Need more vacuum ports

Gofish

Well Known Member
I’m building a tri-power 348 hooked to a 700R4 in a Model A and have more vacuum needs than ports on the engine. The center carb base is cast iron, the others are aluminum. I’m short one large-diameter port. I have ports for the distributor advance, transmission (for lockup), and PCV on the center carb. What I’m short is another large one for either the PCV or the brakes

I don’t want to combine the brake booster and PCV hoses, so I’m thinking of drilling and tapping the intake manifold for another 1/4 NPT fitting. This would be for the brakes and maybe a more convenient source for the trans. It seems pretty straightforward to do this behind the rear carb, as generally shown with the pencil in the top photo. It’s plenty thick there too (the other photo).

So before I get out the drill, how about some better ideas? I’m trying real hard not to be a cut first then measure again guy.


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IMBVSUR?

Well Known Member
Supporting Member 2
Are you using a vacuum switch to lock up that convertor? Have you thought about using a pressure switch on 4th gear in the valve body? The shift points will be controlled by the throttle valve cable. That will give a vacuum port.
 

427John

Well Known Member
Ideally you should drill any additional port down the center so that it intersects both throttle bores especially if it is a large capacity source,but it looks like that may be difficult with the room you have.
 

Gofish

Well Known Member
IMBVSUR?: I have an electric lockup kit in the valve body and it’s activated by vacuum. I could put a switch in to override it,though. My main problem is the larger vacuum feeds like PCV and brake booster.

La Hot Rods: the end carb bases are aluminum and not thick enough to drill a 1/4 NPT (they’re part of a kit to create a Tri-Power). I suppose I could make some super cool aluminum spacers for all 3 carbs and do it that way. Or, get another cast iron base. Wait, I think I have one! RATS, not enough clearance with the distributor. The aftermarket bases for the end carbs have no provisions for vacuum either - the photo shows the aftermarket vs. a gennie version.

427John: I see what you mean. I may be able to sneak a centered fitting in under the float bowl of the rear carb (photo), but it would still just see one bore due to the dual plane design.

I’m beginning to think super cool spacers would be one way to add a vacuum port (would need to be below the rear carb float bowl) and have it see both bores. Raising the carbs can only look cooler too. I’ll think about this a while longer....

Thanks for all the snappy help.

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Don Jacks

Well Seasoned Member
Supporting Member 3
Just remember that the vacuum switch must be attatched to ported vacuum,not manifold vacuum.If set up correctly,the lock up clutch will work in 3rd.and 4th. gear.
 

Gofish

Well Known Member
Don Jacks: I’ll keep that in mind. I think I have 1 ported source.
EDIT: no I don’t have any ported sources. hmm.

DonSSDD: yeah, maybe just change to a manual.

Maybe I’m overthinking this. I think I’ll just add a tee to the 1/4 NPT port on the center carb and see how it works. I can always get the drill out later.

Thanks all.
 
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ragtp66

Well Known Member
I’m building a tri-power 348 hooked to a 700R4 in a Model A and have more vacuum needs than ports on the engine. The center carb base is cast iron, the others are aluminum. I’m short one large-diameter port. I have ports for the distributor advance, transmission (for lockup), and PCV on the center carb. What I’m short is another large one for either the PCV or the brakes

I don’t want to combine the brake booster and PCV hoses, so I’m thinking of drilling and tapping the intake manifold for another 1/4 NPT fitting. This would be for the brakes and maybe a more convenient source for the trans. It seems pretty straightforward to do this behind the rear carb, as generally shown with the pencil in the top photo. It’s plenty thick there too (the other photo).

So before I get out the drill, how about some better ideas? I’m trying real hard not to be a cut first then measure again guy.


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View attachment 80981
Don Jacks: I’ll keep that in mind. I think I have 1 ported source.
EDIT: no I don’t have any ported sources. hmm.

DonSSDD: yeah, maybe just change to a manual.

Maybe I’m overthinking this. I think I’ll just add a tee to the 1/4 NPT port on the center carb and see how it works. I can always get the drill out later.

Thanks all.
The original 348 had the road draft tube not a pcv system you could use the old road draft port with an adapter kit pretty sure show-cars has em, OR move the pcv to the oil filler tubes as GM used to have a filler tube with a PCV bung in it. 58 tripower also used this Y type setup. IMG_1447.JPGIMG_1447.JPGpcv.jpgGMK-4011-259-662_ml.jpg
 

Fathead Racing

Well Known Member
Supporting Member 7
The vacuum switch is designed to interrupt the electrical power to the solenoid that controls the converter lockup. When the switch sees less than 3" or 4" of vacuum it interrupts the circuit and the converter lockup drops out. Was a feature to prevent chugging at low speeds and to assist engine rpm during light acceleration so as to not bogg the engine . The switch could be teed in to the vacuum advance. And I agree with one of our best techs, Cecil. Hollow carburetor stud.
 

Don Jacks

Well Seasoned Member
Supporting Member 3
It was also to release the clutch when the throttle is released.The converter clutch doesn't like being back driven[gearing down].
 

Gofish

Well Known Member
I value the suggestions here. I have a road draft tube for my 348, but I’m currently not using it because 1) didn’t’ want oil drops on the pavement (??) and 2) it interfered with where I routed my Vintage Air heat and AC hoses. At this point of the build I’m forming steel tubing for all the vacuum lines and attaching them to the 1/4 in NPT port on the back of the center carb base plate. I’ll see how it goes. With all your help I have more options.

I’ll post a pic or two when I get the tubing finished in a week or so. I won’t have the car running again till mid summer. Just got back to the project.

Thanks folks.
 

neilswheels

Well Known Member
Slight hijack (sorry Godish) but I’m looking at kinda the same thing, I want a vac takeoff for a vacuum gauge. Center carb being used for PCV, and there’s nothing else, so I was looking at drilling the manifold in the same place, below the back carb, any reason why this won’t work??
 

neilswheels

Well Known Member
Slight hijack (sorry Gofish) but I’m looking at kinda the same thing, I want a vac takeoff for a vacuum gauge. Center carb being used for PCV, and there’s nothing else, so I was looking at drilling the manifold in the same place, below number 3, any reason why this won’t work? Thanks
 

Don Jacks

Well Seasoned Member
Supporting Member 3
Drilling the manifold can work for full manifold vacuum such as vacuum gauges,trans.modulators,ect. will work but not for a source that requires ported .,vacuum such as converter switches.For your case the hollow stud ah la factory would be easier .
 

neilswheels

Well Known Member
Thanks Don. I was thinking more for neatness, a hardline running out the back of the manifold would be more discreet, but would it work if it was in ‘one port’ so to speak, so I assume getting more vacuum when the piston below is drawing in, or am I waaay off?!? Not sure if that makes any sense..
 

neilswheels

Well Known Member
Looking at the carb base, I can see that the tapped hole in the middle carb is ‘fed’ by both sides of the carb, which I assume provides a constant vacuum? So if I tapped the manifold, but only into one side , would that be an issue? Does that make more sense?
 
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