Editor's Note: This article originally appeared in Cessna Owner and Pipers Magazines.
Of all the hangar talk one encounters at airports, the subject of using automotive gasoline in airplane engines is among the most contentious. Some -- particularly owners who have an autogas supplemental type certificate (STC) -- will tell you that it's perfectly safe, and even better for an aircraft engine than running 100LL. Others -- usually pilots who haven't run autogas -- say, "I'll never run that crap in my engine," listing a range of potential problems from vapor lock to deteriorating gaskets.Look to the experts, and you'll find a similar range of opinions. The engine manufacturers are unanimous that auto fuel should not be used in engines originally certificated for avgas. Indeed, Teledyne Continental Motors (TCM) explicitly states that using autogas voids their warranty on new engines and parts. On the other hand, the Experimental Aircraft Association (EAA) and Petersen Aviation -- who issue the necessary STCs -- say that it's perfectly safe. The FAA -- which approves the STCs -- mostly seems to agree, though it continues to prohibit the use of autogas for Part 135 operators while carrying passengers for hire. (Read the rules about using autogas in an Adobe PDF document on the FAA's Web site.) The FAA also wrote a letter a few years ago contradicting some of the claims of the engine manufacturers.What's the real story? In short, for normally-aspirated (non-turbocharged), low-compression engines originally designed for 80 octane aviation gas, autogas is a perfectly viable option that can save considerable money and -- despite what the engine manufacturers say -- won't hurt your engine. That said, there are down sides. To fully understand the issues involved, we'll need to discuss a bit of petrochemistry.
This is a companion discussion topic for the original entry at https://www.avweb.com/ownership/avgas-vs-autogas