You could also try converting the existing engine to an atkinson cycle. It would be tricky depending on the particular application, but a DOHC motor could probably be converted with not much more than a custom or re-ground intake cam. Something tells me it would get more interesting with a SOHC engine since you wouldn't really be able to touch the cam without screwing up the exhaust timing.
The Atkinson cycle uses late intake valve closure (well after the piston has reached bottom dead center) to reduce the amount of air trapped in the cylinder.
This has two effects on efficiency:
1) At low loads where engine efficiency is poor, throttling/pumping losses are reduced.
2) It reduces the compression ratio to prevent knock, while keeping the expansion ratio high. Expansion is where the engine produces power, and anything that increases the expansion ratio improves efficiency. In a dedicated Atkinson engine, the expansion ratio can be increased while keeping the compression ratio low.
Not sure about injection and ignition timing details of Atkinson engines. I suspect that nothing has to change when in closed loop operation, since the A/F ratio is constant at 14.5 to one. With low compression, I suspect ignition timing could be advanced more than a regular engine.
But only Toyota and Honda and Ford know for sure about each individual engine. I only know enough general info about engines to be dangerous.
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