I HEARD IF YOU PUT ANY PISTION IN WITH A COMPRESSION OVER 12.1:1 THAT YOU HAVE TO USE AV GAS. IS THERE ANY TRUTH TO THAT?? IF I PUT IN A 12.5:1 SHOULD I RUN OTHER GAS??

 

i've seen 400ex's with only 11:1 that needed higher octane than the pump fuel could provide. i know more than a few guys that run 11.5:1 and have to mix half pump and half race fuel to keep the engine from "pinging".
if you want to learn more about octane here's a link.Octane link
i haven't met anyone who runs 12.5:1 but i'd expect straight race fuel would be required.