If it's truely a 15 to 1 piston you need to call a race fuel supplier and tell them to send the most expensive race gas they have to you. 116 should be about right.

 

Look the point to it is, they can put whatever they want on the box, that piston may fit 4 or 5 different engines. What I am trying to say is the only true way to find out what's going on inside is by knowing thing entire geometry of the cylinder. You can then get an exact compression ratio. Without it its all just a guess of what the compression ratio is. Do not go by what's on the side of the box

 

^^^^ Good advice right there.

When the bore is small, even a few CC's difference in the head can make a BIG difference in compression.

 

THe fact that he said he has already run 110/91 mix through it would tell me it is definatley NOT 15:1. If it was he would have already melted the piston... I'm going to go out on a limb and say his paperwork is a misprint and it is more likely 11.5:1.

I would however be on the phone talking to whoever you purchased the kit through. They need to clarify what it is they sold you.