Tweet
I have an LSA with an o200A. This engine had 1100 hours when I bought it and topped it with millenium cylinders. With these cylinders I can run the timing at 28 degrees. For the first 10 hours I ran 100LL. After that, I started running 90 octane non ethanol mogas from a station with very high mogas turnover. I lean the mixture in flight and on the ground. I do go on trips and buy 100LL enroute but most of the fuel in the 150 hours I have flown it has been mogas. Usually I will mix 25 percent 100LL to give it some lead but not all the time. I like MMO but do not use it- yet anyway. I cannot tell any difference in the fuels except that it seems to idle better on mogas. The LSA is based at a grass strip in SC with no fuel available so I use the mogas as a convenience. At condition inspections, the spark plugs look great. The only downside I see so far is the staining of mogas but I keep it cleaned off. My prior experience with a stock o200 is that it does not like a constant diet of 100LL. I understand that 100LL has 4 times the lead of 80 octane thus the 25 percent 100LL usage. I have also equipped my LSA for IFR but that is another discussion.
-
-
I guess this relies on having a very high compression engine which is sensitive to relative octane number, which you mentioned you have?Originally posted by Jim Herd View Post
My buddy has 5 aircraft with Rotax powerplants (100hp or 130hp), all use mogas, all have regularly sat for weeks or many months between refilling the tanks. Fuel degradation has never been an issue. I assume mogas in NZ is equally as variable as in the USA.
For what it's worth, I am very interested in the idea of running mogas - but the risks are just too high for me to actually go down that road, when I am only saving 35% of the fuel bill. While 35% sounds like a large number, it is insignificant compared to the benefits of a reliable and long-lasting engine, especially in the hostile terrain where we usually operate our machine.
Comment
-
Short version: I ran my c85 Luscombe on mogas for hundreds of hours with zero issues besides missing the sweet smell of avgas. Now I run my Continental IO360 powered BH on mogas. Unfortunately my plane will sit for months between flights. One time, when the plane had sat for nearly a year, the injectors were fouled with varnish. Cleaned the injectors and went flying. Where I live mogas is just under $4 per gallon and avgas is about $6.50 per gallon. My plane hasn’t tasted 100LL since I was doing engine break-in.Scratch Built 4-place Bearhawk. Continental IO-360, 88" C203 McCauley prop.Comment
-
The winter vapor pressure allowed in gasoline in the US in some cold climates can be really high. The risk is vapor lock. I'm not sure it needs to be that high in places where -20 to -40 is common anymore with modern fuel injection in cars and sleds.Originally posted by Battson View Post
I run a 75/25 mix 90 & 100 in the summer when I'm burning a lot of fuel. Running an O-540 with electronic ignition. Timing is at 23 degrees at WOT SL, 2700rpm, which goes up as MAP and cylinder pressures come back and detonation becomes impossible. It's the 90 octane map recommended by the manufacturer. I switch to all 100 about October because I fly very little in the winter.Last edited by zkelley2; 12-17-2025, 01:30 PM.Comment
-
This video is an interesting story about vapor lock with mogas in an RV-12 with a 912iS.
At about the 1 hour mark, Rian Johnson from Vans talks about their testing with mogas that is more in depth actual testing than I've ever seen done anywhere else.Comment
-
I would guess that your o540 is fuel injected or at least run a fuel pump. In my case, it is all gravity feed with no fuel pump. I also ran 3/8 fuel lines instead of 5/16 on my LSA. I did this for less chance of vapor lock and in case I used a larger engine in the future. With my setup I have flown in cold temperatures at 10,000 ft with no problems. I do try to keep more 100LL in the tanks in the winter just because it stores better.Comment
-
No. You cannot run any mogas with mechanical injection because of the vapor lock problem at the inlet of the very hot mechanical fuel pump.Originally posted by BarrysLSA View Post
I'm carb'd.Comment
-
What do you mean by mechanical injection?
I am aware of lots of aircraft running IO-XXX engines from Lycoming with RSA-style fuel injection system - this is the mechanical, continuous-flow injection system originally developed by Bendix, like I am using.
