ever heard of petrol and gas mixing in the engine and the trobule that puts you in?? -,- .. oil in cylinder kinda the same scene... cng will have triuble reaching the engine and there'll be issues with combustion and therefore reduced performance.. you'll experience reduced acceleration, missing(jerky reactions), weird idles, sometimes even the car turning off... injector rail( the thing which supplys cng to the engine) etc heats up which causes it damage and can affect its life.... the capacity issue if not even in the slightest, an issue!!! lol!