Trek724

Ford recommends 91 octane for the 5.0, but I see custom tunes offered in 87,89, 91 and 93 octane. Would the tunes at 87 or 89 not be good for the motor?

AzPete

Tune = change of how the engine is set to handle what you give it....and how you run it.

The 5.0 will run on any octane fuel...you just loose 10HP on 87 vs 93....research.....

JIM5.0

For custom tunes, you MUST use the octane gas equal to or Higher than what the tune is written for. If you have an 87 tune you may use 87 or higher gas.

Bmr4life

Quick correction, Ford recommends 87 octane AKA regular.

Mishri

The PCM is designed to recalibrate based on which octane you are running (A feature first available to Mustangs in the 2008 Bullitt)

It is strongly advised to only use octane greater than or equal to what your tune calls for in the aftermarket tunes. However the PCM can still adjust if you put the wrong type in. A #8 piston issue has been blamed on 87 fuel used in a car running a 93 tune, it did not happen on that tank of gas though.

Yes, 87 can be used in your stock 2011 Mustang GT.

Joenpb

A better example would be a 91 tune, in which you need to use 91 octane or higher gasoline.