Wanna start an argument among other aircraft owners where passion will ring louder than logic? Either bring up Tail wheel vs Nose wheel or ask whether you should use mogas in your engine.
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 mogas 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 mogas, say, "I'll never run that crap in my engine," listing a range of potential problems from vapor lock to deteriorating gaskets and elevated EGT’s.
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 mogas voids their warranty on new engines and parts. On the other hand, the Experimental Aircraft Association (EAA) says that it's perfectly safe.
What's the real story? In short, for normally-aspirated (non-turbocharged), low-compression engines originally designed for 80 octane aviation gas, mogas is a perfectly viable option that can save considerable money and 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.
Gasoline is a mix of liquid hydrocarbons -- that is, chemical molecules that contain hydrogen and carbon atoms. The simplest such molecules, methane and ethane, have just one or two carbon atoms respectively, and are gases. The two hydrocarbons of most significance for gasoline are heptane, which has seven carbon atoms, and octane, which has eight. Both are liquids at room temperature. "Straight-run" gasoline -- directly as it comes out of a petroleum distillation plant -- consists of 62-64% octane, and the rest heptane. It's said to have an Octane Rating of 62-64.
The octane rating is significant because octane can withstand much higher compression than heptane, and high compression increases power. So to get reasonable power from a lightweight engine for aircraft use, aviation gasolines have an octane rating of 80 or higher.
The octane rating can be increased beyond the simple proportion of octane to heptane by adding anti-knock agents, which delay the onset of detonation. Until recently, the most important such additive, for both automotive and aviation use, was tetra-ethyl lead (TEL). It's found in aviation fuels in the following proportions:
Fuel Grade (Octane Rating) Color TEL per Gallon
80/87 Red 0.5 mL
100LL Blue 1.2 - 2.0 mL
100/130 Green 3.0 - 4.0 mL
115/145 Purple 4.6 mL
A large proportion of low-compression aircraft engines from both Lycoming and Continental were originally certificated for operation on 80/87 octane avgas. Most Lycoming O-235, O-290 and O-320 engines fall in this category, and so do some of the larger O-360 and O-540 engines. Most Continental O-200, O-300 and O-470 engines, and some of the fuel-injected IO-470 and IO-520 engines can run it as well.
So, if you have a low-compression engine, can you just fill it up with mogas and take off? Nope, you've got to get an appropriate STC and despite what you may have heard elsewhere, it is very important to get that STC, even though it usually will consist of one or two pieces of paper, plus new decals for your fuel ports.
Why is the STC important? While unleaded mogas provides sufficient octane to substitute for 80/87 avgas in low-compression engines, there are other differences that can cause problems when using mogas in some engine installations. The two most significant are lower vapour pressure which can lead to vapour lock and incompatibility between some of the additives in mogas and some components (particularly seals) in some aircraft fuel systems.
In order to qualify for an STC, a particular airframe/engine combination has to be rigorously tested, to include either a 150 hour engine endurance test or 500 hour flight test, under controlled conditions. The tests also include checking operation at high ambient temperatures, which can create vapour lock. Some aircraft don't pass -- the Piper Apache and Comanche-250, and Cessna Skyhawk with Avcon's 180HP conversion all failed testing, and cannot legally run mogas.
In a nutshell, by buying the STC you are paying for a bunch of research and testing to verify that it really is safe to use mogas in the airframe/engine combination you have. In a few cases, you may be required to have modifications made or the STC may authorize only premium (91 octane or higher) mogas. For example, Petersen Aviation's STC for Piper PA-28-160, -161, -180, and -181 models requires replacing the electric boost pump and running premium gas.
The testing cannot, of course, guarantee that you won't have problems. Vapour lock is a real issue when using mogas in aircraft engines, particularly at high altitudes on hot days after all, it's made for use at sea level, or at most lower altitudes. Cars don't get up into the flight levels! One way to combat this is to make sure that the mogas you use is fresh (don't fill the airplane up and then leave it in the hangar for a month). Another is to use a Hodges Volatility Tester to assure that the fuel in question won't cause vapour lock at the current ambient temperature.
The engine manufacturers, in their comments on also raise a potentially legitimate concern about variable quality of automotive fuels. Avgas, after all, is far more regulated and subject to a worldwide standard; provided it isn't contaminated, you can be reasonably sure that you're getting pretty much the same thing out of any 100LL pump in the world. Autogas is a completely different matter -- it's regulated differently from country to country and in some cases may have radically different formulations.