[N427EF]

I've been burning Mogas E10 on my IO-540 for the last 100 hours.
I have set up a fuel system with a constant flow return line but otherwise
use a standard mechanical fuel pump and a AFP fuel pump.
I use Avgas in one tank for take off and landing and cruise on mogas.
There have been no problems of any kind even using mogas for take off and landing on very hot days.
My Question:
Why does my engine run better on mogas than avgas?
Specifically I am referring to being able to run the engine leaner on mogas than avgas.Today during my BFR, I demonstrated to my flight instructor how at 65% power I can lean the engine down to less than 10GPH and have it run perfectly smooth and switch to avgas and have the engine almost die on me. Enriching the mixture to 11.5 GPH will produce a smooth running engine again. While I generally don't fly that far lean of peak because of speed loss I find the same engine behavior at less than 11GPH on avgas, not killing the engine but running rough.
There is no noticable change in airspeed going from 11.5 GPH avgas to 10.5 GPH mogas.
Looking at engine analyzer data and GAMI spread I find mogas to be often
at a 0.0 GPH spread and avgas somewhere between 1 and 2 GPH.
Someone help me understand this please.

 

[BobTurner]

When you burn gasoline what really burns is gasoline vapor, not the liquid. Avgas has a lower vapor pressure than car gas, so (to exaggerate) you end up with a cylinder full of little liquid drops of avgas which don't ignite well, especially as the mixture gets lean. With car gas and its higher vapor pressure you end up with a cylinder full of vapor which ignites well even as it gets quite lean. This droplet effect is more obvious in engines with a carb. When I had a 182 the mixture distribution was horrible; due to the inertia of the little droplets they all ran down the manifold, not wanting to turn into the cylinders, so the last cylinder at the end of the manifold was always much richer than the first. And the first cylinder would quit first (too lean) as you leaned the engine. This effect was lessened with car gas, as the vaporized gas "turned the corner" more easily and we got a more even air/fuel distribution. You may be seeing some of this effect too, since even with injection all the fuel is going into the intake manifold, and there may be some "evening out" of the better vaporized car gas. This is why the GAMI spread is zero, which in turn is why you can lean more without one cylinder going too lean and quitting.

 

[bret]

I am no expert, and please correct me if I am wrong, but during my dirt bike days, high altitude hill climbs, the bike ran harder on 87. My hypothesis is that at lower mean efficient combustion pressures, the lower octane mixture flame front travels faster across the combustion chamber? No?.......

 

[rocketbob]

Someone help me understand this please.

Just accept it for what it is. I've seen very similar results with 87 mogas and not much worse on E10.

I've said a few times in this forum that I've found that 100LL needs to go away like the buggy whip and dodo bird. Your engine will run better and cleaner if the fuel system is set up properly as you have done.

 

[N427EF]

Thank you

Based on what little I know about Petroleum science, my guess was the higher vapor pressure of 91 octane mogas and the resulting improvement in its atomization that leads to near perfect fuel distribution.
I am certainly happy with the way it is and I am not planning any changes.
I have not yet balanced the fuel injectors and I am not sure if balancing for avgas will ruin the balance for mogas?
Additionally I find that these numbers are not exactly the same at all altitudes
and with different throttle settings, in fact I have noticed on the GAMI spread
graph that the cylinder that peaks first at low altitude will peak last at high altitude. In any case, I plan on leaving things the way they are, just trying to gain a better understanding of what is going on.

 

[rv7charlie]

There's actually more energy in the low octane gas, too. The additives that slow combustion (prevent detonation) don't make power, so there's less actual fuel in higher octane gas.

Charlie