In message , Elder writes
There's something wrong with your ideas. LPG or Propane is a shorter chain molecule than petrol and therefore will have a lower flash point than petrol, requiring if anything lower compression and certainly not more advanced timing than petrol. Increasing boost pressure allows more fuel to be burnt giving more power, but at the expense of greater fuel usage. So you still end up with only 85% efficiency compared to petrol. Don't forget what happened when the EU tried to get diesel engines to run on any fuel from diesel to petrol, after getting over the injection pump wear issues, it was found that petrol was sticking to the cylinder walls instead of a clean burn and a 25mpg diesel engine would only give about 13mpg when used with petrol.