Posted by Carl on March 11, 1999 at 20:45:15:
In Reply to: avgas lead answer posted by Scott S on March 10, 1999 at 10:21:27:
100LL was supposed to be able to be used in engines designed to run on 80/87 fuel and 100/130 fuel. The problem is that it has 10 times more lead than the old 80/87, and the lead tends to come out of suspension. This causes lead fouling on spark plugs and lead deposits on rings and valves. This is real prevalent on the low compression engines and even shows up in the engines designed to run on 100LL. The first thing noticed is spark-plug fouling. Spark-plugs begin to show signs of decreased performance after 15-20 hours where 50hrs= was common on 80/87. More serious is the loss of compresion due to sticky rings and sticky valves. As the deposits build up on the valve stems and seats, the valves don't close completely and the exhaust gas jetting through the partialy open valve causes erosion or burnt valves and seats. Sticking valves can also cause pushrods to bend. Since the fuel is burning so dirty, the oil becomes contaminated, moisture from the atmosphere and the combustion process is absorbed by the oil, and if the engine operating temps arn't high enough to evaporate it, acids form, causing more abnormal wear in the engine even when not running( this is why you want to run with the proper thermstat and avoid short trips).
Lycoming has a special oil additive they require in their engines, and recommend adding TCP to the fuel ( this helps keep the excess lead in suspention).
I've used a lot of 100LL, and in my personal experience engines not designed specifically to run on it(high power high compresion engines) need a lot of maintenance- frequent spark-plug changes, frequent oil changes, oil and fuel additives, and more frequent overhauls. Generators, chainsaws, outboards, pumps, cars and tractors rarely last a full season (3-4 months here in Alaska).
Running 100LL in built, high performance high compresion engines is not as much of a problem especily if you use a spark plug designed to allow the lead deposits to drop out.
If you just want to increase octane or have and engine that needs a little lead( most of the mid-50s to mid-70s engines or later model boat engines) mixing regular unleaded gas with 100LL will boost the octane significantly, lubricate the valves, and avoid the problems of straight 100LL.
In our airplanes we use a 3-1 ratio(3 gals. reg. to 1 gal. 100LL). Using this ratio increases the octane from 87-89(reg unleaded) to about 97 octane(mixed). I've found I can run one tank of mixed then 2 or 3 tanks of straight unleaded before the effects wear off.
most of the low compression engines designed before the mid 50's don't need to use leaded fuel. Lead was originally added to fuel to lubrcate and cushion the valves when engineers began to increase the compresion on engine to increase performance, not to increase octane, that was a by-product. The valves and seats used in todays engines and available for retrofit are harder and don't need lead as a cushion.