There are severall aircraft which have STC's to run mogas, but the engine manufacturers do not recommend it and neither do the fuel suppliers.
You might just be cynical and think that this is just a way of making more money out of pilots, but the truth is that the fuels are fundamentally different. Mogas is designed to be used on the road and no consideration is given to the requirements of aircraft in its composition, manufacture or handling.
The most obvious difference is the Octane rating. This is actually around 107 MON lean mixture rating for Avgas 100LL, compared to around 85 for the normal unleaded forecourt gasoline. The 95 octane that is often quoted for Mogas is a RON figure and not comparable to the Avgas octane ratings.
Alright, you may have an engine that does not need all of that octane, so why not use Mogas?
Well there are many differences with the fuels, including:
· Increased Ried Vapour Pressure causing greater potential for engine failure through vapour lock.
· Reduced net specific energy of the fuel (less power).
· Increased risk of elastomer swell and softening in fuel system components.
· Some automotive gasoline can contain ether and / or alcohols, which are not permitted for use in Avgas as they can potentially separate out from the fuel.
· Increased incidence of carburettor icing.
· Poor fuel distribution giving the potential of detonation.
· Fuel washing the lubricating oil off the cylinder wall at low temperatures.
· No consideration for aviation in fuel specifications.
· No consistency in the fuel in different seasons and between countries.
· Ability to change the fuel specification without considering the implication for aviation, or informing aviation customers / authorities.
· Poor storage stability when compared to Avgas.
· No control on the storage and handling of the fuel.
· No control on the water content of the fuel.
· Poor filtration requirements when compared to handling Avgas.
The most incidious of this list is vapour pressure. A high vapour pressure fuel will form a stable vapour at lower altitude and / or lower temperature than a low vapour pressure fuel. Vapours forming in the fuel supply pipes can mean vapour lock and fuel starvation.
Add to this that the road fuels change composition between winter and summer, then you really don't know what you are using. The vapour pressure of winter Mogas is high enough to form a stable vapour at around 30 deg C at ground level (less as you climb).
This temperature sounds high for our climate, but tank temperatures of this level are easily achieved by sitting an aircraft in the sun all day - added to this you have the heat from the engine.....
You really do take a risk when using Mogas, you could be lucky, but the truth is that you have to be careful with this - make no mistake, it does kill people.