A guy was telling me earlier that you can convert a diesel to full propane and it will almost double the HP. Can anyone verify this? What about the MPG's? Fuel costs/availability? I'd love to hear more about the subject if anyone knows...

 

No, you can't convert to straight propane. The engine would destroy itself from detonation. There are straight propane/CNG diesels made by Cummins/Westport, but they have special injectors and pumps. Propane really sucks as an on road fuel because it's taxed by the gallon, same as gasoline or diesel. It takes more propane to do the work (roughly half the BTU content of diesel) so the taxes eat you alive. That's why the company I work for doesn't buy propane powered trucks, we run diesels even though a big part of our business is bulk propane delivery.

 

I wondered why the delivery company's got away from using propane. Makes sense to me.

 

Also the propane powered trucks were gas conversions, and when you convert a gas motor to propane you lose about 15 - 20% of your HP and torque. I talked to a guy who was running one for another company, it was a mid '80's GMC with a converted 366 big block. This is in a truck around 30k GVW, so the 366 on gas would have been severely underpowered. Most of what he had to say about it cannot be posted on this web site. Basically the power was and even with a 40 gallon tank he HAD to fill it every day.

 

This is what Cummins/Westport have out right now. Apparently the true propane powered diesel is still experimental, this one has low compression and spark plugs. As you can see from the spec sheet, power falls far short of what a diesel 5.9 can make. Also note that it is properly valved.

 

I'm a mechanic certified in propane/natural gas installations and inspection.
Propane has a HIGHER BTU but has a slower burn rate. That's why conversions for duel fuel vehicles have a timing control box that automatically advances the timing approx. 10 degrees (depending on application) when switching to propane from gas and retards the timing when going back to gasoline. Propane burns hotter, cleaner 'cuz it can be leaned out more which is harder on the heads. (MAINLY VALVES and SEATS) Most conversions done properly have hardened valves and seats installed and the crossover port in the intake plugged with a propane designed gasket set.
If the conversion is done properly and the dual curve ignition box is designed for that application, there's generally very little power/torque loss.
The main disadvantages are that it more harmful to the engine, less availabilty to get fuel (fewer stations than gasoline) and the components are mostly made generic and not specific to each vehicle. You need the components and vehicle designed to accomodate each other for maximum performance. A propane system is also less forgiving in extreme weather (COLD) and has a higher engine maintenance requirement to be dependable enough for cold starts.
Propane is generally cheaper than gasoline to buy but the vehicle and component maintenance cost is higher.