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when the engine is at idle you have a about 20" of hg of vaccum in your intake manifold what you slam on the pedal and put a demand for power on the engine what vaccum inside the manifold drops for a second to 0" of hg until the motor stablizes and creates vaccum again. When you push the pedal to wide of throttle the engine requires an advancment in ignition timeing to stop detonation in the cylinder. when the vaccum drops theres a mechinsim in the distributor that sets the time more advanced to prevent detination and to get the most power out of the engine when it is needed
 

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Discussion Starter · #3 ·
How does it work inside the distributor though? What is the mechanism? Is there some kind of switch with in the distributor that says "without this much vacuum, you do this?"
 

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The Vac advance is a diaphragm with a spring. When vacuum exceeds spring pressure, it pulls a lever back. The lever attaches to a plate in the distributer that the points or pickup coil is mounted to. Rotating the plate alters the timing.
 

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Discussion Starter · #6 ·
wow,
You guys are great!:bigthumbsup
So as the vacuum goes closer to 0", the engine would, in essence, be out of time and vacuum advance corrects this imbalance?
 

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The engine runs better with different timing points for different power demands. The vac advance helps keep the timing at the optimal level for the engine load.
 

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wow,
You guys are great!:bigthumbsup
So as the vacuum goes closer to 0", the engine would, in essence, be out of time and vacuum advance corrects this imbalance?
Not exactly, your engine will create vacuum at idle, cruising, and during part throttle acceleration. During these times the engine can benefit, and tolerate, more timing. So the vacuum canister uses this vacuum to increase the timing of your ignition, usually around 10 deg. or so. When you run at WOT (Wide Open Throttle), the vacuum drops to 0 and your timing returns to it's initial setting, usually around 8-12 degrees, thus preventing detonation. Then the mechanical advance comes into play, but that's a totally different mechanism. A vacuum advance improves throttle response and driveablility, but is totally out of the equation during full throttle runs. If you were running your engine without the vacuum advance connected you'd have terrible part throttle response, and surges while cruising. So to sum it up; the more vacuum (less load) your engine creates the more advance the vacuum canister imparts, the less vacuum (more load) the less advance.
 

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I suggest that you connect the vacuum advance canister to full manifold vacuum. This is when there is vacuum to the can at idle. Most carbs have a vac. advance port that doesn't apply vacuum at idle, only when the throttle is opened. That port needs to be capped off.

You'll get a smoother idle, better throttle response, cooler engine temps, including at idle and better fuel milage. If you've got an engine with a lopey cam, the idle will be smoother with full manifold vacuum. This can be a benefit to those who have a hard time keeping their engine running with a lopey cam and an auto trans.
 

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There is a very good article available that explains this extremely well and thanks to The Greek he provided a link to it previously.

Quoting from his past post,
"It is explained at the following web site. Even though it discusses the application with Chevy engines, the principals are the same no matter who makes the engine.

www.lbfun.com/Corvette/Tech/vettetech.html

Scroll down and click on to "Timing and Vacuum Advance" then click on to the article "Vacuum Explained" by Lars Grimsrud."
 
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