But seriously, in my experience, I'd say that most any naturally aspirated gasoline engine in good tune, with a strong battery and the right oil, will start down to -20F, and most will start when it?s colder, even without a block heater. ?30 F is pushing it, but my old Honda Civic wagon would start up. It?s been a long time since it?s been that cold here though.
I had a turbo Mazda that wouldn't start if it was less than -15 F. I suspect that the low compression ratio of the turbo engine had something to do with that. Diesels also have trouble in very cold weather because of the compression ignition. Sometimes the glow plugs aren?t enough.