There are alot of opinions on this subject.  Here is mine.

I've owned three planes that have had the autofuel STC.

I flew around in my Piper Colt on purely autogas that was available at my home field.  Seems to work ok, no problems to speak of.

I then owned a Piper Tri-Pacer, again I started with autofuel but swicthed to 100LL in the winter because the plane seemed to be easier to start.  When spring and summer came I started to use a mixture of autofuel and 100LL.

Now I own a C172 and I only burn 100LL in it.  Autofuel is not available at my home airport.  There are a few fields that I visit from time to time that have autofuel so I usually have a half and half mix of autofuel and 100LL.

Since autofuel is not generally available at the fields I visit on cross countries that makes it easy to not use it.  And I'm way to lazy to haul it to the airport myself

     Dave

Barry:

When you pulled the carb heat on it richened the mixture significantly.  The engine may have been rough as a result of an overly rich mixture.

Auto fuel and Avgas produce almost the same BTUs of energy.  They result in the same HP output.  Auto fuel has a shorter latency period and as a result seems to burn faster (it really doesn't burn faster, it gets started burning quicker). This results in higher internal cylinder pressures and higher CHTs but lower EGTs.  For this reason, the use of autofuel narrows the detonation margin on your engine.  That is not really a big issue in the smaller engines like it is in the larger TCM and Lycoming engines.

Besides Mogas kinda stinks!  <g>

Walter Atkinson

What octane autogas are you guys talking about?

Just to confirm the opinions of those who feel I'm opinionated:
  Autogas is for Autos and usually, but not always, works OK in them and other ground-based engines that have fresh fuel, and is good for parts cleaner if used outdoors..
  Avgas is for airplanes.  It works just like it's supposed to, stores a long time, causes no serious problems.
  I only use Avgas in my airplanes, and usually also use Avgas in my lawn mower, boat, tiller, power washer, etc. because it doesn't ruin the carburetors during long winter-month storage like autogas does.
  I've noticed a curious thing about autogas users.  They become very agitated, defensive, and/or aggressive when their use of autogas in airplanes is questioned.   Avgas users never do this.
I wonder why this is so.

Thanks guys.. Im going to stick with AVGAS. I feel the same way ..

Thanks

Barry

Alan, I dont think it was directed at you... alot of pilots feel avgas is for aircraft.. however my A&P Swears by AutoGAS and says the 0-200 were meant for the lower oct. fuel and will perform alot better.. he did tell me about the cons as well but said he perfers the autogas and directed me to use it also.. but the hassle of carrying 5 gal of gas to pour in the tanks are anoying.. When I bought the plane he used AV Gas so I guess unless it gets way out of range I will stick to AV..

Del:

* Have you ever noticed that the octane rating of auto fuel offered at the pump is lower at higher elevations. I'm just guessing that there is a relationship between available oxegen and detonation.*

Detonation is a very poorly understood event and we have been doing a lot of research into the issue for the FAA and others.  We recently gave a program on this topic to the FAA engine directorate folks where we demonastated these issues in real time on the engine runningon the test stand.  We can WATCH the detonation occur that way.

The availability of O2 is not a factor in detonation... I have no idea why there would be an adavntage to a lower octane fuel used at higher elevations.  Airplanes certianly use *higher elevations* and we use 100 octane. <g>  (just a thought!)

The detonation margin is altered by the latency period of the fuel.  The latency period is the time it takes for the flame front to become organized and begin to burn after the spark event.  In auto fuel (lower octane) the fuel gets organized and burning quicker than in 100LL.  This makes the effective timing advanced and the peak internal cylinder pressures higher.  This makes the CHT's run higher.  Higher CHTs and higher ICPs result in a narrowed detonation margin.

So, by comparison to 100LL, auto fuel (lower octane fuels) run lower EGTs, higher CHTs, and have a narrower detonation margin.  In lower HP aviation engines this does not seem to present a problem and that's why they can get the STC for those engines.  In the higher output engines this is a BIG No-No.  Ag Operators who run Mogas in their 985s and 1340s find that retarding the timing one or two degrees offsets the advanced effective timing quite nicely.

When PRISM is available, altering the timing will be what will make us able to run these engines on lower octane fuels witohut lead.  We are able to operate the IO-540J2BD, arguably the most detonation-prone engine in the fleet on 95UL (100LL without the lead).

(Mogas still stinks!)

Walter Atkinson
Advanced Pilot Seminars

Del,
I don't have the ability to be overly technical on this subject, I'm tossin' in my 2 cents in worth anyway... 

As the compression ratio increases so does the octane rating.
If the octane rating is too low for the engine's requirements, you're just going to wreck your engine.  Higher octane than necessary is of no benefit, especially $wise.

You may be right about lower octane *requirements* at higher altitudes.  I saw 85 octane gas (auto) once, in Colorado.  Everyone was driving around at 5,000'.  Auto fuels are "blended" for regions...or at least, they used to be.

Our airplane engine fuel requirements were designed for operation at sea level..  You may not NEED 80/87 (or 100LL) to cruise at 5,000 or 8.000', but you DO NEED the octane rating to get you there.
Ever hear an "old clunker" car going up hill, a knockin' & a pingin'?  Often due to someone just too cheap to put in "hi octane".

You're right about the compression ratio remaining constand and the compressed  mixture being directly proportional, but not so with the compression test.  If you squish 80psi worth of air into an airplane cylinder or 3,000psi into a SCUBA tank, the pressure inside the cylinder will be the same inside that cylinder not matter what the altitude.  So pumping pressure into the cylinder as done in a compression check at different altitudes would not be relevant to the air that is "sucked" in during operation and compressed.

Michael

Thanks Micheal and Walter for your replies.

The Mena Triangle? Could be a very interesting book! Or a good article for aviation publications, to make pilots aware, who are unfamiliar with our "triangle".

Kind regards to everone,
Del Lehmann

> The Mena Triangle? Could be a very interesting book!
> Or a good article for aviation publications, to make pilots
> aware, who are unfamiliar with our "triangle".

The Mena Triangle??? That must be that strange place in western  Arkansas where aircraft go in, and mysteriously emerge  later different colored.

Is anyone aware of the difference in boiling points of AVGAS
and MOGAS??? Does VAPOR LOCK scare you?? MOGAS at
higher altitudes may be more subject to vapor lock. Anyone
know???