Marzi. You are allowing a little ignorance to show.
You have your Boings mixed up. 707's haven't flown commercially for decades.
"Flammability" is merely a measure of flash point. Diesel has a much higher flash point than petrol and is therefore less "flammable" but it is absurd to suggest that it doesn't burn as hot. In fact I suspect that the heavier liquids burn hotter.
If avgas burned hotter than avtur, they would use it as a fuel. Trust me, you can burn petrol in a jet engine.
