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The new Audi A8 is supposed to have a 12.5:1 direct injection gasoline engine. It cranks 256 HP from 3.1 liters. The V10 in BMW's new M5 runs a 12:1 compression ratio. (I think it's uses BMW Valvetronic technology.) Those both approach the lower limits of diesel engines. What's astonishing is that Porsche's new red rocketships have supposedly set a record for the highest compression ratio on a mass production engine. I can't remember what it was, but I think it was in the 13's. Yikes!
Of course, you still have to deal with octane. I'm pretty sure all those engines have some pretty sophisticated engine management systems to keep the monster alive. I would think you might be able to get 11:1 out of the EG33, but I'm not sure how well the ECU would adapt.
I wouldn't be surprised if some of the experimental aircraft guys have tweaked the compression. Most aircraft engines are large displacement, slow turning, air-cooled motors. Since car motors are a rather different design, many experimental aircraft use a reduction drive system to enable the use of the upper RPM range. If they're doing that, then it makes sense to toy around with compression - especially at altitude.
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