is propane cheaper then nitrous? maybe just to get a lil umph but not to much.

Its been done. I don't know how well it works or if it is consistant.

Propane does not give any oomp. It is primarily used to "cool" the engine down and control hot spots that occur in the cyclinder. Im sure there are plenty of FAQ's out there on it. I dont think anyone on this site has ever dealt with it in their car although some might know something more. Correct me if Im wrong...

why dont you just use water injection

There's a much better way to get the same effect as propane without all the fancy propane shit....bigger fuel injectors. Bam, you're done.

It's been tried. It works. It's just more fuel in a different form, big deal.

I thought they only do propane on diesel engines?

Nope, but they only SHOULD do propane on diesel engines. It doesnt help nearly as much on normal gasoline engines.