Well, unburnt fuel will stay in the cyl, and when it gets hot enough, it will cause a uncontrolled explosion and more then likely will bend the valves and crack the cyl wall. This explosion is often referred as 'detonation'.if your car says to use premium, you must use premium. People will tell you that the only reason youre supposed to use premium, is because it is a cleaner burning fuel, and this is not true.Premium fuel burns faster then regular, high compression vehicles REQUIRE a fast burning fuel.