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**davzLSA** · 01-15-2021, 10:42 AM

mogas is usually lower octane than avgas so that means your compression ratio in your engine will be lower so you will be running lower horse power. Otherwise your engine will suffer detonation. And as you pointed out you also run the risk of vapor lock.

**whee** · 01-15-2021, 11:11 AM

Petersenâ€™s recommendations pretty much cover it. Bobâ€™s fuel system meets the recommendations and works as is with mogas on a carbureted engine. Fuel pumps in the wings arenâ€™t required because of the ram air vented fuel caps.

1/2â€ fuel lines wonâ€™t do anything to help a carbureted engine.

**whee** · 01-15-2021, 01:43 PM

Iâ€™m pretty familiar with Bernoulli.

The question was specific to mogas and a carbureted 360; 1/2â€ fuel lines arenâ€™t going to do anything helpful in terms of mogas compatibility. The fuel system works perfectly fine with mogas and a carbureted 360.

Less than a handful of BHs are flying with 1/2â€ lines. They simply arenâ€™t necessary. I put 1/2â€ lines in mine for very specific reasons; mogas was not among them.

**zkelley2** · 01-15-2021, 07:44 PM

An alternative to running low compression pistons is to run a custom timing curve on electronic ignition. That's what I did. I run 90 octane(highest pump gas here) and have 2 maps. One for 100LL if I ever run it and want the bit of power back, and one for 90. The difference is 2 degrees retarded at WOT and 1 degree with the power back. It's a lot less HP loss than going to 8 or 7.5:1 pistons. And it's still a HP gain over stock at low power settings.
But our parallel valve 540s were certified on 91/96 to begin with.

**whee** · 01-15-2021, 07:59 PM

Zkelley, Why is a fuel pump required on a Lyc running mogas? If that is they case then I agree, 1/2â€ lines would help relieve that need. I havenâ€™t paid close attention but it seems like the guys needing the engine pump are those with fuel flow transducers that are restrictive.

I have the same ignition system as you and planned to set my ignition maps the same way. However, I didnâ€™t find a need to restart the timing for mogas. In fact I bumped up the timing significantly but never noticed a power difference. To be conservative I reset the timing to factory. Are you able to notice an actual difference?

**zkelley2** · 01-15-2021, 08:40 PM

Originally posted by whee View Post

Zkelley, Why is a fuel pump required on a Lyc running mogas? If that is they case then I agree, 1/2â€ lines would help relieve that need. I havenâ€™t paid close attention but it seems like the guys needing the engine pump are those with fuel flow transducers that are restrictive.

I have the same ignition system as you and planned to set my ignition maps the same way. However, I didnâ€™t find a need to restart the timing for mogas. In fact I bumped up the timing significantly but never noticed a power difference. To be conservative I reset the timing to factory. Are you able to notice an actual difference?

A fuel pump is required if you can't flow enough fuel from gravity to the carb. If I remember correctly, the formula is .55lbs/hr * HP and then you need 150% of that. If you can't get that number, you need a fuel pump. Lycoming engine driven fuel pumps get rather hot since they are attached to the accessory case, which can cause fuel vaporization upstream of the pump at the inlet.
So the importance of fuel line size and mogas is just that you can flow enough to not need that pump. You should be able to easily achieve that on an o-360 with 3/8" lines.

Regarding the SDS ignition, you shouldn't see a HP increase at max power. On 100LL you'd still have it set to factory at whatever max MAP and RPM is for your engine. The advance only comes in at below 65% power where the risk of detonation is more or less gone. You might see a knot or two more.(which checks with the math on HP required for a speed increase.) And if you're doing climbs where full power is now under 65%, you should see a increase in fpm. There'd be a greater increase in climb rate than there would be in airspeed since airspeed isn't really about power.
As far as retarding the timing for mogas, it'd be engine dependant, but with my engine wanting a bit more octane than I'm giving it, talking to Ross, he had me retard the timing slightly for the lower octane. We don't have knock sensors, so we can't go test to detonation. We have to stay far from it. He even gave me a map for 87, and it's a lot lower timing. Running any gasoline possibly available was a feature I wanted since not all of our airports in the bush have avgas available. They all have a pump with car gas. This way I don't have to run my 260hp engine at 230hp all the time and have wildly reduced high altitude performance like low compression pistons cause. It's only that low when I need it to be.

