I've noticed from reading several of Clarence Lee's reviews, that the compression ratio is higher on air chambered engines (currently on the
1.15) compared to non-air chambered engines; comparing the YS 1.20AC vs the 1.20SF. I'm not sure if this has anything to do with it. But I think a non-air chambered engine would have a greater pressurized fuel charge on intake than an AC engine (because of the volume of the AC). A higher compression engine like the YS 1.15FZ would be prone to detonation if it did not have the AC.