These guys have run mogas for years without vaporlock issues preventing reliable operation, sure there can be issues in hot starts etc. There's information about it online, guys with RVs are doing it, also with TCM engines and mechanical injection on Cessnas. Although I understand the Lycoming engine with no return line is more prone to challenges with hot starts. One source of such reference information is the backcountry pilot forum.Comment
-
Mechanical, as in not electronic fuel injection. The typical Benidx fuel injection on lycomings. The fuel pump on the back of the motor gets way to hot for safe operation with the reid vapor pressures found in mogas for safe use at all altitudes. When they tested mogas, they found you can get vapor lock at the pump inlet. Turning on the electric solves this, but the electric is supposed to be a backup, not a requirement for continued safe flight.Originally posted by Battson View Post
Between the suction, the heat and the lower pressure at altitude it can cause an issue. You'll notice that you cannot get a mogas STC for any injected lycoming(checking out peterson, it looks to still be the case). It's not about detonation, as you can get a mogas STC for the exact same compression ratio and max rpm in a carb motor.
The original lycoming SI 1070 excluded any injected motors from the approved list for the ASTM standard of unleaded car fuel they approved, but updated it to include them if the vapor pressure was within spec. Which you cannot possibly know at the pump.
Look at table 2 and read below it.
The class A-4 vapor pressure only exists in summer blends in the states, and I think California all the time. There's no way to know what the vapor pressure of the fuel at a gas station pump is. Depends on when that fuel was ordered and blended.
Just because someone is using it doesn't mean it's safe for everyone. As you know, it's temperature and pressure that determine a boiling point. It would be reasonable to fill up in North Dakota in December when the RVP for pump gas is 15 and fly down to the Southwest somewhere it's quite warm in December and you'd have all the butane boiling out of your fuel just sitting there on the ground.
Electronic injection isn't immune to this, and does cause pressure drop at the pump inlet, but the fuel pumps aren't sitting there at something over 100F making the problem even worse. The RV-12 in the video I posted has a 912iS and still experience partial loss of power due to partial fuel starvation from fuel with too high of an RVP.Last edited by zkelley2; 12-18-2025, 04:43 AM.Comment
-
A follow on to Zach’s informative post. Peterson Aviation (the original mogas stc guy) tried like crazy to get fuel injected Lycomings to pass the required vapor lock tests. Blast hoses to cool fuel lines and pumps, auxiliary low pressure pumps, and I don’t remember what else he told me about. Point was, yes the engine would run on it but vapor lock was always a risk and there was not reasonable way to solve it. And unless you monitor the pressure at the pump inlet you’ll never know how close you are to the edge.
The Continental FI system has a return line for vapor and excess fuel which helps and made getting the STC possible.Scratch Built 4-place Bearhawk. Continental IO-360, 88" C203 McCauley prop.Comment
-
Right - this is all very helpful.
I will attempt a summary comment below, which considers the key aspects above, but please correct me if I missed the point here
I would also note that if those issue(s) occurred individually (or both at once) during critical flights phases, especially during flight in hostile terrain, they would seriously compound the workload and complexity of the situation. Reducing power is not always possible. I could easily see this leading to pilot overload and pilot errors occurring, or a forced landing.It is possible to operate an RSA-style mechanically fuel-injected Lycoming engine on mogas using standard engine controls and ignition systems (i.e. without FADEC or other experimental modifications). A number of pilots do this successfully, in specific conditions.
However, such operations carry significant risks, including vapor lock and detonation, either of which can lead to sudden engine failure without warning. Both the pilot and aircraft must be properly prepared to manage these risks. While the risks cannot be completely eliminated, they can be reduced if the pilot understands the underlying science and operational considerations.
Safe flight with mogas in this configuration requires active, real-time engine monitoring and sufficient pilot knowledge to correctly diagnose the sudden emergence of issues and intervene promptly. Mitigations include reducing power settings to stop detonation and using an electric fuel boost pump to clear vapor lock.
I don't think I will be revisiting my decision to stick with avgas any time soon
Comment
-
I don't think there's any evidence that an engine that can run on mogas regarding octane will experience detonation if you switch from carb to injection. In fact it's probably the opposite because you can actually have all the cylinders at the same fuel air ratio.
Most of the engines in the bearhawks will run on 91 or 93 just fine from an octane perspective. I would refer to SI 1070 for that information.Last edited by zkelley2; 12-19-2025, 12:21 AM.
-
Comment