Mixing 100LL with gasoline

[rwanttaja]

I run a mix of 100LL and auto fuel (no ethanol) in my Fly Baby, and have had no problems with it. I generally try to make every fourth fill 100LL; it has four times the lead of the old 80 octane avgas, so I'm figuring my Continental is getting the right about of lead.

I usually switch to running 100% 100LL in late fall, as I fly less in the winter and avgas is more stable. I've used this system for the ~23 years I've had my airplane, but this spring was the first time I developed apparent lead-fouling. I did some leaning to burn it out, and with the arrival of spring have switched back to car gas. Problem has gone away.

Performance wise, one must remember WHAT the significance of octane is: It is to provide resistance to pre-detonation, or knock. Adding lead is the easiest way to do this, in fact, I believe it's the ONLY economic way to do this above ~93 octane. If one has an engine designed to run on 80 octane fuel, it derives no benefit from a higher-octane gas unless the engine's timing is adjusted to take advantage of it. And if one goes back to 80 octane, one must re-adjust the timing or significant detonation will occur.

Modern car engines have electronic ignition and knock sensors, and can automagically adjust for changing octane levels of the fuel. Don't see that in aircraft engines.

The ironic thing about this is that 100LL fuel actually had LESS energy vs. unit volume than car gas, because of the lead.

Ron "It's a Gas" Wanttaja