On my 93 NX2K I mistakenly set the timing to 20 degrees. It was running great this way. Yesterday I checked it again and moved it back to 15-17. I say that because when the engine fan was running it was at 15 and when the fan wasn't running it was at 17. The car doesn't feel as peppy as it did at 20. Is there a way to safely run 20? Maybe by using only 93 octane gas?