Propane has higher octane but less btu of energy per gallon. That means, in theory, to get the same efficiency for propane vs gasoline your propane engine needs to have a higher static compression ratio. This is why the dual-fuel type propane mixers (like the Impco 300A mixer that I have on my Jeep) are said to be able to do both, but not ideally. The old 70's and 80's 8:1 compression engines are probably some of your worst candidates for propane. I haven't tried it yet, however my propane conversion came with a timing processor that is supposed to remove around 5 degrees of total timing but add roughly 5 degrees of timing around the 2000-2500 rpm mark. They say propane needs a little less total but with it all in sooner.