Vac advance, ported or manifold?

[Ozsum2]

Manifold vacuum is lowest during acceleration and ZERO during WOT.....but ported vacuum is zero at idle and increases with RPM because it is created by air rushing through the venturi of the carburetor and during acceleration the volume of air going through the venturi and it's speed is increasing thus making ported/venturi vacuum highest under conditions when manifold vacuum is lowest or zero.....THIS IS WHY THE VACUUM ADVANCE CONNECTS TO PORTED VACUUM.

Regardless of all the misinformation in this thread total advance is: Initial timing + Mechanincal Advance + Vacuum Advance. The vacuum advance is not supposed to be dropped out at WOT as it would be if manifold vacuum was used, but ported vacuum is highest during WOT and it keeps the vacuum advance in the equation....as it should. Any distributor that has vacuum advance has LESS mechanical advance than a comparable distributor with only mechanical advance in order to keep the total advance the same. Where people run into problems is that after doing engine mods like cams/heads/exhaust which allow for and may require more initial timing they fail to recurve the mechanical advance to reduce it by the same amount they increased the initial timing. ALMOST NOONE EVER MESSES WITH THE MECHANICAL ADVANCE.....and this is the problem that creates all the confusion and misinformation.

Thank you. Finally!!!

 

[Ozsum2]

Nope. Acceleration is your lowest vacuum point, thus your least amount of vacuum advance.

Wrong, wrong, wrong.

 

[Ronstang]

Wrong, wrong, wrong.

Yeah, Max Power should change his name to Min Power if he believes that under acceleration you need the least amount of vacuum advance......


.......I kid, I kid!!!

 

[Ozsum2]

Yeah, Max Power should change his name to Min Power if he believes that under acceleration you need the least amount of vacuum advance......


.......I kid, I kid!!!

No, you are right on. I am freaking here about all this "quality" info.

 

[Max Power]

http://chevyhiperformance.com/howto/97438/

http://www.autosite.com/library/diction/dict0016.asp

 

[Ozsum2]

http://chevyhiperformance.com/howto/97438/

Em, this is a FORD ONLY site.

 

[Max Power]

Did ya read it?

I stated it incorrectly above. Your highest amount of ported vacuum is just off throttle under acceleration, but ported decreases as rpms increase, if I am not mistaken. Ported should also be zero at WOT, I think. I will throw a vacuum guage on tonight and see.

Every source I have read has said that poretd decreases with RPM, there fore rendering the statement from Ronstang from above:

""but ported vacuum is zero at idle and increases with RPM because it is created by air rushing through the venturi of the carburetor and during acceleration the volume of air going through the venturi and it's speed is increasing thus making ported/venturi vacuum highest under conditions when manifold vacuum is lowest or zero""

...somewhat suspect.

 

[bull999999]

Em, this is a FORD ONLY site.

It doesn't matter if it's a Ford, Chevy, or some import, all engines of similar class operate with same princple. Instead of accusing people of lying or smokeying crap for disagreeing with you, why don't you test it out yourself? Just hookup a vaccum gauge and take a look at the readings for the part throttle and WOT between ported and manifold vaccum sources.

 

[bull999999]

Here's more thoughts on ported vs. manifold vacuum debate.

"Thought #1: Ported Vacuum or Manifold Vacuum? Manifold vs Ported, its a never ending debate. Depending on the age of who you ask, and their own experiences you still have a 50-50 chance of one or the other. Ported vacuum is the choice of the factory. It is also my recomendation to start with ported vacuum. Depending on what level of modifcations you have done to your engine, your driving style, and your vehicles characteristics its a point of debate. Each presents its own set of advantages and disadvantages. My suggestion is to just try both and observe how your engine behaves for your driving style."

From http://www.oldengine.org/unfaq/hei.htm

"Some gearheads have near-religious views on ported versus manifold vacuum. Patterson connects the vacuum advance to either source, and chooses the “right” port after test-driving both hookups. He does say that full manifold vacuum should help tip-in response, and this setting made Smith’s Mustang feel crisper off-idle. But if your car idles shaky and “nervously” with manifold vacuum advance, it’s a sign that there’s too much initial advance. So give the ported vacuum a try."

From http://www.carcraft.com/howto/77958/index2.html

"3. The third was an intermediate type. Kind of a mix between the first two. The manifold fed the vacuum advance unit, with the advance curve only giving around 5-7 degree maximum.
and didn't start to have any effect until you let off the peddle a bit or were idling. These units were usually used with a distributor that mechanically started to advance at a much lower rpm range."

From http://www.teglerizer.com/mgstuff/advance_curves.htm

Interest how he didn't notice any difference between the ported and manifold vacuum source except when you let off the peddle or at idle.

"You need to test your particular engine combination for what vacuum source works best for idle quality and partial throttle (light) cruising, along with what advance curve/total advance degrees works the best for overall performance. The vacuum source (ported or full-time) only affects the idle and off-idle characteristics, not the total advance curve - that's the job of the mechanical advance (internal weights inside the distributor). Ported vacuum showed up when emissions became a factor - before that time, full manifold vacuum was provided to the distributor. Many emissions engines use a thermal vacuum control switch to deliver ported vacuum to the distributor until the engine reached operating temperature, then manifold vacuum to keep the engine cooler at idle."

From http://www.chevytalk.com/tech/ignition/Timing_Advance.html

 

[Ozsum2]

It doesn't matter if it's a Ford, Chevy, or some import, all engines of similar class operate with same princple. Instead of accusing people of lying or smokeying crap for disagreeing with you, why don't you test it out yourself? Just hookup a vaccum gauge and take a look at the readings for the part throttle and WOT between ported and manifold vaccum sources.

