Originally Posted By: The_Eric
I've often wondered why we're still using leaded fuel in something like a plane, where reliability is critical. If it is the source of as many issues as you say, it should have been a no-brainer to remove lead a long time ago. Especially when they seem to respond so well to mo-gas or other lead free fuels. I've read plenty of chatter from other pilots who say the same as you and in fact, they're claiming their engines run better all together on fuels other than 100LL.
Detonation resistance is critical in air cooled, hot running, aircraft engines, as any significant detonation leads to outright engine destruction. Remember, cylinder head temps can and do reach 500 deg F in high performance, highly loaded, big bore, aircraft engines.
Unleaded fuels, thus far, have been completely and utterly unable to provide an adequate margin of detonation resistance. While that seems like an absurd statement, consider the requirements. We can, for example use Zylene or 114 octane Toluene to raise octane into the range of acceptability. But getting it started and atomized is nearly impossible without first heating the fuel.
Swift fuels produced a binary fuel of trimethylbenzine (mesitylene) and isopentane, blended to produce about 100+ octane. It meets all of the aviation requirements for detonation resistance. However, Embry-Riddle reported that getting aircraft started on chilly mornings in Florida with 100SF was difficult. More work and tweaking may be necessary.
The bottom line is that up until very recently, it has been near impossible to produce a high octane, non ethanol, viable unleaded.
In fact, I'm still not sure there is a viable replacement.
Certainly, VP Racing fuels produces a number of high octane, non ethanol fuels. None of them are viable aviation fuels, and none of them are high enough octane under high temperature, high boost, big bore conditions. Remember that bigger bores are far more sensitive and likely to experience to detonation.
Even 100LL is sometimes not good enough in some "pedestrian" aircraft engines. I had this happen to me: