LP and natural gas conversions generally have a net loss of horse power, 1 main reason. This is the case because both systems use a regulator to lower the pressure from that of the tank and then "inject" the fuel into the intake prior to the throttle body. The power loss occurs because now your moving your fuel and air through the same space and not injecting it just before the valve in the head, this displaces some of the air that normally would be ingested by the motor.
Propane and natural gas both have much higher octane numbers compared to gasoline, so if you could get enough fuel and air into the motor you could make more power. To my knowledge no one has created an injector similar to the ones used to inject gasoline that could inject propane or natural gas directly, or if they do they arn't cheap and arn't used in the industry as far as I've heard. I know it's been tried in some types of race applications but I doubt it would fly in daily driven applications.
My truck is actually converted to natural gas but was disconnected by the dealer when some warranty work was done with the previous owner. I will make it operable again this summer. I also acquired a home filling station wich is where the real savings are b/c the price of natural gas from your house is dirt cheap about 2/3 less then gasoline.
In case someone asks, my father worked for the local natural gas supplier and worked on a special project with natural gas for vehicles (team NGV). We had many vehicles converted during my childhood. The list includes 2 fullsize 12 passenger 360 powered ram vans, both driven over 350,000 km and no major engine repairs of any kind, 80's chrysler new yorker over 300,000 km, 2000 dodge caravan over 250,000 km, 1995 chevy 3500 4 door dually pickup 454 powered and a Farmall International 706 tractor!!
The benefits are many to this, cost of fuel is a big one, also engine maintenance times can be extended like less frequent oil changes and spark plug changes and wear and tear is almost non-existent. My dad had a contract with a local courier that ran 4 hour round trip runs all day and night long, they had a 360 powered full size van and hit 1 million KM with about 98% natural gas usage, Chrysler Canada purchased the van back with over 1 million Km's and tore the engine down in the R and D facility here in Windsor just to see what it was like inside. It was basically carbonless inside and showed the wear of a motor with 250,000 Km's according to the factory techs. Wich is pretty impressive I think.
Anyway i obviously know more about the compressed natural gas side of things, but I have good contacts for propane info too. Glad to help if I can.
