qualifier......"this chart is based on a street engine with a relatively short cam duration; the gain will be slightly more if the cam duration is longer." -Dave-
I think you could work it backward as well.
Now one thing is if you are worried about detonation, an engine is much more prone to detonate at lower RPM under a load, than at WOT. Simply delaying spark advance goes a long way in running an engine on pump gas. I use performance trends engine software, it will calculate an advace curve for most fuels for your engine combination, simply do the calculation and set up your distributor to that advance curve, has worked for me many years now.