You may be intersted in this discusion on Mogas from the Cardinal Club Digest.
I have a '68 150HP FG with an EAA STC for Mogas. I have flown the
plane now for 1.5 years (about 165 hours) and have used mogas exclusively
with (up to 10%) and without ethanol.
[Does your STC allow mogas with ethanol? Paul]
I have had oil analyses done after every oil/filter change interval (40Hrs)
and there are no measurable negative effects to the engine from using
mogas. I am not brand loyal and buy the cheapest price I can find. I fly
high and in every season, especially winter. I live between lakes Erie &
Ontario so the moisture content of the air is up there in the winter. I use
no additives to the fuel. I do use Avlube in the oil. The cam is fine.
I have done this to prove to many of the local naysayers that mogas is a
viable alternative for the low compression O320E2D engine, and because I
run an environmental firm I can't justify using any leaded fuels. I have
learned a few things which are helpful.
Firstly, the engine will clean itself of most lead deposits when making the
fuel switch but the plugs will need to be cleaned. Once done they remain
very clean if mogas is the only fuel used.
Second, the engine will run smoother, it will take two or three tanks full
to notice the difference.
Thirdly, mogas gets reformulated for each season to enhance starting
characteristics in cold weather. So use your fuel often, don't let it sit
in the tanks from summer to winter. A fuel formulations engineer told me
that 3-4 months storage is no problem, but don't go longer. He also said
that 80/87 octane contains less additive than the other formulations, and
that buying up one grade to get a higher octane rating for an O320 wasn't
worth the money.
[Both 80/87 and 100/130 have almost no additive package. Paul]
Fourthly, ensure that the fuel lines and associated plumbing have fresh
o-rings installed. Particularly the tank caps, selector, shut off, primer
and strainer. When I switched to mogas some of these o-rings started to
leak and needed replacement. They may have needed it because of old age,
but changing the o-rings solved any leaks. Ensure the fuel tanks have not
been "sloshed" with a sealing compound, as mogas will dissolve some
formulations.
I have not had a single drop of water in the mogas. I constantly saw small
amounts of water in the avgas.
[If you really have ethanol in your mogas, then the water is still there,
it is just that the ethanol is keeping it in solution. Not the best, as it
will come back out of solution when the temperature decreases, like at
altitude, and then potentially freeze in the fuel system.]
Ethanol does seem to decrease the energy available per litre (gallon) so as
a result range is roughly reduced by the percentage of ethanol content. I
have not had any vapour lock, hard or hot start problems. I use the prime,
wait, time based on temp, method for starting, as listed by another CFO'er.
Mogas is good but not perfect. Lugging 5 gal jerry cans up onto the wing is
a pain. Avoiding spills over the wing is an acquired skill. Mogas stinks in
comparison to avgas, so sample cups and dipsticks need to be stowed in zip
lock bags so they don't stink inside the cabin. Disposable gloves are
needed to keep mogas from contacting the skin.
Lastly, most of the myths that surround mogas usage seems to comes from
self proclaimed experts who talk lots and have done little. A local flying
club has run its fleet of 150's and 172's on mogas for over twenty years
without ill effect. I suspect that most of the engine damage theory has
come from those who have not used their aircraft frequently and blamed
mogas usage for the engine ailments.