Manifold or ported vacuum?

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Feb 12, 2015 | 09:44 AM

tinman52

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From: northwest MT

Manifold or ported vacuum?

Is the vacuum advance on the DS II supposed to be hooked to manifold or ported vacuum? Seems to idle smoother on ported.......it's a single inlet on the vac can. 4 bbl carb and dual exhaust, otherwise stock engine.

Feb 12, 2015 | 09:52 AM

1986F150six

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From: Sheffield, AL

Originally, ported.

Feb 12, 2015 | 12:23 PM

Harte3

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From: Spokane, WA

Try it both ways and use whichever runs the best.

Feb 12, 2015 | 01:59 PM

AbandonedBronco

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From: Boise, Idaho

Quote:

Originally Posted by Harte3

Try it both ways and use whichever runs the best.

x2

When I had a 600cfm Holley on mine, it liked ported vacuum.
My current 465cfm Holley likes manifold vacuum.

Neither is going to hurt anything except maybe on an emissions test, so pick whichever makes the engine happier.

You'll more than likely have to reset the idle mixture and idle speed when switching between the two, so don't just swap the hose back and forth between the ports.

Feb 12, 2015 | 04:51 PM

Harte3

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From: Spokane, WA

"... more than likely have to reset the idle mixture and idle speed when switching between the two..."

X2

Feb 12, 2015 | 05:07 PM

The Frenchtown Flyer

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WHY I LIKE A PORTED SIGNAL TO THE VACUUM ADVANCE:
If you use manifold vacuum to the distributor at idle the manifold vacuum is high. Hence the vacuum advance is probably "all in". That results in high idle speed which is resolved by closing down the idle plate(s) to achieve the correct idle speed.
Now if the driver puts his foot down on the gas pedal the throttle plate(s) open and the vacuum in the intake plunges [manifold pressure rises abruptly]. Suddenly all that spark advance that was there due to manifold vacuum is gone - there is a big hole - torque drops precipitously - and the vehicle hesitates or stumbles.

Whereas with a ported signal there is less spark at idle - usually between none and 5 inches mercury vs 15 - 20 in. hg. in the manifold. So the throttle is open wider, there is not a large drop-off in vacuum signal as the throttle plate is opened, and acceleration is smoother.

Plus, emissions are reduced. Win / win.

Feb 12, 2015 | 05:12 PM

The Frenchtown Flyer

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Try it on your own vehicle using a vacuum gage. Better yet, use two vacuum gages - one hooked to manifold and one hooked to ported signal - so you can see the difference in real time between the two.