**Converting** a gasoline *car* [not just an engine; a naked engine isn’t such the big deal] to NG is not especially trivial when you consider the req’ments for a tank, proper mounting of same, making sure the valve ass’y is guarded from trunk-junk smashing into it, a line to the engine, proper dressing & temp insulation of same, and the implications of carbueration vs fuel injection, all under the mgmt of a computer. There’s not much question in my mind that this isn’t something to be done by a shade tree mechanic. No doubt there are taxation issues and all that type of darkish stuff, but clearly, some goodly number of car-bombs would result from such conversions carried out haphazardly.
OTOH, *building a car from the beginning* for NG *IS* relatively trivial. *Building a car from the beginning* that can be freely switched back and forth betw gasoline and NG is less trivial. I could be wrong and I’d welcome the correction if I am, but such an animal would almost have to be carbuerated, and I’m not sure if there are a lot of carb’ed cars being produced these days.
Don’t know how much trouble it is to get existing injectors to work OK with the natural gas. A gas will pass more easily through them than a liquid. They are also designed to contain higher pressures than carburetor fuel feed systems are. But the central computer would need to be changed to one that recognizes the new fuel and its characteristics, and that’s well over $200 right there.