Manifold or ported vacuum?
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Welder User
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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.
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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.
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.
Fleet Mechanic
Joined: Nov 2002
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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.
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.
Fleet Mechanic
Joined: Nov 2002
Posts: 1,643
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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.
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