I had asked the question earlier on a different forum and it spun out of control. I've copied and pasted a few of the replies from a guy that worked in a fuel refinery, and also races boats.
It also uses totally different parameters for determing the octane. If Avgas was tested under the same conditions as "mogas"or "autogas", it wouldn't have the octane rating it does. The test procedure is close to the MON test for autogas but with a much lower intake temperature. It DOES NOT have the same octane rating as 100 octane race gas. And it has a much lower REID VAPOR rating. Also, the whole "dry" thing is completely misunderstood. The "dry" in Avgas means it is certified to contain much less water, which will freeze. Any glycol in the fuel is to inhibit any water it does contain from freezing. And there isn't much glycol in it.
And Avgas has a Specific Gravity of about 6.05, compared to say VPs 110 which is 7.30. This means you should be jetting up with Avgas, and be using more. And the "octane rating" of avgas goes up as the mixture gets richer. All gas does, but Avgas even more so, because of the blend. I've got friends with 13:1 980hp big block ford running on 100LL, just by jetting a little richer than he does with Trick 104.
Some people mix avgas 4:1 with pump gas to "stabilize" for lower altitudes. The avgas is harder to vaporize than mogas. The vaporization thing is one of the issues with the supplemental type certificaion, the certification that allows mogas to be used in an aircraft. The fuel lines must be insulated and routed away from any heat source and cooled by outside air etc. or the mogas can flash off and cause a vapor lock. Not a good thing at 10,000 ft So by adding pump gas to the avgas, you are adding fuel with a high enough vapor pressure to flash easier and allows for an easier ignition. It isn't really neccessary, specially if you run mag, because Avgas is still much easier to light than methanol. And remember, airplanes still have to start and make good power at lower altitudes. But the addition of mogas will make the process easier. But will also lower the knock index down slightly. So go lightly.
There is nothing in the compounds that doesn't or didn't exist in mogas in the 60's. Just in different amounts. So maybe its the toluene. As far as comparing av with mogas in evaporation, there are more than a couple of race gas blends that will flat out run it in that department. And they aren't harder on fuels lines than any other fuel. Ask any fuel line supplier which they would rather deal with, Avgas or todays pump, and every last one will tell you, they would rather deal with avgas. There are in fact compounds in todays gas that is not in avgas, and was not in mogas 40 years ago. And these compounds are wrecking havoc with fuel lines in older cars and non-teflon hoses. Even the synthetic rubber stuff. So if its because it evaporates, I can see it. Any fuel will do that to rubber lines. Other wise, there is nothing mysterious in the stuff. Same stuff, just a different blend.
You may not be old enough, but gas in the 60's was cold as hell on your hands, not quite like alky, but cold just the same. Evaporation. Big time. And it would raise a welt on your skin is no time, and burn like hell. You could light it off in a 12 to 1 427 with nothing more than a points and condenser battery ignition.
There is nothing unique about the stuff that makes up Avgas
But while we are on the subject of evaporation, that is the major bitch with the tree huggers. And why they are pushing for an alternative fuel for 100LL. Less than 1% of gas is avgas, yet it is responible, so they say, for 25% of all unburned hydrocarbons released thru evaporation. Wait till they take another look at boats. Sealed fuel tanks and vapor recovery systems will right after the catalytic converters that are already here.
