Join Date: Sep 1999
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Re: vacuum advance question
I don't know JACK S**T about HEI but I think you are barking up the wrong tree getting all twisted up about vacume.
Vacume is a direct result of throttle plate opening. You are trying to suck a given volume ( displacement of engine ) through a variable opening (carb throat)
When the throttle plate is closed as in ideling the opening is small meaning ( hopefully ) high vacume
when it is WOT Wide Open Throttle the opening is large meaning low vacume.
Ported vacume does not come into play till around 1000 rpm or so, after that manifold and ported vacume run In-Synch (And I don't mean the group) no matter how fast or slow you go untill you take your foot off the peddle.
Like I said I do not know Jack about HEI but most older engines I have tuned tell you to disconnect and plug the vacume line, so the timing is not affected by idle speed.
Are you sure you are not out by 180*