Check out the links rob posted, and the description he gave, they will show you how to figure out which distributor you have.
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so with a higher compression engine you want the mechanical advance to come in sooner or later?
Depends on a combination of cam and compression. If you have low compression ratio, you generally want the timing in earlier. If you have a higher compression engine with a big cam, you generally want the timing in earlier. If you have decent compression and a stock or mild cam, you may want it in a bit later if you're having detonation problems. If you're running crappy gas, you may want the timing in a little later.
