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I don't think 30% is a typical figure, at least for peak efficiency. Even for off peak efficiency I don't think 30% is accurate. Every naturally aspirated engine these days is at least 10.0:1 compression. If you consider the simplified thermodynamics of the gasoline engine, there's not really a whole lot of room for improvement there. Optimized spark timing sure but I think at lower speeds this is not really an issue.
If it's direct injected, the latent heat of vaporization will slightly increase efficiency by reducing the temperature and compression stroke energy requirement. If it's turbocharged there's probably a bit more help because you can really bump up the compression there. I just don't see 30% really in almost any circumstance, maybe except very low throttle conditions where the effective expansion ratio is very low.
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