ORIGINAL: Sport_Pilot
High octane gas burns slower
Actually it doesn't burn slower, just takes a higher temp to ignite. Well maybe the flame front accelerates a bit slower at first as it is harder to ignite, but ultimately it catches up. The flame front has about the same speed. Diesel fuel really does burn slower but it wound detonate like crazy. But it takes a much lower temperature to ignite. This makes it great for a Diesel engine as it doesn't need as much compression to ignite. But becaue it burns slower the engine will not rev as high. Diesel also has a higher BTU content, that along with high compression ratios contribute to a Diesel engines high torque.
This subject has been beat up almost as much as "2-stroke oils". The discussion has talked about fuel odor, perfomance, availability, etc. Anyway, good points Sport_Pilot! I'd like to add a bit to your discussion. Regular auto unleaded fuels and 100LL really don't ignite at much different temperatures. Auto gas has a flashpoint below -49degrees F and 100LL flashes at about -35 degrees F. Yes, not the (-) before that temperature! Likewise auto gas auto ignites at about 536 deg. F and I don't have a reliable source for the 100LL auto ign. point. At any rate, gasoline is gasoline and both combust very easily! Sport_Pilot, you described the burn characteristics very well. Its right at the point of ignition where octane has the most effect. From there on, the effect deminishes somewhat. You mentioned diesel fuel .... a way different animal than any gasoline with flash points over 125 deg. F and an auto ignite point over 489 deg. MSDS and other performance info is readly available on the Internet for anybody that is interested. And for those really interested in history, read about the "Ethyl Corporation".
I'm no petro engineer and I'm sure one would set me straight but I did drive a fuel tanker truck many years ago in a prev. life and had to get very familiar with what I hauled. Even a tiny bit of gasoline mixed with diesel can ruin the whole batch .... makes it not pass a flash test. And if you ever contaminate gasoline with diesel fuel then try to run that gas in any automotive application, you will be amazed how it will make the engine knock and make it sound like it is about to self destruct. As a pilot, we are often warned of the dangers of mixing just a little Jet A with 100LL in a recip. engine. Can be fatal!
Anyway, regarding using 100LL in model engines, other than a sweeter smell, there really isn't any good reason to use the stuff. It costs more with few rewards. If a guy has a tank of the stuff nearby, I guess it would be OK. I have 110 racing gas (leaded) in my garage all the time, and it smells and behaves much the same as avgas but I choose to get my model gas at the nearby gas pump ... at half the price. I probably would use 100 unleaded racing gas if I had that on hand.... just for the better smell.