**whee** · 01-15-2021, 09:33 PM

Thanks zkelley.

There is a thread on VAF about the CPI and a guy messing with the timing. Seems like he got a few knots when he advanced the timing. I never noticed an increase but my airplane is more draggy than his RV. Thatâ€™s why I was wondering if you noticed any difference.

Ross wasnâ€™t able to help me much with my timing since it is engine dependent and he doesnâ€™t have any experience with the Continental IO360. My curve is just flat 20deg. I played with it a bit and had it bumped to a flat 25deg but I moved to back after not finding any difference.

Iâ€™d like a curved map but I havenâ€™t looked into how to properly develop one. Iâ€™m not sure where to start since at the elevations around here WOT only gives me 22â€ MP and so I run WOT all the time. Iâ€™m probably overthinking it but thatâ€™s kinda my thing. Haha.

My engine was certified for 100 octane (8.5:1 compression) but seems to be getting along fine with 91.

**svyolo** · 01-15-2021, 10:26 PM

If you are going to change the ingition advance, it is good to read a bunch of info that is out there. If you have injection that you balanced really well, you can run more advance safely. Without that balance, you can get detonation on the more lean cylinder(s). I think SDS and EFII say about an additional 5 degrees. But I think they both recommending running 3-5 degrees less advance for Mogas.

Sharp corners reduce flow, which seems obvious, but they also create turbulence, which can cause fuel vapors to come out of suspension. I think a fuel return is also preferred to purge any vapors that are created.

**noema** · 01-16-2021, 04:35 PM

Thanks for all your input. I take it an O-360 Bearhawk with standard fuel system should be fine on autogas.

If anybody has done some optimization to cover edge cases (say hot day, inadvertently got some fuel with ethanol into the tank) I would be interested to hear about it.

**svyolo** · 01-16-2021, 06:01 PM

I am not sure where you are located. In the states, different states have different blends of fuels, and they can vary quite a bit depending on summer/winter. In general winter blends are more volatile. Winter blend fuel, hot and high, are the worst combination if you are using fuel pumps. There are too many variables to be more exact, unless you go to the trouble of finding out the exact specs of the fuel you are using.

Gravity feed helps out. Ethanol is only bad if something in your fuel system has materials that are not compatible. It will run OK, but over time it will degrade some materials. It degrades some plastics, and I believe brass, but I can't remember for sure. I think it eats carbs.

**Nev** · 12-14-2025, 04:33 AM

Is anyone on the forum running Mogas on their Bearhawks, and if so I would be very interested in your experience with it.

**500AGL** · 12-14-2025, 11:45 AM

On smaller power plants in cooler climates mogas has proven to work just fine. I've run 91 non-ethanol in C90's and O-360's for hundreds of hours, summer (80's) and winter (single digits) without any issue. I did add marvel mystery oil with every gas stop, for lubrication, unless I was adding avgas. A friend has probably run a couple thousand hours of mogas through his O-200. The Caveat to this, in my opinion, is the plane needs to be flown. Not once a month, not every other week, but actually used and the fuel needs to run through it.

Long ago in the Marine industry I was informed about the actual longevity of today's auto-gas. Like most of today's products, gas is no longer made with a shelf life. You have about a week before it starts breaking down its octane rating. In about 2 weeks it's enough of a difference where if you knew you'd try to get rid of it quick. After that it's crap shoot. The marine industry thrives on fuel stabilizers, and we used Star-Tron in all of our marine customers engines. In doing so, we never had a a fuel issue. But, we also informed the customer to drop off their boat with as little fuel in the tank as possible for winter storage, and put fresh gas in next spring. For some that was a 6 month interval, for others it was an 8-9 month interval.

