Poll: Ported v Manifold Vacuum...

[Terlingueno]

Wondering who uses 1) an aftermarket carburetor (Holley, Edelbrock, etc) and 2) whether you use ported, or whether you use manifold vacuum to the distributor.

 

[Goldie Driver]

Factory Q-Jet - the last carb tech revamped the vacuum to manifold. Seemed like it had a bit more oomph. He also disconnected the vacuum lines to the emissions stuff. Seemed like mileage went DOWN.

Replumbed those lines ( EFE, EGR, & Thermac ) and mileage testing continues. That's actually a funny statement 'cuz if this rig gets 11 on the highway it's a day to write home.

 

[Bextreme04]

Wondering who uses 1) an aftermarket carburetor (Holley, Edelbrock, etc) and 2) whether you use ported, or whether you use manifold vacuum to the distributor.

The key thing with this is not the carb. It matters what distributor you have and how you have it set up. If your distributor is set up for a ported vacuum application and you run manifold vacuum... you will have issues. Same if it is opposite.

 

[DoubleDingo]

Factory Q-Jet - the last carb tech revamped the vacuum to manifold. Seemed like it had a bit more oomph. He also disconnected the vacuum lines to the emissions stuff. Seemed like mileage went DOWN.

Replumbed those lines ( EFE, EGR, & Thermac ) and mileage testing continues. That's actually a funny statement 'cuz if this rig gets 11 on the higwhay it's a day to write home.

I had done similar on my 81'. Disconnected and rerouted the vacuum lines. I had to rehook em up for emissions testing, and when I did I replaced all the rubber hoses. It actually ran better being hooked up to all the emissions stuff. And, once it's hot it's running on manifold vacuum if I recall correctly. Checked a photo of my sticker, it shows the line coming off the back of the carb, so manifold vacuum once it's hot.

 

[Ricko1966]

Ported vacuum and manifold vacuum are the same once you open the throttle plates,just a hair. They shut the vacuum advance off at idle to burn late and clean up the tail pipe. I even tell people that can't pass CO emissions,retard the timing a couple of degrees it'll pass because it's still burning the air fuel charge in the manifold. The factory even has a thermo vacuum switch that changes from ported to manifold based off of temperature,it defaults to manifold if the engine gets too hot to help cool it down. I'd run manifold vacuum.. The toothpick on the left is ported the one on right is manifold,as you can see once you crack the throttle they are both open,to manifold vacuum.

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[Ricko1966]

Milage should go up egr partially fills the cylinders with exhaust gas during low load situations. Kinda like if you put a little water in the bottom of a shot glass,you aren't going to drink as much alcohol. Egr has other milage benefits too, but you can research them.Manifold Vacuum on distributor should be a tiny gain due to more efficient timing at idle, and cleaner plugs. EFE and Thermac,a hot engine can run leaner,and actually does just due to differences in density between hot and cold. Also your choke kicks off sooner. All these add up to a couple of mpg.

Factory Q-Jet - the last carb tech revamped the vacuum to manifold. Seemed like it had a bit more oomph. He also disconnected the vacuum lines to the emissions stuff. Seemed like mileage went DOWN.

Replumbed those lines ( EFE, EGR, & Thermac ) and mileage testing continues. That's actually a funny statement 'cuz if this rig gets 11 on the higwhay it's a day to write

 

[Turbo4whl]

• Ported
• Manifold
• Both Check

@Ricko1966 has the answer, Trapped vacuum spark. The picture of the TVS (thermo vacuum switch) he posted also has an air bleed to release the vacuum if the engine stalls when cold. GM didn't waste the money adding that system if it didn't help with the distributor advance.

 

[Goldie Driver]

@Ricko1966 - question for you I just thought of on the way in. My distributor advance is hooked to manifold vacuum via a straight line - nothing else on it. What happens to manifold vacuum during WOT ? More importantly, what happens to the distributor advance ? I am thinking ( now) I need a check valve in line.

TIA !

Britt

 

[Trucksareforwork]

@Ricko1966 - question for you I just thought of on the way in. My distributor advance is hooked to manifold vacuum via a straight line - nothing else on it. What happens to manifold vacuum during WOT ? More importantly, what happens to the distributor advance ? I am thinking ( now) I need a check valve in line.

TIA !

Britt

At WOT, manifold vacuum drops to a negligible level…and so does your vacuum advance. On the other hand, as rpm increases, your mechanical advance inside the distributor adds timing.

The debate about manifold vs ported vacuum is way more about idle quality and off idle performance.

My opinion is the move from ported to manifold vac advance happens as you get into more and more radical cam profiles. A choppy cam usually idles way better with lots of advance, which an appropriately adjusted vac can allows off manifold vac at idle.

The rest has been said: GM engineers knew what they were doing when they put the TVS and other doodads in engines that were made to run with limited initial advance.

 

[elbpony]

The never ending discussion.

I learner there were no pure yes or no, right or wrong. Both sides have pros, cons, dos and donts.

Bottom line, what ever your motor runs best with, its yours.

Back to the question:
Edelbrock 1400
Manifold
delco hei
12mpg, mostly driven between 40-60 mph, little city driving

 

[Ricko1966]

@Ricko1966 - question for you I just thought of on the way in. My distributor advance is hooked to manifold vacuum via a straight line - nothing else on it. What happens to manifold vacuum during WOT ? More importantly, what happens to the distributor advance ? I am thinking ( now) I need a check valve in line.

TIA !

Britt

Manifold vacuum and ported vacuum both drop to almost zero at WOT. Your vacuum advance will go away and it should,that's how its designed,no check valve needed. Your vacuum advance will be going away but your centrifugal advance will be coming up and reaching best total. If the Vacuum advance stayed in you'd be way too advanced,you'd be advanced enough for lean cruise,but at WOT you aren't at a steady relaxed cruise state. Also manifold vacuum will keep your engine cooler and plugs cleaner. As I've said before and showed pictures,the factory used both with a vacuum switch that switched to manifold on a hot engine. The ported vacuum just kills the vacuum signal at idle which makes the advance at idle too late so it burns too hot and cleans up the emissions at idle.

 

[elbpony]

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This graph Shows how vaccuum is at ported and manifold related to throttle position.

 

[Ricko1966]

Ignition Curve Basics: Taking The Mystery Out Of Timing Advance

Originally published in the December 1989 issue of Hot Rod magazine

www.hotrod.com

 

[xm20k]

The 3rd option, Neither
2x 500 CFM AVS 2s, no advance locked at 38 (domed pistons) only manifold vacuum from plenum to PCV.

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