Spark port on my Holley 1946 - manifold vs. ported vacuum

Greetings Ford Six Fans,

Have a Holley 1946 that came with the '80 200 in my '62 Ranchero. In the "Introductions" forum, got into a discussion about which port to use for the vacuum advance on the distributor, and manifold vs. ported vacuum. I felt like the "Introductions" forum wasn't really meant for such an in-depth discussion, so starting another thread here.

Read the article hear in the "Hardcore Tech" forum about manifold vs. ported vacuum. Have seen both in the Falcon Six Handbook and in David's post that the DSII vacuum advance should be connected to a ported vacuum source. The distributor has some upper bushing wear and I'm unsure if the vacuum advance cannister is operating properly, so it may be replaced soon, and the carb will be replaced eventually. However, I'm curious.

If I understand corrrectly, "manifold" vacuum comes from a carb port _below_ the throttle plates (exposed to full manifold vacuum) and "ported" vacuum comes from a carb port _above" the throttle plates (exposed to less than full manifold vacuum). By that logic, the vacuum port labeled as "spark port" on the Holley 1946 carb diagram (from the "Carb Doctor" web site) is a manifold vacuum port, and the vacuum ports labeled as "EGR vacuum" and "Venturi vacuum" are ported vacuum ports (see embedded images).





I currently have my distributor vacuum advance hooked up to the "spark port", which I would think would be full manifold vacuum. This seems contradictory to me, since I'm following the diagram, but everyone says the vacuum advance should be hooked up to a ported vacuum source.

I'm going to check all three of these ports with a vacuum gauge, both at idle and at 2,000 RPM or so. I may try hooking up the vacuum advance to the other ports to check the result. Currently, the vacuum advance is advancing the engine about 20 degrees at idle, which I know is not correct.

Bob the Builder

The distributor has some upper bushing wear and I'm unsure if the vacuum advance cannister is operating properly, so it may be replaced soon, and the carb will be replaced eventually. However, I'm curious.

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I would say the vacuum advance is for sure working since your timing is advancing when you hook it up. Some of the can's are adjustable to vary the amount of advance. What idle RPM are you using? Using too high of an idle RPM would could cause trouble too.

If I understand corrrectly, "manifold" vacuum comes from a carb port _below_ the throttle plates (exposed to full manifold vacuum) and "ported" vacuum comes from a carb port _above" the throttle plates (exposed to less than full manifold vacuum).

Click to expand...

Right basically a carb's Ported Vacuum source can be a "Edited: Manifold, a "Venture, vacuum or a combo of both", if you have the carb off you can turn it over and see were the vacuum source is picked up just above the throttle bade, at the correct idle speed there would be no vacuum. As the throttle is opened up and RPM rises this port is uncovered. A manifold source is picked up below the throttle blades.When the carb is disassemble it's pretty easy to see where the different passages go. If you had some compressed air you could blow some though the ports and feel which tube it comes out of. You may be able to slide a very small dia. wire through it too to see were it comes out. If you have it already together than the vacuum gauge test is the easier way.

bubba22349":2jrpv0kh said:

The distributor has some upper bushing wear and I'm unsure if the vacuum advance cannister is operating properly, so it may be replaced soon, and the carb will be replaced eventually. However, I'm curious.

Click to expand...

I would say the vacuum advance is for sure working since your timing is advancing when you hook it up. Some of the can's are adjustable to vary the amount of advance. What idle RPM are you using? Using too high of an idle RPM would could cause trouble too.

If I understand corrrectly, "manifold" vacuum comes from a carb port _below_ the throttle plates (exposed to full manifold vacuum) and "ported" vacuum comes from a carb port _above" the throttle plates (exposed to less than full manifold vacuum).

Click to expand...

Right basically a carb's Ported Vacuum source can be a "Edited: Manifold, a "Venture, vacuum or a combo of both", if you have the carb off you can turn it over and see were the vacuum source is picked up just above the throttle bade, at the correct idle speed there would be no vacuum. As the throttle is opened up and RPM rises this port is uncovered. A manifold source is picked up below the throttle blades.When the carb is disassemble it's pretty easy to see where the different passages go. If you had some compressed air you could blow some though the ports and feel which tube it comes out of. You may be able to slide a very small dia. wire through it too to see were it comes out. If you have it already together than the vacuum gauge test is the easier way.

Click to expand...

Idle is maybe 1,000 RPM in park. A little high, I know, but I don't think it's so high as to cause the timing to become way advanced. When I say the vacuum advance cannister may not be operating properly, I'm thinking it advances too much, maybe from a weak or broken spring.

Bob the Builder

1000 in park may be a little high usually its about 700 to 750, try to get about 475 to 550 idle RPM in drive (set the parking brake) this is the correct way to set the curb idle speed for an auto trans car the lower you can get idle the better if it will idle smoothly and is easier on the trans when you put it in gear. At 1200 to 1500 you would be out of the idle circuit of the carb.

