In article <[email protected]> [email protected]
(karen.a.beaudoin) writes:
+---------------
| My question is, what exactly is in this AVGAS that makes it different
| from RACE GAS/UNOCAL 76/etc.??  
+---------------
Well, there are a couple of things.  Some are of help and interest to
racing use, others are not.

Perhaps firstly, AVGAS can be fairly high octane.  There are several
catagories of fuel, and they each have two octane numbers associated
with them.  Those numbers reflect the effective octane at two different
test conditions that are to emulate take-off power settings and high
altitude cruise conditions.

The rating pairs are expressed as "80/87" for example, but the lower of
those two numbers works out to be similar to the posted pump octane in you
local gas station (the average of the "motor" and "research" methods).

The generally available blends work out to around 80, 100 and 140 octane
(and the 140 is getting hard to find).  The fuel is identified with a
dye to help the pilot be sure he hasn't been serviced with fuel who's
octane is too low for his plane.  Detonation is no good in a car, but in
a plane it can be fatal.  That's why you see pilots draining samples of
fuel from each tank after service.  The dyes are formulated so that if
they are mixed, they go clear.  So, if the sample isn't the right color
(or has no color) you've been mis-fueled.

The color codes are:  Red for 80 octane, blue for 100 octane and purple
for the 140 juice.  There is also a green dye, used in the so-called
"low lead" version of 100 octane (usually labled 100LL on the truck).
There was some controversy in the aviation circles when the 100LL was
introduced.  It was intended by the petroleum companies to replace both
the 100 and 80 octane fuels and simplify their distribution problems,
but it still has too much lead (4 times the 80 octane amount) for many
planes, which get lead fouling on their plugs.

The point of this is, 100LL is the most widely available avgas, and it
has LOTS of lead compared to mogas regular, since the EPA limited mogas
lead content to the minimum needed to lubracate the valves in older
cars.  It (of course) has even more lead than unleaded mogas.  :-)

Now, why does this matter?  Because the effect of added lead is not
linear.  That is, doubling the amount of lead does not double the octane
increase the first dose got you.  Since unleaded mogas needs to be
constructed from fairly high octane components in the first place adding
a little lead to unleaded mogas can boost its octane quite a bit.  If
you add some 100LL avgas to some 97 (say) unleaded mogas, you can get
octane numbers higher than either gas alone.

The higher octane allows you to run higher compression ratios without
detonation and the holed pistons that go with it.

As to the cost difference, it is mostly due to volumes and distribution.
There is a smaller market for Unocal racing fuel than there is for 100LL
avgas.  It costs more to deal with it, moving it around uncontaminated
and the like.  It's not particularly better otherwise as a mixer.

Another avgas difference is generally much lower Reid vapor pressure.
Since planes fly at altitudes cars never will see (we hope!) the fuel
has to be less volitile to avoid vapor lock.  At the 24,000 feet my
Mooney can cruise, you are above more than half the atmosphere and
regular car gas could froth in the fuel lines to the injectors.  This
means an avgas (or mix) fueled car may be harder to start, and the fuel
distribution in the manifold may be a bit poorer.

So, to summarize (if you've read this far):  Try mixing about 1 part of
the green avgas to 5 parts premium pump mogas.  Adjust the fuel mix
ratios or your compression ratio after examining your plugs (you DO
examine your plugs after a run, don't you?).