> Propane doesn't have a huge calorific value, and I very much doubt that
> such a power increase will result from merely converting to the
> different fuel.

It's not a conversion to a different fuel, its burning both!  They spray
propane into the air intake.  When the diesel fuel is injected, it ignites
the propane as well.  Since diesels run with excess oxygen, there's still
oxygen for the propane to burn.  Sometimes, if the goal is cleaner exhaust
instead of more power, the amount of diesel injected is kept to a minimum
and the primary fuel is propane. (the diesel just acts like a super spark
plug to ignite the lean propane/air mixture.

Bill