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post #7 of (permalink) Old 05-22-2005, 02:57 AM
Way Outta Control
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Re: vacuum advance question

""""Ported vacume does not come into play till around 1000 rpm or so,""""
True - usually.

"""""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.""""

Maybe "in sync," but opposite.

-- Ported gets stronger the faster the R's - to about mid throttle, then drops down somewhat. At WOT it's near 0 again.
-- Manifold vacuum is strong at idle, strong at cruise, then drops under heavy load.
Totally different curves - almost opposite.

And -- there are many different types/curves of ported vacuum - depending on WHERE and HOW it's picked off the venturi bore, at the butterfly edge, above it slightly, above it even more etc, hole size, slotted or not etc.
Each slight change in position creates a different curve.

Several uses that require similar, yet different ported curves: Timing advance, EGR, Cannister purge, bowl venting, etc.

Look at a Thermo Quad - about 6-10 different vacuum nipples, each with a different function and curve.

Since his complaint is too much too soon it has to be slowed down.

Yes, if run without any vacuum, ping should go away - IF it was caused by too much advance too soon. But -- the reason vacuum is used is to advance it during light load for more efficiency - better mileage - kinda important at $2.50 a gallon.
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