I have a stock B20E converted to carbs and have no trouble running it on 92 R+M/2 octane here at sea level. That's nominally a 10.5:1 CR engine, although as actually built by Volvo it's a bit lower.
The Apple Farmer Panamericana race car I built some years back had close to 11:1 CR and it was fine with 92 octane Pemco gas in Mexico, but that had a lot more valve overlap than a stock E engine.
Note that there are rapidly decreasing returns in power from raising compression. There's a lot more gain in going from 9:1 to 10:1 than there is in going from 10:1 to 11:1, and not just because it's a bit less change in percentage in the latter case. It's not a linear function.
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