Re: AVgas in 2 litre?

["Rich Wagner" <drsavage@xxxxxxxxxxxx>]

Michal wrote:

>Oh well, is it safe to use AVGAS in the almost standard 2 litre
>engine? It's minimum octane rating is 100 while the max is 130. I can
>feel the car running better, but it would also burn hotter and maybe
>cause overheating  in the combustion chamber/pistons. (no overall
>overheating has happened yet) I can get AVGAS for about 95 cents
>(about 30cents more than Super) which I can live with, as long as it
>is not going to kill my engine. I may get the car tuned by someone (if
>i really have to) for AVGAS usage, if the engine is safe.

Well, there are really two questions to ask here.  The first is:  "Is it
safe?"  The second is:  "Is it a good idea?"  Whether or not it's safe
depends largely on which grade of avgas you're talking about.  If you're
using 100LL (blue), it's pretty safe to say your engine is not likely to
sustain direct damage.  Higher octane fuels do <not> burn appreciably
hotter than low-octane fuels, so you're not risking burning a valve or
melting a piston.

That said, to attain these high octane levels, these fuels contain <lots>
of lead.  Even 100LL, which is supposed to be a "low lead" fuel, contains
more lead than any auto fuel ever has.  If you're running an even slightly
rich mixture, and running at low power, you're very likely to see
lead-fouling on your spark plugs.  It's also very possible to see lead
deposits build on the exhaust valve seats, and cause poor contact between
the seat and valve, leading to overheated (read:  "valve job") exhaust
valves.  These are very real problems that have to be watched for even in
aircraft engines.  Note that these fuels are made for a very different
application than fuels for an auto.  They're designed to be used in engines
that are generating 100% of their rated power for extended periods, and
operating at 75% of their rated power under <normal> conditions.  (In other
words, for about 99% of their life.)  The high combustion temps associated
with these regimes are what keep the plugs from fouling and prevent the
lead from coming out of solution and plating the exhaust seats.  Your 2
liter engine isn't used this way.  It spends most of its life with the
interiors of its cylinders comparatively cold.  Using 130 octane
exacerbates these problems even more.  Also, these fuels have other
characteristics designed into them--for controling volitility, vapor
pressure, and other factors--that demand specific carburetor and injector
designs.  Your car isn't likely to meter these fuels correctly, so getting
a proper mixture may prove difficult.  Last, if your car has a catalytic
converter on it, any of these fuels will contaminate the catalyst, and can
eventually plug the converter up.

There weren't a whole lot of airplanes that were designed to run on 130
octane fuel, but I have a few pilot's/operations manuals for airplanes that
were, and there are numbers of pages dedicated to the problems of lead
fouling--how to prevent it, how to eliminate it.  In all of them, they
stress exceedingly careful mixture management, and minimizing time spent at
low power settings.  (In my ops manual for the P-51 Mustang, the proceedure
for clearing the lead from the plugs is:  tie the tail of the airplane down
to the pavement; start the engine; run the engine at 60 inches of Hg and
peak mixture for 60 seconds.  "60 in. Hg"!  You're engine will never, ever,
in it's lifetime see combustion temps that high.)

So, will these fuels damage your engine?  Used occasionally, I'd say it's
unlikely.  Run continuously on them, all bets are off.


Rich Wagner
Aeronautical Engineer
Montrose, CO
'82 GTV6 Balocco

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