[Big Dave]

Diesel performance guys do this for quick, cheap HP. What you are doing is making the engine a dual-fuel engine. Propane is nothing more than an auxiliary fuel. Propane won't compression-ignite. The injectors squirt some diesel into the chamber and it acts as a spark plug, blowtorching the propane to ignition. The propane, once ignited combusts very quickly, so if you use propane you can't use a lot of injection advance. Too much injection advance and you bend rods.