Higher compression ratios, higher expansion factor on the downstroke, etc.
Ethanol raises effective octane levels. If you could depend on 10 to 20% ethanol in the fuel, you could require Detroit to up the compression ratio on gas engines to, oh, about 12:1 from the current 9.5:1.
How long on octane are current refineries that they would need to boost it with corn ? Get by the “octane increase” claims and you’ll find the real ethanol contribution is rather pointless.
With much higher compression you’d have to redesign the engine with more strength and weight, thus a negative impact on real mileage. This really becomes a factor on diesels, with far higher compression, but can’t be ignored even in the 12:1 range, IMO