Learn me: Propane Injection. Is it good?

I'm going through another "crazy idea" phase. I need to know the pros and cons of fueling a gas engine with propane. 

I will jump ahead in line and ask, on what engine? Seems it is pretty proven on TDs. No idea on gas. 

I'm sorry. I wasn't clear enough. I'm talking about the system that lets a Gasoline powered engine run completely on Propane. I will edit the original post. 

If it's a carburetor  engine, kits are available and relatively easy to install.

If it's an EFI engine, that is a whole can of worms.

I used to see this some back in the 1970's and early 1980's.  But it was always on trucks so you could put the propane tank in the truck bed.  I'm not sure what conversions in other vehicles do for a place to store the propane.  Since it is stored at pressures high enough that (I assume) you can't use the stock gasoline tank it seems like you would need a higher pressure tank installed somewhere.  Propane is cheaper than gasoline (per gallon) but you don't get as many miles per gallon of propane.  I'm not sure how that works out in terms of $/mile now.  I'm pretty sure you end up with less power from a given engine also.  Doesn't sound like a lot of up-side to this.

Go to gotpropane.com. I just went down this rabbit hole. They have all the answers on that website. Basically, propane is only good if running boost to make up for its gas properties. 

Its stored at around 200psi.  The octane is like 110 equiv so you can run high compression or high boost.  It displaces more air in the fuel mixure than gasoline.  IIRC all the taxis in Japan are required to use propane or something similar.  It does eat up a lot of trunk space in those taxis...  The other big con is, its not easy finding a place to fill up.

Thanks.  I guess that's a non-starter. I wonder if a fella could modify a propane kit to introduce vaporized gasoline to an engine, resulting in a ton of fuel mileage. 

That idea, like Ace Ventura says, is like your large intestine. Smelly, and fraught with peril. 

Propane vaporizes when you let it out of the 200psi bottle.  The "vaporizer" gizmos are heated with engine coolant to keep them from freezing over.  You cannot modify them to vaporize gasoline. 

I think even if you don't do a high compression engine or a turbo engine, you can still get excellent fuel economy with propane.  It will make less power vs gas because there is less oxygen inside the combustion chamber, but the miles per dollar should be good.

I tipped my toes into the rabbit hole long enough to know I didn't want to do it on an old Windsor V8.  The propane vapor displaces some of the space in the intake for the air, so a well designed propane engine has a bigger intake tract and intake valves than a gasoline engine.  It also does best when taking advantage of the high octane with high compression (up to 15:1).  The propane burns hotter than gasoline so you need stuff like sodium filled exhaust valves.  It's best left to the little forklifts that use it so they can be used indoors.