I always thought that most distributors used both mechanical and vacuum advance, and that the vacuum advance kicked in first because the distributor wasn't spinning fast enough to throw the weights out. My understanding has been that, any given engine spinning at idle needs X degrees of advance to ignite the mixture at the proper time in the cylinder. When the same engine accelerates up to a higher rpm, X degrees of advance was no longer the right time because the engine was spinning faster essentially making X degrees retarding the timing. And that's where mechanical and or vacuum advance kick in to rotate the distributor internals to fire earlier in order to iginte the mixture sooner. I understand that vacuum is high at idle because the throttle plates are closed, decreases as they are opened and basically the same as outside at WOT, but the engine hardly ever reaches WOT. Why set base timing then, if you are going to use full advance at idle? Would full advance then become the "base" timing? I guess I just don't understand why the manufacturer would recommmend base timing settings and have mechanical or vacuum advance if it's possible to just go with full advance at idle....why the two if you can just go full advance until vacuum falls off? It's just totally backwards of what I've always read, heard, and been told. I'm not saying it's wrong at all, just confuses me. Is there an article on this particular subject I could read, or is it an easy explanation? What's the thinking there? I appreciate any more light you can shed on my darkness. Thanks, Josh
