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Re: Why Use Manifold Vacuum for Vacuum Advance Distributor? [Re: GomangoCuda] #1048065
08/07/11 12:29 PM
08/07/11 12:29 PM
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This subject comes up often enough that both of those articles should be put in the Tech Archives.


In theory, there is no difference between theory and practice. In practice, there is.
Re: Why Use Manifold Vacuum for Vacuum Advance Distributor? [Re: Lee446] #1048066
08/07/11 12:34 PM
08/07/11 12:34 PM
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Quote:

Again, this is wrong information. At wide open throttle regardless if it is ported or manifold vacuum there will be no vacuum or very little vacuum. The ported vacuum is there for just idle or very close to idle conditions. Once you are on the road there is very little difference between the two. I still think that using ported vacuum is better for more situation as long as the distributor advance slots have been shortened to allow much more mechanical advance then is the norm on the stock dist.




Ported vacuum is shut off at idle and the same as manifold vacuum as soon as the throttle plates start opening.

Quote:

As I said, ported vac is generated by the flow of air thru the venturi, the higher the flow, the more vac. That is exactly how the end carbs on six packs and the secondaries on Vac secondary carbs are able to open at high/full throttle. If they used manifold vac, they would never open. That GM guy gives a good explanation, it has always been a confusing subject.




No. Ported vacuum does not come from the venturi. It comes from right above the throttle plates so that as soon as you come off idle it is the same as manifold vacuum. You are correct about the vacuum source for opening a sixpack or for that matter any vacuum secondary Holley carb is from the venturi. Two entirely different things.

Quote:

The ONLY difference between "ported" and manifold is the ported passage is just above the throttle blades. When the throttle is closed it closes the port = no vacuum = no vacuum advance at idle. As soon as you crack the throttle past the port it then sees manifold vacuum. This was an emissions era invention.

The manifold vacuum port passage is below the blades so it sees vacuum or the lack of it all the time.





This is correct.

Last edited by GomangoCuda; 08/07/11 12:42 PM.

In theory, there is no difference between theory and practice. In practice, there is.
Re: Why Use Manifold Vacuum for Vacuum Advance Distributor? [Re: GomangoCuda] #1048067
08/07/11 01:45 PM
08/07/11 01:45 PM
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540DUSTER Offline
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the reason the vacuum drops on ported vacuum is that as the blades of the throttle are opened the air is moved to the center of the bore;away from the drilled hole in the wall of the bore.Have you ever noticed that in a fast moving river the water on the edge of the flow will be slower or even move backwards than the water in the middle of the river?

Re: Why Use Manifold Vacuum for Vacuum Advance Distributor? [Re: Lee446] #1048068
08/07/11 02:17 PM
08/07/11 02:17 PM
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Quote:

As I said, ported vac is generated by the flow of air thru the venturi, the higher the flow, the more vac.






??? by saying that you saying that at WOT you should have the most vac. then. no? thats wrong.

vac advance only works at part throttle where you have vac. at WOT the vac advance is just along for the ride. your dist. is fully mechanical at that point.

Re: Why Use Manifold Vacuum for Vacuum Advance Distributor? [Re: 540DUSTER] #1048069
08/07/11 02:37 PM
08/07/11 02:37 PM
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The vacuum that opens the secondaries on a 4 barrel or Six pack comes from a hole just below the start of the venturi, not down by the throttle plates. If you have a Holley its kind of inline with the boosters. You have to look hard to see it. Once the plates open a certain amount the ported vaccum goes away. I run my VA off manifold vacuum, the main benefeit is the advanced timing @ idle. My 440 makes about 16" @ 900 rpm with about 25 degrees advance. Sent my dist to Don @ FBO who beleives maifold vacuum is correct. You can't just plug it in one or the other and hope one works better, it has to be setup properly.


Paul Reber
Re: Why Use Manifold Vacuum for Vacuum Advance Distributor? [Re: Twostick] #1048070
08/07/11 03:19 PM
08/07/11 03:19 PM
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Quote:

Quote:

Quote:

Quote:

Ported vacumn is generated by flow velocity thru the venturi of the carb.





That is correct. And intake manifold vacum will be right around o at wide open throttle where ported vacum will be high. Thats why ported is used so much as it will give more advance with more throttle. . Ron




Again, this is wrong information. At wide open throttle regardless if it is ported or manifold vacuum there will be no vacuum or very little vacuum. The ported vacuum is there for just idle or very close to idle conditions. Once you are on the road there is very little difference between the two. I still think that using ported vacuum is better for more situation as long as the distributor advance slots have been shortened to allow much more mechanical advance then is the norm on the stock dist.







