Oh come on...
The issue is 9.1 CR and 91 octane. No one seems to be proposing unlikely stuff like 10:1 and 87 octane. Lower compression and/or more octane is less critical. So, only one combination to check.
Dan, I was answering the question posed by David Z: "How much detonation margin is left with 91, 93 or 94 octane fuels on 7.5 pistons? 8.5 pistons? 9.0 pistons? There is a standard detonation test and experience shows it offers a reasonable level of safety. From what I've read and understood, nobody has conducted this detonation test with 91, 93 or 94 octane and 9.0 CR. Maybe it will pass the test, and I'd like to know too."
My background is engine development, engine building and dyno testing, even built my own engine dyno from scratch. If you want to have a COMPLETE picture of what David asked above, you need to to do the same sort of testing that Lycoming has done and which you were previously advocating for.
If you want to know the effect of CR and detonation margin with all variables including octane, you'd be doing DAYS of testing with 3 different engines.
If you just want to know at 9 to 1 on 91 octane, the task is much simpler. We already have a pretty good idea of what that involves from folks already doing something similar. Michael gave some idea on what works for him.
For only a slight power loss at high MAP, you can run mogas by retarding spark under those conditions. This has the added advantage of reducing CHTs in the climb on a hot day. Some even run LOP in the climb above 5000 feet by pulling the prop back and LOP while flying formation.
Once MAP is low enough, timing can be re-advanced for best efficiency.
I've always advocated to be conservative. Mogas quality is more variable than 100LL. Better to chop a couple more degrees of timing off and suffer a few hp loss than get close to the edge. I think Michael is doing just that and doesn't worry. Mojave isn't a cool place in summer so that's a worst case scenario as far as IATs and CHTs go.
For folks using mags or EIs with non-programmable timing curves, I'd think twice about 9 to 1 pistons, in fact, I'd come out and say don't do it if you plan to run mogas. The cruise compromises resulting from the required ignition retard at high MAP would be noticeable.
Higher CR is useful if you fly high a lot where ambient pressure is low. You want to squeeze that thin air as much as you can up there. Dave Anders was running 12 to 1 at one point and high altitude performance was awesome (100LL). You can limit MAP and timing down low to take advantage up high. Hopefully with the next gen EFI, we'll be able to raise CRs even more and retain detonation margins as they do in the automotive world where we see turbo engines running over 15 psi boost with 10.5 CRs on 87 octane fuel. We won't see those kinds of numbers on a standard Lycoming combustion chamber of course but I believe we'll be able to push CRs a bit higher than what we generally see now.
Ron, Greg and Michael have shared some of their mogas experiences here. It can and is being done already and actually for a very long time (39 years) on many hundreds of aircraft using the Petersen STCs with fixed timing. Nothing new at all like you make it out to be.
https://www.autofuelstc.com/approved_engines_airfames.phtml
A point more compression isn't a big deal with variable timing EIs.