What 2-bbl BBD port for connection to dash-mounted vacuum gauge?

[MoPar~Man]

I have a clone BBD which is probably based on 1977+ design. On the back side of the throttle plate are 2 large ports - one goes to PVC, the other is capped (I'm told this capped port might be for brake booster on later cars, mine comes from a port on the intake manifold). In between these 2 large ports but on the carb body is the smaller port for choke pull-off.

My basic question is, does the choke pull-off port give a good (full) manifold vacuum signal? Or should I tap into the unused larger port?

I could get this from the small nipple that comes off the brake booster, but this comes from intake manifold runner for I think CYL 7 so there is likely going to be bounce in this signal if I take it from there I think.

 

[CBODY67]

Seems like there was a particular emissions control device which attached to a port that measured venturi vacuum rather than manifold vacuum?

Where does the hvac vac come from? You could always tee into that supply (line for the vac actuators) to run your interior vac gauge?

Just some thoughts,
CBODY67

 

[MrMoparCHP]

On my 67 Barracuda the factory used the port on the intake manifold for the vacuum gauge, same port as the power brakes and AC controls.


Alan

 

[cbarge]

Your choke pull off has constant vacuum.
You can tee into it for your vacuum guage.
Did that on my 67 Monaco for my remote guage for fine tuning. ( console has the Tach)
Did you ask sonething similar about this in another thread few months ago??
Carter BBD for 318 - vacuum port identification on clone "Super 6" carb

 

[Toolmanmike]

A vacuum gauge should read constant vacuum (below the throttle plates on the carb or from the intake manifold)

 

[MoPar~Man]

Your choke pull off has constant vacuum.
You can tee into it for your vacuum guage.
Did that on my 67 Monaco for my remote guage for fine tuning. ( console has the Tach)
Did you ask sonething similar about this in another thread few months ago??
Carter BBD for 318 - vacuum port identification on clone "Super 6" carb

I did, but I didn't think I asked about best place to connect a full-time vac guage. Referencing that post, I wonder if the large pipe in the front-center (canister purge port?) might be a good candidate - if it has direct path to the center hole on the underside of the throttle plate. Taking it off the intake manifold port, which comes off a runner to a specific cylinder I think, would not necessarily give me a smooth signal. The '67 Barracuda had a factory vac guage option? I find that - curious.

I don't have A/C, nor any interior vacuum-operated devices - exept for the rear shelf flow-through air gate, which comes off the brake booster plastic fitting, which yes I could tap that but I think it's too heavily influenced by the vacuum signal from a particular cylinder (ie too bouncy).

 

[Toolmanmike]

I did, but I didn't think I asked about best place to connect a full-time vac guage. Referencing that post, I wonder if the large pipe in the front-center (canister purge port?) might be a good candidate - if it has direct path to the center hole on the underside of the throttle plate. Taking it off the intake manifold port, which comes off a runner to a specific cylinder I think, would not necessarily give me a smooth signal. The '67 Barracuda had a factory vac guage option? I find that - curious.

I don't have A/C, nor any interior vacuum-operated devices - exept for the rear shelf flow-through air gate, which comes off the brake booster plastic fitting, which yes I could tap that but I think it's too heavily influenced by the vacuum signal from a particular cylinder (ie too bouncy).

Does it look something like this? The plugged one on the bottom right is constant vacuum as well as the larger one on the right. The plugged one on the left in the 9:00 position is ported vacuum and only has vacuum at off idle position. The laghe one at the 11:00 position should be for PCV.

 

[CBODY67]

If you want to talk about a "smooth" vacuum source, rather than "floating", you have to consider that EACH intake stroke brings a negative pressure pulse into the system. Same on the exhaust side of things, except it is a positive pressure pulse! The fact that it all happens so fast makes it SEEM to be smooth rather than otherwise. Every vacuum gauge I've seen, while not that many, had a brass inset into the sensing tube, near where that metal tube has the slip-fitting for the vac hose source of vacuum. That dampens any spikes in the vac supply to the gauge.

On Chrysler products and maybe some others, the "gauges" in the center of the floor console were NOT all tachometers. If it was an automatic transmission car, it was a vacuum gauge disguised as a "Performance Gauge", where lower vac readings meant "more performance" as higher readings meant "cruising performance". I believe that an automatic car could have a tach if it was ordered that way, though . . . have to check the order guides for verification. No vac gauges in the instrument cluster, just tachs and gauges.

Might need to look in the MasterTech series at www.mymopar.com to see where the emissions canister hooks to the carburetor, but it should be full manifold vacuum at that port. Realize, too, that "ported vacuum" also happens "below the throttle plates", just that that location is at the throttle plate at hot base idle and under the throttle plate at higher rpms. FWIW.

