Quote:
Originally Posted by CSHADES
...you want more advance as the rpms increase that is why you go with ported vacuum.
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This is not correct. Mechanical advance takes care of more advance as the rpms increase. Vacuum advance is to correct the timing for the thinner mixture at high manifold vacuum. Thinner mixtures burn slower. It is a matter of combustion chemistry.
Consider that ported vacuum *immediately* goes to full vacuum advance as soon as you crack the throttle. Why? What changed? Nothing.
Ported vacuum was part of emissions controls introduced on January 1, 1968. I know. I was there and working on cars when it changed. From 1938 through 1967, the Big Three exclusively used manifold vacuum for advance. The GTOs, the Camaros, the Mustangs, the Plymouth Max Wedge, the hemis, the 427 side-oiler, all those cars used manifold vacuum, as did all the tiny engines like the 195 straight-six Chevy and the 225 slant-six Chrysler.
It wasn't because they were stupid all those years and didn't know how engines worked. They changed to ported vacuum in 1968 to get a late spark at idle, so unburned charge would dump into the exhaust manifold, into which they pumped air with the A.I.R. pump in order to burn up pollutants that caused smog in stopped traffic in big cities. As soon as you wanted some power, the advance had to go all in, to use the full charge in the cylinder, so you got full vacuum advance as soon as you cracked the throttle.
Read this post:
http://67-72chevytrucks.com/vboard/s...d.php?t=689321