From: leed@hpgrla.gr.hp.com (Lee Devlin)
Newsgroups: rec.aviation
Subject: Auto Gas STC questions
Date: 19 Aug 92 20:40:28 GMT
Organization: Hewlett-Packard Avondale, PA

I found this posting in the Aviation Digest and haven't seen any
follow-ups so I thought I'd go ahead and post my answers and perhaps
generate some discussion.  I know how the poster must feel since I have
had trouble posting in the past.  In fact, I have to log into a computer
nearly 2000 miles away from my division to get my postings to escape to
the world.  (It used to work locally but now we have Information Technology
professionals handling it :-(.
----------------------------------------------------------------------

>Date: Mon, 17 Aug 1992 08:50:08 PDT
>From: Eugenio_Allevato.El_Segundo@xerox.com
>Subject: Auto Gas STC .... Another Try
>To: Aviation@mc.lcs.mit.edu
>Message-ID: <"17-Aug-92  8:50:08 PDT".*.Eugenio_Allevato.El_Segundo@Xerox.com>

>        I did not get any responses on my previous inquiry, so I thought that I
>would try again.  Surely, the net has to have a few owners running mogas!!!

I will more than make up for the silence with this lengthy reply :-).

>        I am starting my investigation on Auto gas conversion and have the
>following questions:
>
>        1)  Besides the paperwork, what is mechanically required to accomplish

Much of the work has already been performed for you in the form of
testing done independently both Petersen Aviation and the EAA.  The EAA
got the first auto fuel STC on August 5th, 1982.  The STC costs 1$ per
horsepower and generally speaking, no modifications are required.
Please be aware that only certain engine/airframe configurations are
approved and you should contact the EAA or Petersen to make sure there
is one for your plane.  Petersen has evidently done some experimenting
with replacement fuel pumps, etc, so their STCs sometimes include
additional hardware.

>        2)  Suppose that I go on a cross-country and take-off with full tanks
>of auto gas,  3 hours later I land and refuel;  can I mix av gas with auto gas?

Auto gas and aviation gas are completely miscible because they are both
composed of hydrocarbon chains from around C5 through C12 (that's analytical
chemist-speak for hydrocarbon chains containing somewhere between 5 and
12 carbon atoms).  There has been much mis-information published
about them separating like oil and water which is just plain nonsense.
Incidentally, these fuels are colorless until the dyes are added so
don't confuse the color stratifications of the dyes for separation of
the fuel mixtures.

>        3)  Where do I get information (price, agency/company?,) regarding the
>conversion?

You can get free information packets from the EAA at 414-426-4843 or at
Petersen Aviation at 308-237-9338.

>        4)  Any net wisdom on the conversion?  Reliability,  etc....

Well, if that's not flame bait, I don't know what is.  The main
difference between auto gas and av gas is the way that the manufacuturer
enhances the octane number.  I should also state that I've done some
simulated distillation tests using ASTM method D3710 on many gasolines
and have found that av gas is generally composed of fewer types of
hydrocarbons than auto gas.  Keep in mind that there are hundreds of
different kinds of hydrocarbon chains between C5 though C12.

Auto gas uses aromatics like benzene and toluene as well as oxygenates
like MTBE, ETBE, TAME, and (Heaven forbid!) C1 through C4 alcohols to
enhance octane number.  Alcohols have lost popularity as octane
enhancers due to their destructive effects on certain sealing materials.
In a random sample of 20 gasolines we tested last summer, we found no
alcohols in them at all.  We used ASTM Method D4815 for these tests in
conjunction with an atomic emmissions detector which could reliably
dectect any oxygenated compounds down to the ppm range.  However, there
is a simple method for testing for alcohol in gas using a special fuel
tester that only costs a few dollars.

Av gas uses tetraethyllead to enhance the octane number.  Some people
claim that lead deposits, besides fouling plugs, have beneficial effects
on internal combustion engines primarily in the area 'lubrication' of
the valves.  Other claims center around the lead helping to seal valves
in their seats which prevents valve seat erosion.  It seems there is
much debate over the unleaded fuel<->valve erosion connection and some
people believe that it was only a problem with certain GM engines but is
repeated over and over like so much urban legend.  However, aircraft
engines have hardened valve seats so this lead sealing is not a benefit
from that standpoint.

A frequent concern with using auto gas is that its Reid vapor pressure
changes from winter to summer.  The formulation of auto gas is changed
in the winter to make it more volatile at low temperatures to promote
easy engine starting.  This fuel would not be acceptable in the summer
due to its propensity to vaporize in the fuel lines and cause vapor
lock.  This is more of a problem with planes utilizing fuel pumps than
it is with those having gravity feeds and of course, high altitudes can
contribute to vapor lock due to lower ambient pressure.  However, if
you're flying your plane in the summer with gas you put in it in the
winter, you don't fly enough to deserve your own plane :-).

>I have a PA28-180 with the Lycoming O-360

More than any other factor, the compression ratio of the engine
determines how low an octane number is acceptable.  It seems a shame
that by increasing the compression ratio to get about 10% more
horsepower out of an engine, you're forced to use fuel that can cost
almost twice as much.  Unfortunately, the EAA doesn't hold an STC for
the PA28-180 (they have them for the -140, -150, and -160 though).
Petersen has several STCs for aircraft that are not on the EAA's list
including the PA28-180 and PA28-181.  This STC is $420 and requires the
removal of the factory boost pump which is replaced by two boost pumps
which are included with the STC.  It takes about 5 hours to install.
You can then burn 91 minimum grade octane.  That requirement will add
about $.20 per gallon to your expected cost of 87 octane auto gas :-(.

I have also noticed that 80/87 av gas is steadily increasing in price
and at my airport it currently costs $2.11, or $.20 more than 100LL.
It's also getting hard to find.  Some of us may have no choice but to
use auto gas in the future.

I have been using auto gas for about 70 hours of flying by taking
several 5 gallon cans out to the airport with me each time I fly.
Sometimes, I just top it off when I go by the airport.  I've found that
my plane runs better on auto gas.  There's no lead fouling and since it
only burns about 6 gallons an hour, and has a 36 gallon capacity it's
not hard keeping it full.  I do miss the convenience of taxiing up to
the pump.  Double handling the fuel is no fun either.  But I figure if
I fly it 150 hours a year, it saves about $900 in fuel bills.

At the gas station, they just think I have a thirsty lawnmower:)


Lee Devlin    N3IWP    |    HP Avondale Division  | phone: (215) 268-5697 
Piper Colt N4986Z      |    Rt 41 & Starr Road    | email:
Spirit of rec.aviation |    Avondale, PA 19311    | devlin@hpavla.avo.hp.com
SWPLII
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