[Christ]

Stop Stop Stop STOP.

Don't re-adjust your timing... instead, do the following:

1. Seafoam your engine. If it has more than 50,000 miles on it, this is one of the essential things to do. Follow the instructions explicitly.
2. Change to a colder range plug. One range colder could stop the detonation on it's own.
3. An often over-looked quickie, good for a slight boost in economy and power (not even noticeable) is indexing your spark plugs. Mark them where the electrode's open side is. When you put them in, place that mark as close to the intake port as possible. Voila, you're done.

Do those things in that order, and pinging should stop. If it does not, you need to consider cooler intake charge, or faster intake velocity. (Get both... cut a big hole in the bottom of your intake box.)

SeaFoam is one of my favorite products, and I firmly believe that it works as well as anyone has said. Unless they said it added a turbo and intercooler for them.. or something. Then they're full of it.

You could also try running the thinnest oil that your climate will allow... or running thinner oil in the summer. Also check/clean your PCV system... the oil getting sprayed into your intake stream causes detonation.