I love Avgas for its better ability to not vapor lock as much, clean evaporation on fuel spillage, octane rating and long term storage abilities.
I don't like avgas when pilots don't run their engines often enough, hard enough to get to temperature, or lean enough to prevent lead deposits and build up.
And naturally I don't like the price.

I love autogas in some areas for having non-ethanol both available and less expensive.
I don't like autogas for the residue, the requirement to use immediately, and the smell and oiliness when having to handle it.
But by nature, people, and by pilots, are cheap. We often subscribe to the lowest cost even if it's not ideal for the long term.

If I am at a location that offers both, either on the field, or just down the road, if the cost difference is less than $1/gallon, I'd suggest buying the Avgas. That $60 or less will make no detriment to your future flights, and may provide a benefit. It's just harder to make that decision when you're adding fuel 3-4 times a week.

The above is related to carb'd Lycomings and Continentals. Injected aircraft engines have more systems requiring lubrication and I would heavily lean to avgas.

So here's my unbridled opinion:
- I like non-ethanol mogas, provided it's used in a timely manner with a proper MMO mixture, in carb engines with standard compression (less than 8.5:1).
- I have seen the results and spoken to mechanics that have torn down engines and I believe in MMO.
- But non of this matters if you don't fly the plane. If you spend more money on avgas once a month, it doesn't affect much. The longest running, best performing, and least amount of maintenance is on engines that run alot. Like all the time. Never fly less than an hour at temperature. Adjust your cooling to ensure you oil temp is at least 180 degrees. If you can't get to 180 don't even start it.

With airplane ownership comes great responsibility. A recreational GA pilot at a minimum can fly 100 hours a year. One that loves to fly can fly 200 hours a year, every year. Engines loves this. Burn the fuel. Take yourself flying, no matter what fuel you put in.

**jaredyates** · 12-14-2025, 01:29 PM

I tried it for a little while and didn't have any operational problems, but there were a few drawbacks.

First, there are no airport pumps dispensing it in my area, so I had to obtain a tank and transport the fuel to the plane. If I start adding the cost of that effort, it gets large fast.

The biggest problem for me is stability and shelf life, like Peter says. Tying to keep all of the various engines in my life running is a constant battle because car gas goes bad so quickly.

I found that the car gas was much more willing to stain the airframe in places where it might drip or come in contact. It smells terrible, both on the airplane and if any of it gets on me. Avgas doesn't have either of these problems.

All of these are factors that go into a cost-benefit analysis, which also includes the price. For me, non-ethanol fuel is about $1 to $1.50 less per gallon than avgas. If the cost difference was high enough, I could probably put up with the downsides of car gas, but it isn't there yet.

I considered getting a load of Swift fuel, but it was quite a bit more expensive than Avgas in the quantities that I'd be able to get it. If I could find an unleaded fuel that didn't leave behind deposits upon evaporation, that costs somewhere near the same as Avgas, I would probably buy a bunch of it.

**Jim Herd** · 12-14-2025, 02:36 PM

I would like to add to many of the good comments above. I just finished an authorized Rotax training class in Hurricane, Utah. There are 2 levels – basically, normal TLC including oil change, and then removal/inspection/reinstallation of all the engine modules. There is a third class involving repair of the modules, which I did not take. So I have studied the fuels issues extensively in the past, and this class added some knowledge I will share. And yes, there is always controversy surrounding this subject.

