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<< Lancair Builders' Mail List >>
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Brent's statements regarding the quality and octane rating of auto fuel sold
at gas stations would have had a lot of validity 20 years ago and some
validity just ten years ago, but have little now. Neither is 100LL the "holy
grail" that he thinks it is.
100LL actually contains a lot of lead, but derives it's "Low Lead"
designation because it's formulated with less lead then it's heavily leaded
predecessor. I choose to burn auto gas in my 8.5:1 engine, not because of
cost, but because it runs better burning auto gas and because I don't want
it fouled with lead. Many other pilots have told me that their engines also
run better on auto gas than 100LL.
Octane -
There are two methods of measuring octane for auto fuel, the
"Research" (R) method and the "Motor" (M) method. Research yields a higher
number than Motor. Many years ago, the EPA decided to eliminate the dual
numbers (forcing a marriage of apples and oranges) by adding the two and
averaging them as [(R+M) / 2].
100LL is measured on the Research scale. When leaded auto gas was
available, you could buy 100 to 103 octane (Research) at some gas stations.
Currently available 93 [(R+M) / 2] octane auto gas would measure about 97
octane, if measured on the Research scale.
Octane requirements are dictated primarily by compression ratio;
however, aircraft piston engines do require a higher octane margin to guard
against detonation than a more evolved automobile engine (for many reasons
that would be too lengthy to list here) primarily because of their cruder
design (large bore pistons yeilding a wide flamefront that requires two
sparkplugs spaced far apart) and because pilots must (sometimes erronously)
set the mixture control themselves :-). So, in using auto gas you do give up
some of that margin of safety.
Purity -
The EPA has mandated increasingly strict evaporative controls and
auto gas has been greatly cleaned up because of electronic fuel injectors. I
can't go around with analysis equipment so, yes, I have faith that what I'm
purchasing is as stated. However, there are a lot of "No Name" stations out
there and it's true that someone can always profit by adulterating or
watering down a product so I wouldn't advise purchaing auto fuel for an
airplane from other than a modern, leading brand station (with a high
turnover rate). The 100LL at your airport could have been in the tank for a
Long time.
Also, all of the old, leaking metal storage tanks at the auto
stations have been replaced with fiberglass tanks. That is not yetr true of
many airports. Their old metal tanks could well be leaking, giving you a
dose of water and other contaminents with your 100LL. As for
cross-contamination in the auto fuel pipeline system, if 50 or 100K gallons
is refined and pumped through a pipeline that has residuals, the
contamination would probably not even be detectable.
As I stated previously, I burn only Amoco. They do not store gas at
their refinery (preventing cross'contamination) but ship directly to
dealers. In addition, Amoco prevents their dealers from putting other than
Amoco in their storage tanks by charging them only for gas that's pumped,
not what is delivered. So, if an Amoco dealer put anything else in his
tanks, he would pay twice.
Please note that I've refrained from using any expletives or
thinly-veiled personal critisim.
Bill Rumburg
N403WR (Sonic bOOm)
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