[oil pan 4]

Ok getting back on track.
If fuel vapor engines were so amazing why cant an engine be built that runs off say propane (which has a boiling point of around -40) a substance where you know there is no possibility that its anything other than a super heated gas at room temperature when under 1atm of pressure and get significantly better btu/mile economy than a gasoline engine?

The best propane engines I have found get maybe 5% better fuel economy than their gasoline counterpart. That difference could be due to the propane mod specific engines running higher compression and different ignition advance than their gasoline counter parts because propane burns in an engine almost like 105 octane gasoline.

I don't consider simple small engines that have been converted from gasoline to propane a fair comparison, where no other changes are made aside from fuel induction system on the converted engine. The converted engine should at the very least get an ignition timing advance too.