The ONLY difference between "ported" and manifold is the ported passage is just above the throttle blades. When the throttle is closed it closes the port = no vacuum = no vacuum advance at idle. As soon as you crack the throttle past the port it then sees manifold vacuum. This was an emissions era invention.

The manifold vacuum port passage is below the blades so it sees vacuum or the lack of it all the time.

A simple test to confirm this is get 2 vacuum gauges and hook one to each port. You will find that they will be basically the same everywhere but idle.

Kevin





I agree I used the wrong words as I was refering to venturi vacum but I said ported.
Ported is actually manifold vacum but it is placed above the throttle plate so you have no vacum at the port at idle.
Venturi vacum is from airflow thru the carb venturi which increases with more airflow. It is how the vacum carbs work from venturi vacum. Mopar also ueed a venturi vacum signal to the EGR amplifier in the 70's as it was the signal it got and it told the amplifier when to put intake vacum to the EGR valve.

Another thing to remember is intake and ported vacum will be strongest on closed throttle decell and then at idle. And when at part throttle the intake and ported vacum signal is still strong as thats why the vacum advance can help part throttle power and milage. But when the throttle reaches a certain point close to and at wide open throttle the vacum drops to about 0 because with the throttle open there is no way to create a low pressure area under the throttle.

Many cars in the 70's with all the emission devices used the dual vacum advance which at times also put manifold vacum on the other side of the vacum diaphram to retard timing at idle. That way they could open the throttle more to maintain idle speed letting in more air thus leaner mixture at idle. They also used Decell valves that would advance the timing under decelleration as another emission device.

Bottom line is most vacum advances use ported vacum which is intake vacum taken above the throttle plate and would give alot of advance at part throttle but when going to wide open throttle the vacum advance would drop off and the mechanical advance was what took over.
I was wrong in stating most vacum advances use venturi vacum as some did but not alot. Ron

Re: Why Use Manifold Vacuum for Vacuum Advance Distributor? [Re: 383man] #1048071
08/07/11 03:38 PM
08/07/11 03:38 PM
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540DUSTER Offline
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there is ported vacuum at idle,at lease with all the mopar dist. I have ever messed with.Get a service manual and look it up for yourself.Might not be alot but it is there.

Re: Why Use Manifold Vacuum for Vacuum Advance Distributor? [Re: 540DUSTER] #1048072
08/07/11 04:17 PM
08/07/11 04:17 PM
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I'd be real hesitant to accept what works on some other brand of engine as gospel for all engines.

GM may very well have used ported vacuum for whatever reason that is completely inapplicable to MoPar engines. During the emissions time frame that GM engineer speaks of Chrysler used lean burn whereas GM used air injection and cats well before Chrysler had to. So something in the engine design was very different between the two.

I can also tell you this, my 65 Cuda uses ported vacuum as did my 64 300, both well before tail pipe sniffing became the norm.

Last edited by Supercuda; 08/07/11 04:18 PM.

They say there are no such thing as a stupid question.
They say there is always the exception that proves the rule.
Don't be the exception.
Re: Why Use Manifold Vacuum for Vacuum Advance Distributor? [Re: Supercuda] #1048073
08/07/11 07:06 PM
08/07/11 07:06 PM
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Don't know what I was thinking, Y'all are correct, if ported was venturi vac, you would have 50* plus at WOT. Mea Culpa!

Re: Why Use Manifold Vacuum for Vacuum Advance Distributor? [Re: 383man] #1048074
08/07/11 07:56 PM
08/07/11 07:56 PM
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Quote:

My question would be, what are the pros and cons of hooking up a vacuum advance to manifold vacuum?




Quote:

"ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.






If your car has a mandatory smog-test every year, use Ported vacuum. Otherwise use Intake vacuum.

Re: Why Use Manifold Vacuum for Vacuum Advance Distributor? [Re: BigBlockMopar] #1048075
08/07/11 08:39 PM
08/07/11 08:39 PM
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I really appreciate the articles. I understood the reasoning behind the use of vacuum advance, but I never understood why base timing was so retarded on engines when we know they like more initial timing. The emissions info clears it up. I still see no reason to use ported vacuum for the advance.

A test that I want to do now is hook up my mighty vac to the vacuum can and see when full vacuum advance is achieved. Interesting. I am assuming that the adjustable cans adjust the amount of advance, and not how much vacuum it takes to get full advance.


