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Old January 4th, 2018, 01:58 AM
James James is offline
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Join Date: Jun 2013
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Default Re: Regarding vacuum advance and timing

That an interesting article and I agree with it. However there is one engine configuration it does not touch on, it is deceleration comparison between manifold and ported vacuum.

If you are cruising down the highway and you take your foot off of the gas pedal, with manifold vacuum it can climb up to 21-25 " of vacuum. This would place the distributor can in full advance while decelerating. In my opinion (I could be wrong on this) you have a rich idle mixture (due to higher vacuum through a carburetor idle circuit) getting approximately an extra 15° (on top of whatever the centrifugal advance bring in) of ignition advance possibly causing the peak pressure to build before TDC and could be making the engine/car slow down faster. Could also contribute to washing the cylinder down with fuel (from the extra rich idle fuel mixture).

With ported vacuum under the same conditions the distributor can does not move and stays a 0°. Only the centrifugal advance is providing ignition advance base on rpm. This will could allow the engine/car to coast further.

Another point to bring up if you are using ported vacuum and you engine idle is set properly. Then you switch to manifold vacuum to supply your distributor can you may have an engine racing (idling fast) because you are not able to bring the idle back down where it belong. With an automatic transmission this is not acceptable.

I too have seem people disconnect their distributor can thinking they had more engine power. In reality they make more frequent stop for gas.

Disclaimer: The results you get may vary and you may or may not notice any differences.

Myself I always use ported vacuum, even in my hi performance cars/trucks because they are not for racing they are daily drivers.
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