You really need to get over the "smoking thing" It was an attention getter on my part and wasn't meant to imply anything. Unless of course, you do. Vacum advance increases with throttle folks. That is what it is there for. It is nill at idle. And NO, I don't waste my time ready Chebby stuff. I never said anyone was lieing. I said I didn't believe certain things which is my perogative. Why don't you do as you suggest I do and see for yourself. Why would an engine need Vac advance at idle? Hmmmmmmmmmmmm?

 

[bull999999]

You really need to get over the "smoking thing" I was an attention getter on my part and wasn't meant to imply anything. Unless of course, you do. Vacum advance increases with throttle folks. That is what it is there for. It is nill at idle. And NO, I don't waste my time ready Chebby stuff. I never said anyone was lieing. I said I didn't believe certain things which is my perogative. Why don't you do as you suggest I do and see for yourself. Why would an engine need Vac advance at idle? Hmmmmmmmmmmmm?

1. Read my post above about ported vs manifold vacuum.

2. I already answered your question about benifits of vac advance at idle and even supplied an E-mail address for Performance Distributors so you won't have to take my word for it. Since they build distributors for a living, you'd figure they should know something about vacuum advances.

3. I guess you missed my post about testing out the both ported and manifold vacuum ports at various throttle positions.

4. My bad about getting pissed at the "smoking" commment. No, I don't smoke but one of my pet peeve is when someone disagrees without giving a detailed reason why.

 

[Ozsum2]

1. Read my post above about ported vs manifold vacuum.

2. I already answered your question about benifits of vac advance at idle and even supplied an E-mail address for Performance Distributors so you won't have to take my word for it. Since they build distributors for a living, you'd figure they should know something about vacuum advances.

3. I guess you missed my post about testing out the both ported and manifold vacuum ports at various throttle positions.

No, I didn't miss it. I simply wasn't looking for it. Some dist have no vac advance and only mechanical. That wouldn't help then much at idle.

 

[bull999999]

No, I didn't miss it. I simply wasn't looking for it. Some dist have no vac advance and only mechanical. That wouldn't help then much at idle.

You've got a vaild point there, although I still believe you can take advantage of vacuum advance under certain applications as listed above but I guess we can agree to disagree at this point.

 

[Ozsum2]

You've got a vaild point there, although I still believe you can take advantage of vacuum advance under certain applications as listed above but I guess we can agree to disagree at this point.

I am quite certain that we are not alone.

 

[351LX]

Ronstang, I used to believe that too.Everyone on the internet had a different theory with the blind leading the blind so I set out to find the facts.That is why I hooked two vacuum gauges to ported and non-ported sources myself and drove around to see the effect.Have you ever done this?If you do you will see that past part throttle they are exactly the same.I also took the opportunity to follow the passage in my carb from the ported outlet to see where it went.It went to the baseplate just above the throttle blades, no where near the venturis.The majority of mis-information on this thread is coming from you.If vacuum advance worked the way you say it does you would have about 60 degrees of advance at WOT which would result in serious pinging or engine damage.Before you respond, please take a vacuum gauge, hook it to a ported outlet and tell me what your vacuum is at WOT.I believe you will be surprised.

Manifold vacuum is lowest during acceleration and ZERO during WOT.....but ported vacuum is zero at idle and increases with RPM because it is created by air rushing through the venturi of the carburetor and during acceleration the volume of air going through the venturi and it's speed is increasing thus making ported/venturi vacuum highest under conditions when manifold vacuum is lowest or zero.....THIS IS WHY THE VACUUM ADVANCE CONNECTS TO PORTED VACUUM.

Regardless of all the misinformation in this thread total advance is: Initial timing + Mechanincal Advance + Vacuum Advance. The vacuum advance is not supposed to be dropped out at WOT as it would be if manifold vacuum was used, but ported vacuum is highest during WOT and it keeps the vacuum advance in the equation....as it should. Any distributor that has vacuum advance has LESS mechanical advance than a comparable distributor with only mechanical advance in order to keep the total advance the same. Where people run into problems is that after doing engine mods like cams/heads/exhaust which allow for and may require more initial timing they fail to recurve the mechanical advance to reduce it by the same amount they increased the initial timing. ALMOST NOONE EVER MESSES WITH THE MECHANICAL ADVANCE.....and this is the problem that creates all the confusion and misinformation.

 

[Max Power]

I hooked a vacuum guage up last nite and as expected, there was a decent amount of ported vacuum just off idle. It climbed a bit until about 2000-2500 rpm, then dropped off. At WOT, it went to zero, just like manifold vacuum.

351LX is correct.

 

[Ozsum2]

I hooked a vacuum guage up last nite and as expected, there was a decent amount of ported vacuum just off idle. It climbed a bit until about 2000-2500 rpm, then dropped off. At WOT, it went to zero, just like manifold vacuum.

351LX is correct.

And it will. The thing is you had to get off idle to start the vac up. The vac with the manifold is happing AT idle. At 2000-2500 rpm, I'm running at like 60 mph, so I have ported vac when I need it the most. Who drives arong WOT all the time and lives to talk about it anyway? I don't need to stick my finger in a glass of water to tell me if it is wet or not.

 

[Max Power]

Who said you did?

You said ported increses throughout the rpm range. It doesn't. You are wrong.

 

[Ozsum2]

Who said you did?

You said ported increses throughout the rpm range. It doesn't. You are wrong.

Yes, it does. Keep reading and open your mind.

 

[Ozsum2]

Nope. Acceleration is your lowest vacuum point, thus your least amount of vacuum advance.

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Wrong again!