Originally Posted by gpsman1

The way I see it, ethanol adds oxygen. With E40, I'm adding about 8 times more oxygen molecules than gas with MTBE.

The O2 sensors in the CC detect more oxygen. Per look-up table, the computer says "unused oxygen is coming out the tailpipe, thus I need to add more fuel to take advantage of this oxygen."

Isn't that how the feedback loop works?
When I moved from E40 to E10, I reduced the oxygen. At the same time, same RPM, same wheel speed, fuel trim went down 75%. ( from +16 to +4 ) The cars runs a fuel trim of zero with pure gas.

Looks like in my car the Fuel trim adjusts, and the spark timing doesn't.
Maybe if I trick the car to run more lean, the spark will adjust?