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Re: Fuel question

Different rating system is correct, but it's awfully close to automotive.
Most pilots don't even know this, but the fuel is designated by it's lowest
test number.  There's actually an "octane" range.  The old red dyed fuel we
previously used was 80/87.  The new blue stuff I occasionally semi-legally
pumped for the race guys is 100/130.  With the Red 80 fuel going the way of
bi-planes many owners of older airplanes are buying auto fuel conversions,
which quite often consist only of paperwork specifying the minimum "octane"
fuel that can be used from the pump.  With 100LL (low lead) avgas ranging
between $2.20 & $2.50/gallon or so even higher octane auto pump gas is
-----Original Message-----
From: Jim Adney <jadney@vwtype3.org>
To: type3@vwtype3.org <type3@vwtype3.org>
Date: Friday, July 16, 1999 1:12 AM
Subject: Re: Fuel question

--It is my understanding that avgas uses a completely different rating
system for octane, so you cannot compare the octane numbers. I have
NO idea why this is so.

> Actually it's usually not even the octane content that is
> referred to with the number.  It used to be but it's more efficiently done
> by other chemicals now.  Pumps by us here in no-VW-show-country (Indiana)
> declare the number on the pump to be an anti-knock rating rather than the
> popular octane nomenclature.

Under the automotive system the o7/zĀting is measured by
comparing the resistance to knock of a particular fuel to that of
pure Octane (straight chain 8 carbons, 18 hydrogens.) If they are the
same then the fuel is given an octane rating of 100.

It turns out that there are 2 popular ways to make this comparison
and they end up giving different numbers. The 2 are called the RON
(Research Octane Number) and the MON (Motor Octane Number.) The
number that is usually put on the pump in the US is the average of
the 2 = (RON + MON)/2, but the number that VW specifies as required
is the smaller of the 2, so this explains why we can almost always
"get away with" a couple of % less than what VW seems to ask for.

Jim Adney, jadney@vwtype3.org
Madison, Wisconsin, USA

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