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I tuned a few high compression K24's to run on 87, but it absolutely guts their low end torque, and they're very temperature sensitive. Best I can tell, OEMs get away with high compression on regular (without resorting to Atkinson cycle) by having very sophisticated knock detection, and very high resolution ignition tables. The ND Miata knock control is done via ionization detection through the spark plugs, and ignition control down to a tenth of a degree, just as an example.
There's very little downside if you're keeping revs up. Most of the ignition advance is lost below 2500rpm.
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