Enjoy!
CBODY67

 

[MoPar~Man]

Does it look something like this? The plugged one on the bottom right is constant vacuum as well as the larger one on the right. The plugged one on the left in the 9:00 position is ported vacuum and only has vacuum at off idle position. The laghe one at the 11:00 position should be for PCV.

The smaller (plugged) port - I find that RPM goes up a little if it's not plugged, even at idle. Air flowing through this port is not going to bring any extra gas with it, so if RPM goes up does that mean I'm improving the A/F ratio? I'm leaving it unplugged if I'm essentially creating a more "correct" ratio (presumably less rich). Less fuel useage at idle? If capping this port made no difference to RPM then I'd be using it for the vac guage.

The larger port (between the mixture screws) I have that port also, it's also capped, does it connect to the nearby center hole in the base of the throttle body? Maybe I'll try capping the smaller port and un-capping this port and see if again I get RPM increase.

The (small?) capped port you show at the 9 position, I have that port but it's the same (large) size pipe as the port at 11. I have one of those capped, the other going to PVC.

 

[CBODY67]

I suspect the idle speed increase when one port is unplugged has MORE to do with the fact the engine "wants more air" rather than making a more perfect air fuel ratio.

 

[MrMoparCHP]

On the 67 Barracuda the "Performance Gauge" (vacuum gauge) is part of the instrument cluster.

My car car, connected with the Cleaner Air Package valve.
Same port was used for Power Brakes and AC

Alan

 

[Toolmanmike]

The smaller (plugged) port - I find that RPM goes up a little if it's not plugged, even at idle. Air flowing through this port is not going to bring any extra gas with it, so if RPM goes up does that mean I'm improving the A/F ratio? I'm leaving it unplugged if I'm essentially creating a more "correct" ratio (presumably less rich). Less fuel useage at idle? If capping this port made no difference to RPM then I'd be using it for the vac guage.

The larger port (between the mixture screws) I have that port also, it's also capped, does it connect to the nearby center hole in the base of the throttle body? Maybe I'll try capping the smaller port and un-capping this port and see if again I get RPM increase.

The (small?) capped port you show at the 9 position, I have that port but it's the same (large) size pipe as the port at 11. I have one of those capped, the other going to PVC.

If a vacuum port is not used you DO NOT leave it open. It is either used or plugged.

 

[Captainfury]

On the 67 Barracuda the "Performance Gauge" (vacuum gauge) is part of the instrument cluster.

My car car, connected with the Cleaner Air Package valve.
Same port was used for Power Brakes and AC
View attachment 724500


Alan

That is a beautiful engine bay- I dream about engine bays that look like that! So clean, no grease or miscellaneous wires to confuse.

But wait- is that a ladies g-string to the right of the coil?????
Now I'm dreaming of your engine bay just that much more!!

 

[Gerald Morris]

The smaller (plugged) port - I find that RPM goes up a little if it's not plugged, even at idle. Air flowing through this port is not going to bring any extra gas with it, so if RPM goes up does that mean I'm improving the A/F ratio? I'm leaving it unplugged if I'm essentially creating a more "correct" ratio (presumably less rich). Less fuel useage at idle? If capping this port made no difference to RPM then I'd be using it for the vac guage.

The larger port (between the mixture screws) I have that port also, it's also capped, does it connect to the nearby center hole in the base of the throttle body? Maybe I'll try capping the smaller port and un-capping this port and see if again I get RPM increase.

The (small?) capped port you show at the 9 position, I have that port but it's the same (large) size pipe as the port at 11. I have one of those capped, the other going to PVC.

I find the best place to read engine vacuum to be the big fitting at the rear of the intake manifold near center, and used to run a small vac gauge off a tee just up from it, shared with the little line to my heater. These were the ONLY vacuum lines off my manifold, and the only other off the entire engine was the vac advance line off the proper port on the front of my carb. Consequently, my vacuum readings were consistent whether off the little dash gauge or one of my mechanics gauges when I wanted to fine tune my timing or something.

One advantage of reading vacuum from a larger port back from the throttle plates lies in the way the intake manifold forces sundry ripples in air pressure to cancel each other out as they reach the center of the system at a port large enough to permit this sort of wave addition from all parts of the system. Thus applies perturbation theory to practice. Using a little larger vac line, permitting more volume will further damp any ripple, especially if you give it a bit of length. Pray, don't take such advice ad absurdum, like, with a cubic meter vac box attached or such!

 

[66/440]

Why not Tee off one of the lines on the manifold vacuum tree?