This article talks about the differences between aviation fuel and fuel used in automobiles ("Avgas" vs." Mogas").
AvgasYou have to scroll down a ways because it also includes information about jet fuels as well.
According to this article, the basic chemistry of the two fuels (at least for 100LL) is nearly identical. The main difference is the addition of lead in avgas. While it is obviously helpful in controlling detonation (aka self ignition, in the article) it also lubricates valve seats and guides, increasing their longevity. I think its also important to remember that these engines are typically air cooled, so they are likely to run much high cylinder head temperatures than in an automobile. Apparently, avgas is also is required to meet specific quality standards unique to aviation fuel, and its vapor pressure is also lower than automotive fuels. This is to reduce the incidence of vapor lock at higher altitudes (as klhansen mentions).
Question: Do the any of the Chevy inline 6's like the 235 use hardened valve seats, or valve seat inserts? If not, that might be one reason the mechanic recommended avgas - in order to reduce valve seat wear. My GMC 270 has inserts from the factory, so I have never faced this problem. Do the Chevy guys who run unleaded fuel typically use additive(s) in order to the address this issue?
WRT to any catastrophic damage to the engine or fuel system (thrown rods, melted fuel lines, damaged pistons, etc.) I agree with EdPruss. These sound like urban legends to me - akin to "my neighbor/cousin/uncle/barber once ran premium fuel in a car designed for regular, and it ran so hot that it caught on fire and burned up". If these issues did occur, something other than 100LL avgas was the cause. Once again, the primary difference is how the formulation resists detonation. It does not burn hotter, nor does it contain significantly more BTUs per gallon than regular fuel. Its like insuring a $100K house for $200K. Just because you have the extra insurance, doesn't mean its any more likely to burn down.... (Well... unless my brother in law owns it, maybe...
One possible side effect, however, could be if you drive your truck in cold climates. The low vapor pressure may make it difficult to generate enough fuel vapor to start the engine when cold.
I would actually be much more concerned about the effects of using ethanol laced fuels than Avgas.
I know a guy who has been using Avgas in his '43 GMC for years, with no issues. He runs it because he lives near an airport, and there is a lack of stations with ethanol free fuel in his area.
Lastly, and I know this will be an important point to all of the responsible citizens on this site, there's no road tax on avgas, so (at least in my state) you are technically cheating the government out of revenue. Hope you will be able to sleep at night...
Best Regards.....
