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  #31 (permalink)  
Old 11-07-2004, 02:08 PM
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Well,I absolutely disagree. First off,because ported vac is set above the venturi,the amount of vacuum is increased by the amount of air traveling threw the carb,at a low rpm crawl the vac is barely increased,at idle near unmeasurable,knock it into passing gear and watch it jump up. For me,a big hill is the finest place to tune a car,next to a wheel dyno, if you are detonating it is from one of four sources,inadequate fuel,initial time too adv"dist turned to far",too light of springs"detonates only once the rpms are up",or too light of horn,can be changed to a heavy one or an adjustable one"they use an Allen screw threw the vacuum port". The reason the vac adv dist is desired over a full mech one is you can Taylor the speed of the curve at a lower rpm for town driving,this is why someone with a big stall convert would likely choose a mech only one because no extra advance would be needed because the low rpm load would be alleviated. The motor needs all the advance it can get without detonating at all given rpms,any less and you loose power and efficiency,remember,when it comes to the advance,efficiency is power,no two ways about it.

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  #32 (permalink)  
Old 11-07-2004, 04:23 PM
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Quote:
Orignally posted by rifraf
First off,because ported vac is set above the venturi,the amount of vacuum is increased by the amount of air traveling threw the carb
Just so I am understanding you correctly are you saying that at full throttle you have vacuum at the ported vacuum port?
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  #33 (permalink)  
Old 11-07-2004, 06:27 PM
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There is no vacuum at full throttle.If that is what he's saying he is wrong.bm
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  #34 (permalink)  
Old 11-10-2004, 01:16 AM
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No,I am not wrong. At an idle your needle may not even register on the gauge,if it does its a small amount,you see, as the air speed increases,so does the vacuum,I think you are confused,mixed up,or I dont know what,full manifold vac"under the butterflies",or any passage leading there will read zero or close to it under load.
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Old 11-10-2004, 07:48 PM
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Reply to rifraf

Ok I read several different articles by the likes of Hotrod and the Ford forum and it seems you are the only one that thinks that you can have vaccum advance at full throttle.

Disregard the ported or full story for the moment but neither had any vacuum advance at full throttle.

Maybe you know something the rest of the automotive publications or tech writers don't.

If you do please accept my sincerest apologies for my ignorance.
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  #36 (permalink)  
Old 11-11-2004, 12:48 PM
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Hey,don't get offended,Im trying to help. Im saying there are two sources of vacuum on most all popular carbs,its a fact,one is above the venturi and one is below. One drops vac at a load"the one below,and the other changes according to air speed,"increases". I tune a lot of rods,very highly noted ones,one featured on the cover of national dragster being given the nhra/valvoline award for best in class. What article do you speak of? Maybe I can learn how to pick up an extra 1/2 second,maybe Ive got something to learn,after all,if its in an article,it must be the last word.
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  #37 (permalink)  
Old 11-11-2004, 02:11 PM
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YIKES!!!

This is a little touchy/sticky situation. I think both of you are right to a point. Some of it is mis-understanding (IMO). This is a subject that people usually have their own opinion on and stand on one side of the fence or the other. Personally I think some cars (combos) run better with full manifold and others run better with ported. I think either can be "made" (tuned) to work. Even the "pros" don't see eye to eye on this issue.

Tmod, at full throttle you are correct there is zero vacuum. In order to have vacuum you have to have a difference in atmosphere and inside the manifold. Once the throttle is fully open the "pressure" is the same inside and out.

rifraf, I know exactly what you are trying to say as well. I agree with a lot of what you have said. I personally run no vacuum advance but, I fall into the category of high stall and don't care about economy.

I guess my question is, is there really a "right" answer? To me the right answer is do what works for you and makes your car run the way you want it to.


Royce
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  #38 (permalink)  
Old 11-11-2004, 02:27 PM
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Ok,as I have said also,two ways,and I have an opinion only,admittedly a strong one,but based on a lot of personal research,but now a question has been raised and disputed. Question being,does ported vac drop like manifold vac does under a load?
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  #39 (permalink)  
Old 11-11-2004, 03:55 PM
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rifraf,
Now the question you posed is tricky. You will see vacuum at the ported vacuum port under full throttle. The reason is air is rushing passed the port on the way into the manifold. This will cause a vaccum at the ported vacuum port (IMO). At this point there should be zero manifold vacuum. What makes it tricky is that is really not "manifold" vacuum you are seeing. I guess vacuum is vacuum.

