I'd just like to add that there is a difference between 'pinging' and 'detonation' Usually detonation is louder, becuase it is the mixture exploding before a spark, or shortly after the spark as mentioned before. 'Pinging' is basically when you are running too far advanced for your combo, heads and all. if your have 2 fuel that burn at the same rate, with your ignition too far advanced, changine fuels wont help at all, the flame is burning and creating the most pressure before the pistons has gone over tdc, slamming it back down onto the rod bearing, and piston pin. Very bad thing, it ruins bearings and other things. One example, look at an engine with a high advance rate, its hard to start, because the cylinders are trying to make power before tdc, and it is trying to reverse the motor. As the rpms speed up, the piston is going up faster, and with mechanical advance, in a perfect world, the most power comes just as the piston is rounding tdc, getting the most power from the combustion. So basically if you are running high compression, with a small cam its all octane related, if you are running a fairly mild engine, basically 8.5:1 compression, and you have just done your timing, it is probably too much advance, and you should back it off a little