There is considerable danger in experimenting with different fuels. The greatest danger is detonation, which can not be detected by the human senses until the engine is destroyed, and that may take just a few seconds. This is one reason all engines should have a full engine monitor, with alarms to draw attention if the indications go whacky (especially CHT) due to the onset of detonation.
There is great variability in chemical formulation of mogas, within each country and around the world. And with seasonal changes in the chemical mix. And cars have far less demands on fuel properties than do aircraft. That’s why auto petrol was able to remove lead decades ago, but avgas is still struggling with that for many reasons.
Distribution controls are also highly variable with mogas, as is the age of the fuel dispensed at the pump, due to flowage rate & retail tank refill frequency.
Ethanol is problematic, even with fuel systems that are “compatible”. That’s because it likes to absorb water and it is highly volatile and will evaporate as we fly high, or with high temperatures.
Mogas in the USA usually has 5% higher energy density than 100LL.
Octane ratings are calculated differently around the world, so be careful with comparisons.
In the USA, mogas can generally not be trusted beyond 2 weeks! I thought it was 3 months, based on my research over a decade ago. but the Rotax instructor said it is now only 2 weeks. They have mogas right on that airport, so I asked if he recommended I fill my 916iS with mogas. He inquired when the last delivery was received, and then said NO, do not use that mogas! Maybe he is excessively cautious, but you will get the general message.
Avgas is much more “stable” & “trustworthy” in many ways. The chemistry is more consistent and quality-controlled around the world, and it is generally certified to be good for at least a year. In practice, I think it is fair to say it will usually be good for 3 years, based on aircraft I know that have not moved for several years, then put back in service.
The Rotax 916iS in my Carbon Cub UL is approved to run on avgas or mogas – with no pilot or engine changes needed. It is only 1350ccs and turbo boosted to over 50 inches MAP, and it is FADEC. This is amazing, and it tends to give the impression that we can do the same with Conti or Lyc. Not so! The Rotax has an incredible array of sensors and computers that monitor & control every variable of the combustion process. For example, there is a “knock sensor” (as do most modern cars!), so with any onset of detonation the engine will immediately know and adjust parameters to stop the detonation. Specifically, it will retard ignition, change fuel injection, and maybe reduce rpm & throttle.
My Bearhawk Patrol has a Lycoming-Titan O360 with none of the above! It does have a full engine monitor, but I consider experimenting with different fuel to be playing with fire. So I stick with 100LL.
Rotax 916iS calls for 100 hour oil & filter change, but that goes to 50 hours if I run 30%+ of the time on 100LL. And oil changes are a huge chore cos I have to remove top & bottom clamshell engine cowls. Nonetheless, I am running exclusively 100LL for all the reasons above.
94UL is rarely available in the USA, but quite acceptable in the 916iS. In a conventional Lycoming or Continental, it may or may not be suitable depending on exact engine configuration and any modifications. The problem with 94UL is that over 80% of the fuel used in small piston planes in the USA is used by engines that require 100 U.S. octane. Providing both 100LL and 94UL is not commercially viable at most airports, so only 100LL is generally available.
In other regions of the world, and at specific airports, the fuel situation can be very different. For example, an airport that is overwhelmed with small experimental engines that can take mogas, may prudently provide a fuel other than 100LL, with a high turnover of inventory, and enjoy a workable profit. Or even a club environment that has no profit incentive.
Of course, the perfect solution is unleaded avgas that will satisfy all our engines – 100UL. But it will be years before that becomes ubiquitous around the world. And even then, remote areas such as Africa will probably still only have Jet A and mogas.

And finally a couple of editorial comments. My Bearhawk has some features that are BETTER than the Carbon Cub UL! I moved to the Carb on Cub UL because, as an engineer, I am attracted by the sophisticated Rotax engine concepts. The 916iS has all the attributes of a modern car engine, whereas the Lycoming is similar to the engine in my 1959 Frogeye Sprite. I enjoy the challenge of the high-tech Rotax, even though it does have considerable downsides. For example, in Nevada, USA, there is almost no professional mechanics willing to work on Rotax. Also, the entire engine concept is sooooo different & soooo complex that you really don’t want to let a conventional mechanic loose on a Rotax! That’s why I took the formal Rotax classes. Even changing the oil is very different, with easy opportunities to cause big trouble if you do it incorrectly. Rotax is also very restrictive & dictatorial, so it can be hard to jump through their hoops, even for getting parts.

Bottom line – my Lyc-Titan O360 is not approved for mogas, and my Rotax 916iS is approved for mogas, but I will use it in neither. And since these two wonderful aircraft perform almost identically, and sere the same aviation mission, I should get on with selling the Bearhawk! Oh, and I heard a rumor that Virgil might be considering offering a Rotax engine some time in the future. That would make the brand a world-beater! IMHO.

From Jim Herd

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Mogas/Autogas Considerations

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