LDUBS said:
I'm going to admit the octane discussion is way over my head. But, I do have a question. If the leaded octane fuel is such a big maintenance issue, why does aviation gaso have so much lead in it?
In aircraft engines the valve seats and valve material can't be changed without getting a STC (Supplemental Type Certificate), which is very expensive to get. So the same materials are used for replacement parts at overhaul as what the engine was originally certified with. Those certifications (called a Type Certificate) were obtained back in the 40's and 50's when the engine was originally designed and certified.
So, unfortunately, they still require the lead for valve seat lubrication, and these are air-cooled engines that run cylinder head temps of 435-450F at full power, 375 at normal cruise power. Lubricating oils formulated for automotive engines can't withstand those kinds of temperatures, and neither can automotive fuels - vapor pressure is too high with automotive fuels. There are STC's you can get for some engines (like the Continental O-200) to allow use of automotive gasoline. But the STC limits the altitude the aircraft can be operated at, and shortens the TBO (Time Between Overhaul).
The general aviation industry is badly in need of more modern engine designs that can use unleaded fuels. But it is too expensive to get the certifications for it, and the general aviation market can't bear the cost. Commercial aviation switched to turbine power long ago. But general aviation can't bear the cost of turboshaft engines either - a typical small Pratt&Whitney Canada PT6 engine costs $1.1 million, where the piston equivalent costs ~$80,000.
Running 100LL in your outboard at the cold temperatures that outboard engines run at will only cause extreme lead/carbon buildup on spark plugs, piston crown, combustion chamber head and exhaust ports. It will build up in the top ring land and seize the ring, resulting in a scored cylinder.
Unfortunately, this also happens to aircraft engines. Take like a Seneca V with twin turbocharged Continental TSIO-360's, the TBO is 1,800 hrs. But the engines will have to be "topped" at around 1,000 hrs, meaning the cylinders and pistons are replaced with new to make it to TBO, without removing the engines from the airframe. The cost to "top" a pair of TSIO-360's is around $37,000, and major overhaul is ~$57,000 apiece. The cost of the airplane is $1.15 million new for a 2022 model. This is just a light personal twin. If they replaced those two piston engines with a couple small PT6 turbines, the cost of the airplane would be over $3 million. Turbine powered GA personal airplanes are very expensive. A single-engine TBM940 is a $4.5 million dollar airplane and it costs $744/hr to stick the key in the switch and start it.
https://air.one/aircraft-showroom/daher-tbm-940
This is an accepted part of the cost of flying airplanes - nothing about them is cheap. It's not an accepted part of operating outboard motors.
Some people still do run avgas in certain 1960's muscle car engines. But if you put that engine on a dyno and compare 100LL avgas to Sunoco Optima 95 race gas, you'll find you get way more power with the race gas. 100LL avgas is 112,387 BTU/gallon (18,700 BTU/lb). The old MTBE formulated automotive gasoline was 124,880 BTU/gallon. E10 87 regular automotive gas is 114,100 BTU/gallon. So basically, 100LL avgas does not even have the energy content per gallon that today's regular E10 87 octane pump gas has.
