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Replaced the old vacuum advance on my ’55 President’s 259

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  • Ignition: Replaced the old vacuum advance on my ’55 President’s 259

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  • #2
    Doing your research really paid off.......Well done!


    • #3
      "My research turned up that a working one will benefit most in the low RPM range and have a positive effect on gas mileage."

      VAcuum advance provides additional ignition timing at Small throttle settings ( low manifold vacuum) , at any rpm. Highway cruising with semi-modern cars often cranks in 10 crank degrees or more.
      That is in addition to the rpm controlled centrifugal advance which presumably is already near optimum ( well, pretty good anyway) for full throttle ( ~zero manifold vacuum) operation. That means without V-Adv the cruising ignition timing can be over 10 degrees "retarded" from the optimum for that rpm and load. Think of all the threads where folks have reported being pleased by the results advancing the base timing (and thus every point on the operating curve) 2, 3, or 5 degrees.

      In the 80s For his race engines Smokey Yunick advocated no vacuum advance and sometimes even locked timing ( no centrifugal). I believe the rational was simplicity and solid reliability.

      For passenger cars his recommendations were far different. See the "Smokey 350" attachment of 1990 recommendation for 1980 Chevy V8 road van 305, not 350).

      A dial back timing light and tach let's me make a map of my actual centrifugal advance curve with the car in the driveway. Add a Mity-Vac vacuum pump and I can map the actual vacuum advance curve too if I keep the test rpm constant. Only Then do I know where I stand compared to the specs in the shop manual, which is always my starting point. "First, make it stock." (After I confirm the TDC marks on the damper are accurate. ) TDC can also be confirmed with decent accuracy with the engine assembled, and in the car).

      Twisting the distributor a little is not going to compensate very well for seized centrifugal advance, or, as I believe you are about to discover, an in-operative Vac adv.
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