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If we're talking stock tunes, new cars typically run lean or ultra-lean at low rpm... at those rpm, the engines are more prone to "knock" on poor gas, which leads to the ECU pulling timing and perhaps running richer. The octane boosting from ethanol allows the engine to run leaner and stay lean.
At high rpm, the engine typically enters power enrichment, most of them sticking between 12:1 and 13:1, so you don't have that problem.
At least, this is why I think it affects low rpm more... but that would require validation.
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