Air to fuel ratio... U want to be within a certain range to keep it Safe on ur motor.
If u don't know ur AFRs ur Rollin the dice.
I'm willing to bet ur slightly Leaner on the ethanol which would explain it getting more mpgs and running better. Too lean (not enough fuel) and motor goes boom. Ur basically running a higher octane but not adjusting fuel...I bet it's on the lean side but doesn't sound like that big of a problem in ur situation. If u get better mileage on e-30 as opposed to e-10 than that def what's goin on.
I dump the fuel after it sits for a while because my race car is on a very specific map/tune and the slightest change in octane will throw my afr curve off. Gotta have fresh corn bread in my oven.
Ur good though... E10-30 is not e-85( when it comes to eating lines and precipitating lol). A lot more stable than the shine.
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