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If you know your engine is stock, go by the octane rating listed in the owner's manual. Typically, in the '60s and early '70s, compressions were 10:1. In the '80s, it was in the 8s. Recent Corvettes have higher compressions that are happy on 92-octane, thanks to engine electronics. If you run 100-octane Sunoco GT Unleaded, your engine-management system can provide you with additional power.