# Vacuum Advance Ported or Unported?



## ajluzi1957 (Mar 22, 2017)

Good Morning All,

I have read conflicting information on this. Can somebody help me and tell me if on a 73 Pontiac 400 engine, should the vacuum advance for the distributor be connected to to the ported vacuum on the carb (rochester 4bbl) or manifold vacuum.

Thank you


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## Lemans guy (Oct 14, 2014)

It was probably designed to be ported, as by 73 emissions issues were stronger and the ported vacumn retards the spark at idle, which burns off more pollutants

because it makes the exhaust hotter....

depending on your state laws and the car requirements for emissions, it will run cooler and better on full manifold vacumn if the vacumn advance and distributor are properly set up, better gas mileage and stronger runner....

if the original base timing setting was low like 4 or 6 then it is likely ported as that is where the car ran at idle with retarded spark for burning off the emission,

the centrifigal advance would then pick up the remaining 30 or 32 for toal adavnec and the vacumn advance probably had too much as well...:nerd::nerd:

sorry I didn't have the exact for your but someone here will.....


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## ajluzi1957 (Mar 22, 2017)

thanks for the response. The car is a 69, but the PO dropped in a 73. This is a non emissions vehicle in my state, so that will not be an issue. I will plug into the manifold and time it that way to see how she runs.


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## Lemans guy (Oct 14, 2014)

Oh for sure use full manifold Vacumn on the vac can, if it is an HEI dist use the Standard Motor parts SMP VC 302, it pulls 10 degrees advance at the crank and hook it to full manifold vac, you need to get your centrifigal dialed in with base to 36. And then of course set the springs to come in early....


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## ajluzi1957 (Mar 22, 2017)

Thanks! It is still set up with points, but i will research total timing on the forum and work on that


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## ajluzi1957 (Mar 22, 2017)

Back to being confused again.... I was looking up information on total timing on this forum and came across a reference that Bear put on here from a chevy article on "Setting Ignitions Timing Curve" which stated the following:

"Vacuum canisters advance according to engine vacuum, which is why they must be connected to a ported vacuum source. Ported vacuum is drawn from just above the throttle blades to make sure the vacuum canister does not advance at idle."


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## Lemans guy (Oct 14, 2014)

It is correct to say that ported vacumn, that is above the throttle plate, is used to make sure there is no vac advance at idle.

That is correct, that is why ported is used, to reduce timing advance at idle.....

but the reason the car manufactures did that was to reduce oxides of Nitrogen by creating a hotter exhaust temp at idle, which then burns those off and makes your car run hot.

if you run full manifold vac you will get a smoother idle, crisp throttle response from the already advanced timing with a light pedal applied and why would you want to give that up?

some guys like ported, but it was created for emissions not performance...

look up google...Timing 101 written by a GM engineer about timing on these muscle cars....give you some background on it all:nerd::nerd:


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## ajluzi1957 (Mar 22, 2017)

Thanks Again! Saw that article last night and will set it up that way


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## ajluzi1957 (Mar 22, 2017)

Sorry to bring this up again, but I need a little help. I want to use full vacuum as was suggested. The specs on the car call for 9 degrees BTDC at idle, so I plugged the vacuum advance, set the idle, timed the car to 9, then plugged in the vacuum advance, and the idle increases substantially. Sorry for the novice question here, but I'm assuming that I was supposed to plug the vacuum advance to set initial timing correct? If so, when I plug it in, do I just go back and reset the idle speed? I would think that timing is way advanced at that point...


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## geeteeohguy (Feb 2, 2008)

You did it right. After re-connecting the vacuum line, you need to adjust the idle speed back down to specs, and then adjust the carb mixture screws. Pontiacs are set up with manifold vacuum at idle so that they run cooler at low speed and in traffic and have better throttle response at low speed. You are okay.


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## ajluzi1957 (Mar 22, 2017)

Thanks!


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## Lemans guy (Oct 14, 2014)

yes Geeteeoh guy is always spot on...you have it!

I run mine at 10 base and 10 on the vac can...for 20 BTDC at idle very smooth,.....and cool....


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