http://www.afdc.energy.gov/fuels/fue...ison_chart.pdf
Energy content of gasoline: 112,114 - 116,090 BTU/gal
Energy content of propane: 84,250 BTU/gal
A gallon of propane has 72.5730% to 75.1467% the energy of a gallon of gasoline.
I don't know what comparison you're trying to make as regards your "BTU comparison of the 2 fuels". You'll note it's energy per volume (BTU/gallon)... removing volume gives you nothing to compare.
The propane engines run higher compression because they can... propane has a higher effective octane than gasoline.
Likewise, a relatively slower flame speed (gasoline - 16.5 m/sec, propane - 0.43 m/sec), means the spark timing in a propane engine is advanced as compared to a gasoline engine.
Propane's flammability limits are 2.5% to 9.5% in air, with 4.2% by volume for maximum heat production. Gasoline's stoichiometric ratio is 1.4285% by volume.
So despite having to use more propane per volume air (and remember, your engine is an air pump... with a pretty much fixed amount of air it'll pump for any given throttle position and corresponding engine RPM), and despite the fact that propane has lower energy content (on a volume basis, the only way to compare apples to apples), the fact that an engine running propane (notwithstanding it being reconfigured to have higher compression and more spark advance to be compatible with the fuel used) gets better fuel efficiency is a testament to the fact that gaseous fuels burn more efficiently than fuels that are in liquid form and must then be evaporated (and remember, in a gasoline engine, due to the fact that the fuel has a relatively higher condensation temperature as compared to propane, when cylinder pressure rises, some of it
will condense on the relatively cooler head and cylinder wall surfaces, it's a simple matter of physics that Peter Rotgans (Peterrr) demonstrated mathematically right here on ecomodder.com).