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Vacuum Advance

To: "Dan DiBiase" <>, <>,
Subject: Vacuum Advance
From: "R. O. Lindsay" <>
Date: Thu, 01 Feb 2001 10:46:08 -0600
Hello gang,
   The original designs for vacuum advance used a port on
the carburetor that was connected to a small opening in
the throat of the carburetor.  At idle, that opening is almost
completely covered by the throttle plate.  Little-to-no
vacuum is then present on the port when the throttle is closed.
As the throttle is opened, the vacuum rises (depresses?) and
the vacuum advance module goes to work.  My vacuum module
is a 5-17-10 model.  That is, no action until 5psi depression
(meaning no advance at all when the throttle is closed) and a
maximum of 20 degrees <crankshaft> advance when the
depression is at 17 psi. or greater.
   Now for the problem.  Newer designs have a vacuum port on
the inlet manifold rather than the carb.  That port shows
pretty strong vacuum even at idle.  My car has BOTH ports.
Which method of applying advance is BETTER for my engine
and my application?  Emissions are not an issue.  Recalling, the
car is a '70 B-GT to be used for primarily for spirited street
driving.  It is equipped with dual HS4 carbs.
   Thanks for all constructive advice.



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