Stock Tune Ethanol Limit

[shogun32]

the ECU constantly monitors for knock and O2. The factory tunes also have limits as to how far/fast it can advance spark etc. A few gallons of E85 won't do anything. If you want to take advantage of the higher octane you need a tune if for no other reason than to move the limits and ramp rates on timing and injector pulse width etc. A full tank of exclusively E85 is probably a lousy idea but shouldn't be a death warrant for the engine.

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[AZ18yote]

This is why I use a Flex tune in many situations. Granted it's made for switching between fuels and mixing. The E85R tune is good to 70% alch I believe. Anything lower, then you'll want to use a flex tune. They have some safety measures built in like that to the tune to compensate for pump variations. It's good to find a good E85 station that is consistent.

This. Flex tune checks all the boxes.