There are two basic ways doing a propane conversion: straight (dedicated) and bi-fuel (commonly known as dual fuel).
Straight propane systems are best suited to older non-computer controlled engines with a good supply of fuel or to off-road applications. A good example of an older straight propane conversion is the 1977 Pontiac conversion I did several years ago.
A dual fuel propane injection conversion (like my 2003 SVIS Impala conversion) is the only type that can meet North American emission regulations. With the computerized integration of fuel injection and ignition systems of modern engines, dual fuel injection conversions also perform seamlessly and completely eliminate any possibility of intake manifold-destroying backfires.