383, Hemi 4-Speed, AlterKtion, D60
Re: Why Use Manifold Vacuum for Vacuum Advance Distributor? [Re: 62maxwgn] #1048076
08/09/11 10:30 PM
08/09/11 10:30 PM
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Taking vacuum from a ported area provides a better vacuum signal at the restriction of the throttle bores. It is accurate. The signazl for a distributro is much better at the throttle bore. The restriction creates the added velocity. Sorry but your wrong.

Re: Why Use Manifold Vacuum for Vacuum Advance Distributor? [Re: YO7_A66] #1048077
08/10/11 01:46 AM
08/10/11 01:46 AM
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Because of my limited knowledge, I am a bit confused. I thought ported vacuum was manifold vacuum because it screws into the manifold. So where does a "manifold vacuum" hose connect exactly? Also, what is the "can". I know of vacuum ports and vacuum amplifier on my MP 360/300hp crate motor. Where should my vacuum advance be connected? Idle is a little crappy right now. Thanks.


Facts are stubborn things.
Re: Why Use Manifold Vacuum for Vacuum Advance Distributor? [Re: larrymopar360] #1048078
08/10/11 07:15 PM
08/10/11 07:15 PM
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Manifold vacuum is just as it says. Taken from the manifold. Ported vacuum is above the throttle plates. Remember the constriction at the throttle bores creates velocity. cacuum it this pot is governed by the size of the bore. As the throttle plates opemna small slot on the side of the bore in the primaries is less restricted and the value of the port vacuum declines.As you would expect from the engine rpms increasing.

Re: Why Use Manifold Vacuum for Vacuum Advance Distribu [Re: Wild_Hemi] #1048079
11/04/11 12:49 PM
11/04/11 12:49 PM
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What is the difference between "timed" and "ported"? Are they the same just different terminology?

After doing some more reading, this Winter I am going to buy a distributor that is setup for manifold vacuum advance (which comes with a lower rated can).
I like the idea of having more timing at idle (especially in D at a stop) and if the distributor is already setup for manifold vacuum, then the idle should not fluctuate as others have witnessed.

Thanks for all of the replies in the past on this issue.

Thanks


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Re: Why Use Manifold Vacuum for Vacuum Advance Distribu [Re: YO7_A66] #1048080
11/04/11 03:06 PM
11/04/11 03:06 PM
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Timed and ported are the same.
Just set your timing for best idle performance, in gear and nuetral, and shorten your advance to get that initial, and best full mechanical.
Then hook your vac back up and see if it pings and bucks good.

Re: Why Use Manifold Vacuum for Vacuum Advance Distributor? [Re: Supercuda] #1048081
11/06/11 06:15 AM
11/06/11 06:15 AM
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Quote:

I'd be real hesitant to accept what works on some other brand of engine as gospel for all engines.

GM may very well have used ported vacuum for whatever reason that is completely inapplicable to MoPar engines. During the emissions time frame that GM engineer speaks of Chrysler used lean burn whereas GM used air injection and cats well before Chrysler had to. So something in the engine design was very different between the two.

I can also tell you this, my 65 Cuda uses ported vacuum as did my 64 300, both well before tail pipe sniffing became the norm.




Exactly. GM did things differently then Chrysler and articles about why and how they did it is irrelevant.

Re: Why Use Manifold Vacuum for Vacuum Advance Distributor? [Re: 5thAve] #1048082
11/06/11 10:58 AM
11/06/11 10:58 AM
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So, if the laws of physics are different between GM, Ford, and Mopar, how about some info as to why Mopar did it the way they did.


Steve
Re: Why Use Manifold Vacuum for Vacuum Advance Distributor? [Re: Strawdawg] #1048083
11/07/11 01:19 PM
11/07/11 01:19 PM
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There's so much contradiction typed with confidence that it's making a Green bearing discussion look unanomous.

Re: Why Use Manifold Vacuum for Vacuum Advance Distributor? [Re: Fury Fan] #1048084
11/07/11 01:44 PM
11/07/11 01:44 PM
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LOL...that is true.

The facts are pretty simple here. NOx became the big target in that period and lower compression, egr, and retarded (reduced) timing were the tools available in those days to try to reduce levels and the implementation was very crude by today's standards given the lack of computer control.

All the manufacturers had similar versions because the science was the same for all. It should be a moot point today as most of us are after performance and are not running timing curves similar to what the factories had to impose to achieve the reduction.

Performance was a secondary consideration and the results definitely reflected that.


Steve
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