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post #16 of 19 Old 02-16-2008, 09:34 AM
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A stock 258 with emissions intact will be hooked to manifold vacuum via a CTO port. Once the coolant reaches a certain temp, the CTO switches over to ported vacuum. So the answer is both, depends on engine temp.

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post #17 of 19 Old 02-16-2008, 10:11 AM
Mike Romain
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Originally Posted by illinicj View Post
So what is a stock 258 vacuum advance hooked to? Ported or manifold?

With no emissions computer, Ported.

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post #18 of 19 Old 02-16-2008, 10:24 AM
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I agree with algmmech with everything except the part about the vacuum increase at the manifold port as the throttle just starts to open, then the vacuum decreases when throttle is increased. But then again, I drive with a heavy foot and have no reason not to believe him. I do a lot of tuning with a vac gauge as well.
I just reread what I wrote, I'm sorry if I mislead anyone about this. What I meant to say, as throttle is gradually opened under no load vacuum will increase. The first initial opening will cause the gauge to drop and then increase. Say for instance, with the engine idling at 500 rpm's vacuum may be at 18" when rpms are increased to say 1500 and held there vacuum will be probably about 20".

The vacuum advance is basically a way for the distributer to monitor when the engine is under load. We ignite the air fuel mixture as the piston is still moving up on the compression stroke, becuse of the amount of time it takes to completely burn the fuel. The mixture should be a controlled burn and not an explosion. We want maximum combustion pressure when the piston is a few degrees after TDC. This will give us maximum power. As the piston is moving faster, we have to ignite the mixture even sooner so the maximum cumbustion pressure is still a few degrees past TDC. With the engine under load there is higher combustion pressures and temperatures that causes a pressure front to be pushed toward the ignited mixture, when these fronts come together we get detonation. This is the metallic pinging sound heard under load. If we ignite the mixture later as with less vacuum advance under load, we can get more of a controlled burn. Ther is also preignition, which is the igniting of the mixture before the spark plug fires. Both detonation and preignition can cause engine damage and actually try to push the piston down while it is still trying to move up on the compression stroke.

This description is rather brief, sorry but this can get really detailed, books have been written on this subject. We can start talking about quench and squash chambers and how the flame front travels across the piston. Ther just isn't enough space for this now.

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post #19 of 19 Old 02-16-2008, 10:59 AM
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Originally Posted by CJ Chet View Post
Manifold vacuum is taken BELOW the carb and is constant. Port vacuum is variable and controlled by the carb (taken off of the carb itself).
That's half right...

Manifold Vacuum can be one taken one of two places...
1. Would be Plenum vacuum, taken just below the carb throttle blades.
This is total manifold vacuum, and will be "Relatively" constant, high anytime the throttle blades are closed and the engine isn't heavily loaded.

2. Would be taken from a runner out on the manifold closer to the cylinders.
This will be a 'Pulsing' type of vacuum and will only be a high vacuum when the intake valve is open.


The correct answer should have been,

Manifold Vacuum is the vacuum created by the collective low pressure cells created when the pistons drop during the intake strokes.
IT is a constantly changing and pulsing vacuum that responds to engine speed, engine loading and throttle opening.

Manifold vacuum is very high at idle and any time the throttle blades are closed,
Is available at high volume for powering things like vacuum assist brakes and heating/cooling passage controls.

Spark Ported Vacuum is a vacuum sample that is taken at the Venturi int he carb or fuel injection throttle body, and is a very smooth type of vacuum signal that responds only to the amount of air being drawn through the carb...
Which makes it ideal for most ignition vacuum sources.

This is a low volume source because any changes in the amount of volume at the throttle body can drastically change the fuel/air mixture in a carb.
Only Ignition vacuum sources and carb internal passages usually use this type of vacuum.
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