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Full Vacuum or Ported
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   Forward Look Technical Discussions -> Engine, Exhaust, Fuel and IgnitionMessage format
 
matte
Posted 2017-06-28 9:40 PM (#543082)
Subject: Full Vacuum or Ported



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Searching on the net shows so many different views and arguments. Both sides of the argument has good points.

My engine is a 1967 383 with a 600 Edelbrock 1406 and has a mild cam...

Just wanted to see peoples thoughts and experience from here about Ported Vs Full Vacuum

Also should which ever one isn't being used be plugged?



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plymouth
Posted 2017-06-29 9:19 AM (#543117 - in reply to #543082)
Subject: Re: Full Vacuum or Ported



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Ported vacuum is for your vacuum advance, which is the left side fitting on your 1406. Use the full vacuum fitting to hook up a vacuum gauge to tune the carb.
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BigBlockMopar
Posted 2017-06-29 2:13 PM (#543141 - in reply to #543082)
Subject: Re: Full Vacuum or Ported



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Intake vacuum makes an engine run(idle) cooler. Gives a snappier throttle reponse.
Ported vacuum is mainly a smog relic just to have retarded ignition which causes hotter exhaust at idle so more (possible unburned) fuel is burned in the exhaust.

At mid/high rpm it doesn't matter anymore as both follow the same curve.


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1960fury
Posted 2017-06-29 2:58 PM (#543149 - in reply to #543141)
Subject: Re: Full Vacuum or Ported



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BigBlockMopar - 2017-06-29 2:13 PM


Ported vacuum is mainly a smog relic just to have retarded ignition which causes hotter exhaust at idle so more (possible unburned) fuel is burned in the exhaust.




i heard that before but fact is ported vacuum is the way fl cars came from the factory (at least 59-61) there must be a reason for this but smog its probably not.
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BigBlockMopar
Posted 2017-06-29 3:10 PM (#543153 - in reply to #543082)
Subject: Re: Full Vacuum or Ported



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True. Maybe they found the idle could be more stable with ported vacuum.
Full intake vacuum can alter the ignition advance a bit due to possible variations in engine vacuum.

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matte
Posted 2017-06-29 6:21 PM (#543175 - in reply to #543153)
Subject: Re: Full Vacuum or Ported



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At the moment I am running full and have a pretty good idle however after the car has been for a good run, the idle starts to get rough. I might as well try ported and see what happens. Although I do worry a little because my engine is a hot engine and ported will increase it..

Should I plug the side that is not being used or is there no difference?

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Powerflite
Posted 2017-06-29 7:55 PM (#543182 - in reply to #543175)
Subject: Re: Full Vacuum or Ported



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Yes, plug the unused port. I find full vacuum gives a more snappy response so I usually use that. But one drawback to it is that when your vacuum advance goes bad, you end up with a serious vacuum leak that will really make your motor run crappy. But this is somewhat good because at least it will give you a solid indication that it has gone bad.
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Cataldo
Posted 2017-06-29 9:26 PM (#543191 - in reply to #543082)
Subject: Re: Full Vacuum or Ported


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http://nastyz28.com/threads/ported-or-manifold-vac-for-vac-advance....

Here is a link that further explains it, but as others have stated Manifold Vacuum generally will make the motor run better, my experience is that it usually helps. Not often does a motor run better on ported.
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matte
Posted 2017-06-30 4:28 AM (#543203 - in reply to #543191)
Subject: Re: Full Vacuum or Ported



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Thanks guys. I might have to try both and see what happens..



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matte
Posted 2017-06-30 4:29 AM (#543204 - in reply to #543149)
Subject: Re: Full Vacuum or Ported



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1960fury - 2017-06-29 2:58 PM

BigBlockMopar - 2017-06-29 2:13 PM


Ported vacuum is mainly a smog relic just to have retarded ignition which causes hotter exhaust at idle so more (possible unburned) fuel is burned in the exhaust.




i heard that before but fact is ported vacuum is the way fl cars came from the factory (at least 59-61) there must be a reason for this but smog its probably not.


Yeah I read that too and then continued with it throughout the 60's and 70's..

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plymouth
Posted 2017-06-30 1:48 PM (#543223 - in reply to #543204)
Subject: Re: Full Vacuum or Ported



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Wouldn't having the vacuum advance hooked up to manifold vacuum cause it to run too far advanced at idle? Suppose you run 10 degrees before top dead center with ported vacuum, wouldn't the timing be higher than the 10 degrees specified in the manual with the advance unit hooked up to manifold vacuum?

Edited by plymouth 2017-06-30 1:49 PM
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57chizler
Posted 2017-06-30 2:45 PM (#543231 - in reply to #543223)
Subject: Re: Full Vacuum or Ported



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Yes, but at idle there is virtually no load so the engine can tolerate the extra advance.
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1960fury
Posted 2017-06-30 4:13 PM (#543236 - in reply to #543231)
Subject: Re: Full Vacuum or Ported



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57chizler - 2017-06-30 2:45 PM


Yes, but at idle there is virtually no load so the engine can tolerate the extra advance.


so are you running manifold vacuum? i just wonder why they came ported from the factory (this also requires extra machining) and why do they say to pull and plug the vacuum line for timing adjustments at idle? this make no difference with ported vacuum.
maybe they used manifold vacuum before and this was left by mistake in the service manuals? if so, why did they switch to ported vacuum?
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plymouth
Posted 2017-06-30 5:02 PM (#543240 - in reply to #543236)
Subject: Re: Full Vacuum or Ported



