I put 91 octane in my car, I thought I had a 91 tune but I don't. Will this mess up the engine, if I put the car back to stock? Or shold I put it on 87, 89, or 93 octane?

 

load a tune that has a lower number, then the
octane rating you can regularly purchase

 

I would install either your 87 or 89 tune (the latter for more performance). You probably could go back to stock without any problems (don't know what all you've done to your car), but those tunes, I'm assuming, are designed with more performance for your car in mind, so why not just use them?!?

Then on your next fill up, put in whatever octane you want and install the appropraite tune.

 

As long asthe octane in the tankis higher thanthat required by the tuneyou've loaded it's not a problem.

 

+1 you will be fine. Only if you uploaded a 91 tune and put in say 87 octane then you would have a problem