We have posted in the past the ill effects of being forced to use E10 gas ( feds require E15 by mid year) for performance as it takes more volume of E10 to produce the same level of energy then straight gas.

Where straight gas at part throttle would require a 14.7:1 AFR due to E10 causing a leaner AFR for best performance the NON WOT engine conditions for EPA would require at the least the PCM tuned for a richer AFR of 14.2:1

We obtained straight 91 octane gas to compare it to other tests where E10 was used to see what the effects are

The PCM had been tuned for E10 so that the fuel trims which were at 8% too lean back to around zero

Leaving that tune in and using straight gas and the engine being 11.3:1 compression we see where the fuel trims were at zero with E10 that now with straight gas the average fuel trims are now -5% rich with a maximum of -11% too rich.

Notice WOT timing was at 29 degrees, zero knock and misfire.

Clearly gives a image that in using E10 your PCM would be running 5 to 10% too lean ( leaner in hot weather) which induces engine knock and lower timing to reduce HP and Tq

Since E15 will be required by EPA around mid year you can expect your engine to be running even leaner and less performance unless you tune the PCM to solve the issues of using ethanol