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Hi Guys,

Since I wanted to check the timing on our 347 the other day, I noticed that we had the vacuum line hooked to full manifold vacuum. Not thinking much about it I went to the instructions for our distributer (billet Pertronix 111) and it calls for ported vacuum. I have a basic understanding of both types but are they calling for ported strictly for smog requirements or is there another reason they want it ported?
 

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You may get some ignition ping just off idle if using full manifold vacuum. Manifold vacuum will give you full advance at idle, depending on the cam, and less advance as the throttle is opened.
Some people prefer it set up that way and some smog equipped engines require it, but I usually use the ported vacuum that will give you more advance at cruise, and only your initial mechanical advance at idle.
 
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Manifold vacuum will give you better MPGs at cruise and a better idle with a large cam. I love it, run my engines that way. But you need to search, this is much about it. It's been discussed over and over. I tune my advance at WOT without vacuum as it is nil when hooked to manifold. Works well.
 

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Discussion Starter #4
You may get some ignition ping just off idle if using full manifold vacuum. Manifold vacuum will give you full advance at idle, depending on the cam, and less advance as the throttle is opened.
Some people prefer it set up that way and some smog equipped engines require it, but I usually use the ported vacuum that will give you more advance at cruise, and only your initial mechanical advance at idle.
Funny you should mention about slight detonation off idle....I do have some occasionally.
 

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Please explain to me guys how part throttle,cruising, and MPGs are different between ported vs manifold vacuum. To my understanding they are the EXACT SAME at all conditions except when the throttle blades are closed. Or am I off? Once the throttle blades are open the 'port' above the blade is exposed to full manifold vacuum and there should be no difference. What am I missing?
 

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Funny you should mention about slight detonation off idle....I do have some occasionally.

The hesitation is probably your vacuum canister transitioning (unsmoothly) from zero vacuum to full vacuum as soon as the throttle blades open if you are using ported vacuum.
 

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Manifold vacuum will give you better MPGs at cruise and a better idle with a large cam. I love it, run my engines that way. But you need to search, this is much about it. It's been discussed over and over. I tune my advance at WOT without vacuum as it is nil when hooked to manifold. Works well.
Not to hijack the thread, but I have a similar question posted today. With no vac, I am at 36 degrees at 3.000 rpm. Hook vacuum up and I jump to 42.9 degrees at 3,000 rpm. I'm using a port off of my Holley street avenger
 

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Please explain to me guys how part throttle,cruising, and MPGs are different between ported vs manifold vacuum. To my understanding they are the EXACT SAME at all conditions except when the throttle blades are closed. Or am I off? Once the throttle blades are open the 'port' above the blade is exposed to full manifold vacuum and there should be no difference. What am I missing?
According to this graph you are right!

 

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Discussion Starter #9 (Edited)
The hesitation is probably your vacuum canister transitioning (unsmoothly) from zero vacuum to full vacuum as soon as the throttle blades open if you are using ported vacuum.
Just switched it over to ported and set it at 14' initial and a total of 35 ish. No hesitation and no rattle. Tried a few more degrees and it would detonate--considering it was 85' when I tuned, it should be safe. The racer in me wants to drain the tank and run a few gallons of C16 and really lean on it but sanity should prevail and be happy with what we have.
 
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Cruising at 60 mpg the throttle is just cracked . manifold vacuum will be very high.
 
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The problem with eh graph is cruising doesn't give you port throttle. So there an acceptance in the variables here that isn't true.
 

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How much manifold vacuum will be measured in the intake manifold at wide open throttle? That answer is why ported vacuum has the advantage for making more power. Air passing over an orifice creates the vacuum.
 
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How much manifold vacuum will be measured in the intake manifold at wide open throttle? That answer is why ported vacuum has the advantage for making more power. Air passing over an orifice creates the vacuum.
Then I guess all the Engineers who designed distributors without vacuum advance, like the 289 Hipo, were idiots? You can't make that statement unless you truly understand how ignition advance can be tuned, derived and used.
 

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According to this graph you are right!




Let me expand what's going on in this graph. (Nice data logging btw!).

Ported vacuum was developed to reduce hydrocarbons. Pulling timing will reduce hydrocarbons. Problem is pulling timing also adversely affects MPGs while cruising and also power. So the manufacturers needed a way to turn the vacuum off to the vacuum advance while idling but allow full vacuum to the vacuum advance while cruising (turn it back on). To do this they drill a hole right above the throttle blade in the throttle body. This gives zero vacuum at idle but as soon as the throttle plate is moved allows full manifold vacuum to pass through during part throttle, cruise, WOT, ect.

Hook two vacuum gauges up (and yes I have done this) one ported one manifold. The ported will read zero at idle the manifold will read 15-20(whatever). Now touch the throttle and you will see the ported gauge match the manifold gauge.

There should be no disparity at any point above idling. Even light throttle they will match. The only thing the ported vacuum will do is turn off the vacuum signal by being above the throttle plate at idle not exposing it to manifold vacuum.

If your car starts/idles better (and some will) with ported, run it. If it's better with manifold, run it. But any disparity above idle is all in your head. There should be NO difference. NOTE: idle rpm will increase with manifold over ported(due to more advance with manifold vac). Manufacturers would many times use small vacuum 'switches' to switch to manifold from ported if the a/c turns on or the engine gets warm to increase fan speed.

In the above graph you can see the vacuum signals match perfectly until the throttle position goes to zero giving zero vacuum at the ported location.
 

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The problem with eh graph is cruising doesn't give you port throttle. So there an acceptance in the variables here that isn't true.

Anything above idle at a ported location will give full manifold vacuum at the ported location. Unless you are cruising with your foot off the gas there should be no difference. Ported vacuum is just an 'off' switch for vacuum at idle. Nothing more.

Found a website that's a good read:

http://www.lbfun.com/warehouse/tech_info/timing & vacuum advance/vacuum_explained.pdf

Obviously this guy knows more than I do and can explain it better.

EDIT: I suck at links. I don't think that one is working.
 

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Then I guess all the Engineers who designed distributors without vacuum advance, like the 289 Hipo, were idiots? You can't make that statement unless you truly understand how ignition advance can be tuned, derived and used.
I understand the question is about choosing manifold or ported vacuum to a vacuum advance fitted dizzy. If talking about a better type of advance system that may be for another topic.
 

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Just switched it over to ported and set it at 14' initial and a total of 35 ish. No hesitation and no rattle. Tried a few more degrees and it would detonate--considering it was 85' when I tuned, it should be safe. The racer in me wants to drain the tank and run a few gallons of C16 and really lean on it but sanity should prevail and be happy with what we have.

Some engines will simply start/idle and transition off idle better with ported as they don't like the extra timing. Others won't. Your advance curve should be the same regardless.

It's possible you are running lean when transitioning off idle and your engine doesn't like the extra timing with manifold vacuum during the transition off the idle circuit.
 
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