Two things cause detonation. Too much timing advance and/or not enough fuel. Get it fixed right away or return it to stock tune. Continued detonation will either break the rings or put a hole in a piston. Its also hard on the crankshaft and rods.

 

never saw 86... thought federal minimun was 87

 

I've only ever seen 86 in Lubbock and Amarillo. In Dallas 87 is normal

 

Some of the mountain states sell 85 as the minimum. The higher altitude lets the engines run better on lower octanes.