"Spark Port" is most definitely a ported source, not manifold.
The photos in those pictures are labeled differently than the Ford manuals.
I am currently running my dizzy off the "venturi vacuum port, works fine. There's is no discernible difference between that one and the EGR port on the passenger side. You can hook up a vacuum guage and compare to verify this.
RPM at idle should not change when hooked up to ported vacuum or not.
What is the timing with no vacuum to the dizzy?

JackFish":1uding08 said:

"Spark Port" is most definitely a ported source, not manifold.
The photos in those pictures are labeled differently than the Ford manuals.
I am currently running my dizzy off the "venturi vacuum port, works fine. There's is no discernible difference between that one and the EGR port on the passenger side. You can hook up a vacuum guage and compare to verify this.
RPM at idle should not change when hooked up to ported vacuum or not.
What is the timing with no vacuum to the dizzy?

Click to expand...

Hi Jackfish,

It's good to find someone who's actually running this same carb! I expect that you still have all the emissions / electronic management features hooked up? (I do not)

Is there any chance you could scan or copy the Holley 1946 diagram from your Ford manual and send it to me? I have a Ford manual for my '62, but of course the carbs were totally different back then.

Using the port labeled "spark port" on my diagram, there is definitely a large (about 20 degree) timing advance when connected, and RPM actually goes down when connected. I can feel the vacuum on the port labeled "spark port" with my finger; not so on the ports labeled "EGR vacuum" or "venturi vacuum". I intend to test with vacuum gauge.

I set initial timing (vacuum advance disconnected) at 12 degree BTDC, but have it backed off now to about 2 degrees BTDC because the vacuum advances it so much.

Thank you,
Bob the Builder

bubba22349":3oryr8cq said:

...basically a Ported Vacuum source is a "Ventura vacuum"...

Click to expand...

Not really, unless you're referring to the Pontiac Ventura, in which case I have no idea. :mrgreen:

Venturi vacuum is taken from right at the venturi (like in the dreaded Load-O-Matic distributors), and increases with airflow (engine rpm). Ported vacuum is taken from just above the throttle blade, and is non-existent at idle but then jumps up high as the throttle is cracked open and then drops as it opens further towards WOT. Manifold vacuum is taken from anywhere below the throttle blade (carb or manifold), and is highest at idle and goes down as the throttle opens. The ONLY difference between Ported and Manifold is at idle, otherwise they're same/same (the distributor's ignition curve will probably be different, but that's another thing)

With Ported vacuum, hooking up the dizzy's hose won't change anything (timing or engine rpm) - at normal idle.

With Manifold vacuum, hooking up the dizzy's hose will increase the ignition timing, which will increase the engine rpm. Normal procedure is to adjust the carb mixture and idle speed, disconnect/plug the hose, set the static/initial timing, re-adjust the carb mixture and idle speed, connect the hose, then re-adjust (re-check, really) the carb mixture and lower the idle speed to spec.

Unless you've got an early carb with the Spark-Control Valve for the dreaded Load-O-Matic, you won't have a way to use Venturi vacuum, only Ported or Manifold.

Excellent reading in manifold vs. ported vacuum from a former GM engineer who loves and deals with ignition: http://www.corvetteactioncenter.com/for ... vance.html

Read it all, if you want to learn a LOT about ignition timing, but his bottom line is that "for peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively."

Of course, you'll also want to make sure your distributor's ignition curve isn't one of the factory emissions ones he speaks of, which leads us to recurving your distributor - several good stickies around here on that topic...

The long and the short of it is this: If you don't want to recurve your dizzy, then try Ported and Manifold and use whichever one you like best. If you recurve (or at least confirm your ignition curve (not all that hard to do, really), then use Manifold Vacuum and be done with it.

JackFish":2miep4i4 said:

"Spark Port" is most definitely a ported source, not manifold.
The photos in those pictures are labeled differently than the Ford manuals.
I am currently running my dizzy off the "venturi vacuum port, works fine. There's is no discernible difference between that one and the EGR port on the passenger side. You can hook up a vacuum guage and compare to verify this.
RPM at idle should not change when hooked up to ported vacuum or not.
What is the timing with no vacuum to the dizzy?

Click to expand...

Tonight I hooked up a vacuum gauge to each of the three vacuum ports on my Holley 1946:

"Spark" port - 15" at idle, 15" at 2000 RPM, staying at 15" as RPM increased further
"EGR" port - 7.5" at idle, 15" at 2000 RPM, increasing slightly as RPM increased further
"Venturi" port - 0 both at idle and at 2000 RPM

I hooked up the DS II vacuum advance to the "EGR" port. Now it doesn't advance too far at idle, but it advances at higher RPM.

Bob the Builder

That might work