if you only have 91 octane gas where you live, then only use the 91 octane tune...

the diablo tune, for 93 octane gas, has higher timing/more spark advance for higher octane fuel... so if you run that tune on only 91 octane gas, you'll get knock...
I still can not find this "91 octane tune" after ...

yeah the diablosport tune is safe for 91 and 93 octane also known as premium fuel. For people with 93 octane tune they can add a couple degrees of timing (10% advance) to the performance tune. otherwise they will just use the tune as is if they have 91 octane in the tank.

monte is right that you ...

confused... I was informed that the performance tune was a 93 octane tune.

I have a u7198 and just updated the latest software. However, I could not find an option for a 91 octane tune (CA). The options I saw were Performance tune and Factory HP tune. How do I access a 91 octane tune and second how do I disable skip shift (02 ls1 Camaro) Thank you!