Vacuum advance - ported vs direct manifold
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Joined: Oct 2004
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From: Southern CA
Vacuum advance - ported vs direct manifold
The following, contains two articles:
http://www.chevelles.com/forums/show...63&postcount=2
The first, has been floating around the internet (with several different authors) for at least ten years.
Other than "0" vacuum at idle, and low initial spark timing, does this "creative writing exercise" contain any facts?
Norm
http://www.chevelles.com/forums/show...63&postcount=2
The first, has been floating around the internet (with several different authors) for at least ten years.
Originally Posted by John Z
After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it ........
Norm
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From: Southern CA
Ported vacuum dates back to the first use of vacuum advance, during the thirties, and was used until computers took over.
Emissions requirements date back to '55 at the earliest.
Norm
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Joined: Jan 2007
Posts: 490
From: Gladstone, OR
OK Norm, I gotta' ask, is the part about the mixture being lean under cruise conditions correct? I'm guessing no. I would assume the mixture is right at cruise speeds, vacuum is high, load is low and the engine can tolerate more timing.
Don
Don
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From: Southern CA
It's why I started the thread.
Yes it is. High vacuum closes the economizer valve. which restricts fuel flow to the jets.
The right mixture for cruise conditions is lean, which causes increased MPG on the highway.
Not exactly. High vacuum leans the mixture, which requires more timing. Fortunately, that same high vacuum supplies the the right amount of advance, at the right time.
Under load, there is less vacuum, the power valve opens, enriching the mixture, and the advance is reduced by the appropriate amount.
Power goes up, MPG goes down. And vice-versa.
Norm
Under load, there is less vacuum, the power valve opens, enriching the mixture, and the advance is reduced by the appropriate amount.
Power goes up, MPG goes down. And vice-versa.
Norm
Just the facts...
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From: THE GREAT WIDE-OPEN
when did the nox emission standard come into play?
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From: Southern CA
........ when did the nox emission standard come into play?
The kit included a cap for the ported source at the carb.
Norm
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Joined: Mar 2007
Posts: 50,770
From: Northern VA
is Fast Enough ...
Joined: Jul 2008
Posts: 1,308
From: dogtown
Just to clarify, this was a Californina state mandate. People from the rest of the country likely aren't aware of it. I'll never forget the ordeal that a friend of mine experienced where he had moved to SoCal with a Dodge. He had previously installed a fully mechanical distributor. CARB made him remove that distributor and install an original vac advance unit just so he could then install the mandated NOX device that rendered the vac advance inoperative.
Got to luv california ...
I had a freind that put a "Smog" motor in his car just to pass smog ..
We also have smog check points where they set up a sniffer and pull
your car over if it's running dirty ...
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