Avgas vs Mogas vs Pulp

[flindersflyer]

AvGas ........ MoGas .......... PUlp. What is recommended for a Rotax 912 or 914? Which fuel burns cleaner and leaves less deposits in the engine? I'm new to this game but am curious as to whether one fuel is better than the next. Anyone out there enlighten me?

 

 

[GDL]

I always use at least 89 Octane MoGas (boat gas specifically because it has no ethanol and few additives in Canada) in my 912UL. I know at least 91 Octane is recommended for the 912ULS, the higher the octane the better. I know very little about the 914. I am also experimenting with the new AeroShell semi-synthetic oil that is designed for both MoGas and 100LL. Said to be able to absorb much of the lead from the 100LL. MoGas can be a pain to find when traveling from airport to airport. I have never had a problem with MoGas volatility at altitude, which I think others have. I have been to 4000 meters.

 

My oil and plug changes are done at recommended 100 hours (or less) with MoGas. Need to be done (according to Rotax distributor) at 50 hours with 100LL. My experience with MoGas is that it leaves not many deposits. But it may also depend on what has gone into the fuel and how long it has sat, and other factors I don't understand.

 

 

[facthunter]

The rotax is designed to use unleaded fuel. Some have reported some loss of compression after using 100LL It is said that the engine does not run hot enough, being liquid cooled, to make the 100LL work as designed, on the valve seats and an uneven build-up can result. This appears a temporary problem if reverting to Mogas. Mixtures or the occasional use of avgas would seem to be OK.

 

ALL bar the 80HP engine should use premium in australia. I would consider it optional on the 80 HP. without detriment.. Nev

 

 

[GDL]

Hi FactFinder. I haven't heard that about the 100LL. I would have thought the engine would run hotter because of the higher octane value of the 100LL. Do you have any links to that info? I would be interested in seeing the comments. My partner in my plane used 100LL in a cross Canada trip (~70 hours and 10,000 km)with no known ill effects. But I do prefer MoGas for better plug life. And the techs at Rotax agree with you that the 100hp 912S needs premium MoGas.

 

 

[facthunter]

Higher octane doesn't make engines run hotter.(I used to think it did too). In theory the fuel burns slower but it is only milliseconds and doesn't even effect spark timing. Avgas is formulated for aircooled aero engines, that run head temps MUCH hotter that the Rotax engines .There are reactions that rely on high temperatures being reached. Bromine is added to deal with the lead deposits, and is responsible for the whitish colour in the exhaust pipe .( which is a dead giveaway as an indicator of it's use.). Most of my information was obtained from BP Australia from the chief of their fuels technology section, in relation to valve seat deterioration in other engines. that I was involved with.

 

Rotax don't recommend it. I personally know pilots who have had loss of compression in their Rotax's. after using it.( an engine that usually gives no trouble in that area, at all. The problem is that the lead deposit ( that acts just like flux when you are brazing and doesn't melt till high temps are achieved) builds up thicker than it should, and sets like glass on the seat. IF a small piece dislodges, the gas at high pressure and temp blasts past and can erode a piece of the valve or seat, and cause it to fail ultimately. Nev

 

 

[Guest davidh10]

Apart from what has already been said, read the Rotax Service Bulletin on fluid selection. It is the official word.

 

 

[GDL]

It is interesting the various information floating around - some official, some not - about what Rotax engines need for fuel. The Canadian distributor is not far from where I fly so I talk to them at times. They do not have a problem with 100LL but do not recommend a steady diet of it. FactFinder, thank you for that info. I did not know about much of the flame aspects of 100LL & its impact on Rotax engines. Makes one think about what goes into my fuel tank.

 

 

[crashley]

I run premium 98 when possible and mogas when it is not this is what rotax recommends

 

 

[GDL]

Hi Crashley. In Canada, any of the high octane MoGas fuels have at least 10% ethanol in them. This is definitely not recommended because of what it does to the seals and soft components in the carb. We do not have much choice except what is called boat gas (which has less additives and no ethanol). Out boat gas is mid-grade (89 octane although I do not know if our octane is the same as yours). This is the best we can hope for. My engine is the 80 hp and I have not had any problems. I have not heard any problems with the 100 hp but I am less familiar with those engines.

 

 

[snakenjac]

It would be nice to only use Mogas as recommended, however when doing long distance touring it becomes impossible as Avgas in most cases is all that will be available. If planning ahead, it may be possible to have Mogas arranged for arrival at fuel stops, however if an alternate is forced on us, see if the Mogas provider will do it a second time ........ unlikely.

 

From what I read, it is simply a matter of pumping Avgas when it is all that is available, and doing early oil changes.

 

 

[JG3]

I once did a long trip on Avgas, and after 70 hrs got a bad mag check.... The first plug that I pulled the gap was completely closed by lead deposit. In all the others the cavity between the insulator and shell was full of deposit.

 

 

 

 

[onetrack]

One of the more common features of plug fouling by lead is when the insulator on the centre electrode becomes coated with deposits, and this makes the spark "sidetrack" down the centre insulator to ground, without jumping the gap.

 

It doesn't take much of a deposit coating on the insulator to do this, either. Iron deposits are the worst. The insulator can appear relatively clean, but the coating - often recognisable by pronounced discolouration - is a highly conductive coating.

 

http://www.moto-parts.co.uk/sparkplugdiagnosis.pdf

 

 

[OZJohn]

Apart from the valve and plug fouling mentioned above the issue with using Avgas in Rotax is the lead buildup in the gear box which affects the clutch. You should change oil and filter every 25 hours if using Avgas. The gearbox needs to be serviced at 600 hours if using Avgas and Rotax recommends using Avgas no more than 30% of time. Better to use unleaded.

 

The problem with using Aeroshell semi synthetic oil in Rotax is that it is not suitable for the gearbox and clutch.

 

John.

 

 

[Keith Page]

I do not know where to find the information these days, however look about :Google". They are fantastic articles with great information.

 

There are some excellent articles written by the Rotax expert of the UK. One bit of information I will take from those articles and treat it like gospel,

 

"Use 95-98 do not use leaded fuels as the engines are not designed for lead and will cause grief". The lead WILL cause a lot of grief.

 

See in one post above the plugs are not a joy.

 

KP.

 

 

[BirdDog]

My 912ULS (2006 model) runs perfectly well on Caltex 95 unleaded. I use AvGas - ONLY when I have to, and that's not very often.

 

 

[Downunder]

Original post 2011. .....

 

Rotax spec 80 hp UL engine min 91 RON fuel.

 

100 Hp ULS min 95 RON fuel....... many prefer 98 incase the fuel octane degrades. RAA produced a paper indicating fuel octane does degrade in vented tanks over time and states some solutions to the problem.

 

Other countries use different octane rating standards. The USA uses MON. The numbers tend to be slightly lower than our RON rating.

 

91 in the usa maybe fine for a ULS but putting 91 in a ULS here is a no no....