Thought I would share this with everybody.I keep in my race log book.  http://www.motorsportsracingfuels.com/OctaneCalculator.html

This is just a general rule of thumb that was on the net
Compression ratio = Octane
5:1 =72
6:1 =81
7:1=87
8:1=92
9:1=96
10:1=100
11:1=104
12:1=108
The best way to tell if you need a higher octane (or less timming) is when you rebuild your engine, when you remove your rods and the upper bearing falls out of the rod,you had to much timming in it or you were getting pre ingition. That will always bend the upper rod bearings to were they just fall out. That is why it is so important to check the factory timmimg marks.

One other thing. If you are using aluminum heads you dont need as much octane as the above chart. That chart is for cast iron heads.

Great, we sure need more cars. Good luck

He is right with aluminum heads you dont need as much octane as the above chart.