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I've watched some of the old Chrysler tech films and in the late 40s they were using ported vacuum.
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plymouth
Posted 2017-06-30 5:07 PM (#543241 - in reply to #543240)
Subject: Re: Full Vacuum or Ported



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Here's a video. http://www.mymopar.com/index.php?pid=402
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1960fury
Posted 2017-06-30 5:56 PM (#543253 - in reply to #543241)
Subject: Re: Full Vacuum or Ported



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great video, thanks for sharing.
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Powerflite
Posted 2017-06-30 6:17 PM (#543256 - in reply to #543241)
Subject: Re: Full Vacuum or Ported



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Just because you see the port coming from above the throttle plate does not necessarily mean that it is a ported vacuum source. There could easily be an internal line that runs it below the throttle plate. But the video definitely refers to ported vacuum.

Here is an article written by HotRod magazine on it. http://www.hotrod.com/articles/vacuum-advance-tech/

"A suitable amount (whether manifold or ported) can noticeably increase part-throttle response without any negative aspects. But because most of our engines are modified in some manner, neither form is necessarily “correct” for any vehicle. Since vacuum advance offers such distinct advantages for any car driven on the street, we suggest experimenting with both forms to determine which works best for your specific application. Once you find the right combination, you’ll wonder why you waited so long to perform your own vacuum-advance experiment!"

Personally, I have always had better results with full manifold vacuum when using aftermarket carburetors, but I usually just keep it at the stock position when using the stock carb since there isn't usually an easy way to add a full vacuum connection.



Edited by Powerflite 2017-06-30 6:25 PM
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matte
Posted 2017-06-30 7:03 PM (#543261 - in reply to #543082)
Subject: Re: Full Vacuum or Ported



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That was a great video. I've learnt a fair bit from those old Chrysler Tech Recordings...

I have been running full vacuum because that's the setup it came with when I bought it. It does have a quick response time and am happy with it for most of the part. I just have some weird tiny issues that could be due to wanting to be on ported vacuum.. I guess I'll only know if I try both. My concern is how the temp rises when ported and the old girl gets hot in the Australian summers(Dec-Feb) and city traffic. I'll let you guys know for interest. Thanks again.

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matte
Posted 2017-06-30 7:04 PM (#543262 - in reply to #543082)
Subject: Re: Full Vacuum or Ported



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That was a great video. I've learnt a fair bit from those old Chrysler Tech Recordings...

I have been running full vacuum because that's the setup it came with when I bought it. It does have a quick response time and am happy with it for most of the part. I just have some weird tiny issues that could be due to wanting to be on ported vacuum.. I guess I'll only know if I try both. My concern is how the temp rises when ported and the old girl gets hot in the Australian summers(Dec-Feb) and city traffic. I'll let you guys know for interest. Thanks again.

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1960fury
Posted 2017-06-30 7:09 PM (#543263 - in reply to #543256)
Subject: Re: Full Vacuum or Ported



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Powerflite - 2017-06-30 6:17 PM

Just because you see the port coming from above the throttle plate does not necessarily mean that it is a ported vacuum source. There could easily be an internal line that runs it below the throttle plate. But the video definitely refers to ported vacuum.




??? you mean to let us think it is ported but in reality its manifold vacuum, or the bore above the throttle plate is not meant to be a ported vacuum source? what else?
it is pretty easy to find out if its ported or not and all oe carbs have a ported vacuum bore, extra machining that wouldn't have been done if they were supposed to run manifold vacuum.
i've always been running ported and couldn't wish for a snappier take off from idle.


Edited by 1960fury 2017-06-30 7:13 PM
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Powerflite
Posted 2017-06-30 11:52 PM (#543281 - in reply to #543263)
Subject: Re: Full Vacuum or Ported



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All I meant to say is that you can't infer it from the position on the outside. It depends on where it is routed to on the inside. But you are right that it is easy to tell; just see if it has a lot of vacuum at idle.
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BigBlockMopar
Posted 2017-07-01 4:21 AM (#543287 - in reply to #543082)
Subject: Re: Full Vacuum or Ported



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Engine vacuum is directly related to the load on an engine.
High vacuum = low load
Low vacuum = high load

With a ported vacuum source for the ignition, at idle, you're basicly 'hiding' the engine load for the ignition system. Ignition advance stays put at its initial setting.
Is this a good thing? The factory might have thought so, but that may be related to other issues like dependability or a more easy way of getting an acceptable tune on (various) engine-types before they left the factory.
Personally, I don't think it's 'better' (Engine runs hotter, plugs may foul quicker).


With ported vacuum,
when the throttle is opened and engine rpm's rise, more airflow passes through the carb and slowly more intake vacuum is exposed to the port and ignition system.

With full intake vacuum, ignition advance is high (initial adv. + vacuum adv.).
Once the throttle is opened, engine vacuum drops, which results in ignition advance dropping so basicly the ignition sees initial adv. + mech. adv. (+ some vac. adv. depending on sensitivity of vac. canister) at that point.



Having said all that;
Ignition "advance" is basicly a bad thing, as it causes a counterrotational force in the engine/on the pistons before TDC.
Any engine that run better with less advance, is built with a more efficient design (mostly headdesign) which defines how quick a fuel mixture will burn inside the engine (or more detailed: how quick the flamefront, right after spark, can travel in the head's chamber).

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