Hi David.
Here's an extract from the UK Light Aircraft Associations technical guidelines on fuel.
"With these engines, many of which were originally produced many decades ago, there have
been a variety of different valve seat and valve materials used over the years, and there is a
possibility that some combinations in the field might suffer problems with valve seat recession if
deprived of the dry-lubricating effect of tetraethyl lead in leaded fuels. To guard against this
possibility it is recommended that all users of Continental and Lycoming engines cleared for
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unleaded Mogas use should either use a fuel mixture with 10% 100LL in it or run a tankful of
100LL through the engine at least every 75 running hours to lubricate the valves and valve
seats."
The 24" max manifold effectively means that I cannot operate @ more than 80% of rated power on mogas. The max EGT is a cross check on not getting too lean (provided of course you are not already LOP). These measures are aimed at reducing the chance of detonation.
Regarding the F.I. setup, My company designs and makes automotive F.I. systems. We have also supplied aviation F.I. component parts and ignition systems for 40 years - This puts me in a good place to assess the suitability of component parts for ethanol bearing fuels. In my experience, a LOT of legacy aviation fuel systems are not ethanol fuel tollerant.
Just to be clear - My original post never said that "this would work" or suggested that others did the same, just describing what I do and why then sharing the results - "warts and all"
Regards,
Richard
Richard,
You are correct about components and compatibility, some are and some are not.
The article written by the UKLAA is completely wrong. Seriously this is not true. But it is a myth that has stood the test of time as they do. Simply put TEL is there for one reason only, detonation or anti-knock purposes. The old cast iron heads had a machined seat, when unleaded came along they got recession all of a sudden because the shorter latency of the combustion even ramped up the internal cylinder pressures and temperatures. Change the timing and you can offset that.
Lycoming and TCM engines always had hardened seats, so that was never an issue. But detonation margin for certification purposes is an issue and on turbo charged engines a very real issue in normal flight.
Think of it like this, the fuel is in a liquid vapour state on the inlet valves, and the tiny minuscule amount of TEL (which is not lead) is in that liquid, surrounded by a solvent. Not a very good lubricant. Once combusted, the TEL is transformed to lead oxybromide and comes out past the valve, in the form of a salt. Again, a very bad idea for a lubricant. Literally the TEL could not d anything to help the valve guides or seats even if it wanted to. And on the seats it causes more problems, but that is another story.
Getting TEL out of fuel is very good for the cleanliness of the engine. No risk there.
Memory burst?..In my hangar is an RV7A with O-360, standard installation. Beautiful plane, and Jamie has tried premium mogas with a hot engine on a hot day, on the ground it would not taxi or even take off. I hate to think what would have happened had he been able to take off when the heat soak got him. - BE CAREFUL FOLKS, especially in warm climates.
