Does a custom or handheld tune lose its gains when you switch to running E85 instead of premium?

A gasoline tune isn't set up for E85, so it can't use E85's advantages — and it may not fuel it correctly.

• E85 needs about 30–40% more fuel than gasoline, and the tune must be adjusted for it (Performance Racing Industry).
• E85's knock resistance allows more timing (and boost), but only once the calibration is made for it.
• Without flex-fuel logic, a tune is built for one ethanol content. Running a different fuel than it was calibrated for means the fueling and timing no longer match.

If you want to switch between premium and E85, you need a flex-fuel tune with an ethanol sensor, or separate tunes. See fuel, E85 & boost.

Want to do it yourself? See who tunes it, and how to learn.