Do you agree?

(by the way I had a Camaro named rifraf my first 70 VERY fast)

Royce
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  #40 (permalink)  
Old 11-11-2004, 04:26 PM
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Reply to rifraf,

I am not offended, I just have a different opinion then you.

The only way I know of to straighten this out is try it myself.

As soon as I get my Carb back on the truck I will give it a try. I already have a vacuum gauge in the truck and I will plumb in another gauge hooked to the ported vacuum port.

Also I searched several articles not just one and they all came back with no vacuum advance under full throttle even when hooked to the ported port.

Last edited by Tmod; 11-11-2004 at 04:34 PM.
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  #41 (permalink)  
Old 11-11-2004, 07:25 PM
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Ok,first off,I realize there are two ways to set up a dist,and in some cases I myself use the other method because it fits the purpose better,but rarely on street rigs,I also have used methods not mentioned with check valves and solenoids,Im not going to go into that though. The port I am speaking of gives near zero vac at an idle and more as the air speed increases. I do not view vacuum as vacuum because there is a time and a place when the need for advance or retard is a benefit or a detriment,I feel the amount of air passing threw a carb is a direct representation of what the motor is doing and equal to what is needed at a lower speed for advance. Next off,what do you mean by tricky? Im a bit lost,please read the prior posts on the prior pages as well before ya say.
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  #42 (permalink)  
Old 11-11-2004, 08:36 PM
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I've been running ported. Sure, maybe 30 years ago everybody ran full manifold vacuum. All that's going to do is give you more advance in a couple of different RPM ranges. The gasoline we are using today is garbage compaired with what they had to push the limits with back in the late '60's. No wonder people were running advance for days! I'm brushing the 11:1 line (I think its about 10.6 now that I take a closer look, at that's static anyway) with my iron headed motor. I don't mess around with full manifold vacuum. Like royce is saying, you can have an equally fast ride without running any vacuum advance at all! Don't you think royce would hook his up if it was going to gain him .2 seconds of his ET?

If you tune your motor right and dial in your distributor and curve correctly you won't have any increase in performance going from ported to full manifold. Ported or full, vacuum advance is just there to keep your plugs clean and give you a better cruising burn. And I certainly think given every motor is different, a blanket recommendation such as "full manifold vacuum is always better" is a load of horse *.

Anyway, that's my .02

K
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  #43 (permalink)  
Old 11-11-2004, 09:17 PM
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Well,you wont get any more overall power either way if you have your curve set to a proper overall advance,the reason I prefer ported vac is for the drive ability and snap in the lower places,when you go to stab the throttle off idle,having the advance retard sucks for me anyway.
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  #44 (permalink)  
Old 11-12-2004, 09:50 AM
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Obviously there is a difference of opinions here. I've been told by numerous hotrodders to use full manifold vacuum... and I've also been told by other hotrodders to use ported. If you can get it to run the way you want it to run then why would it matter what way you do it? Try it with full manifold vacuum, tune it properly and see how it feels. Then try it with ported vacuum, tune it properly and see how it feels. We'll never get a clear answer here and I don't believe there's any emperical evidence to back it up anyway. And even if someone posts a dyno slip showing how they gained 6hp from going to ported, I bet someone else will post a dyno slip saying they gained 6hp going to full vacuum. Just do what works best for you.
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  #45 (permalink)  
Old 11-12-2004, 02:19 PM
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Quote:
Originally posted by fried_guy
If you can get it to run the way you want it to run then why would it matter what way you do it? Try it with full manifold vacuum, tune it properly and see how it feels. Then try it with ported vacuum, tune it properly and see how it feels. We'll never get a clear answer here and I don't believe there's any emperical evidence to back it up anyway. And even if someone posts a dyno slip showing how they gained 6hp from going to ported, I bet someone else will post a dyno slip saying they gained 6hp going to full vacuum. Just do what works best for you.
Fried Guy summed it up very well. "Just do what works best for you", whatever your motor likes, no two are alike even if you build them identical. I just wish people just did not have blinders on all of the time, and think that there way is the only way. That is what makes this so cool, because what you learn from one motor, may not work for the next one, so having more than one answer makes you a better mechanic, IMHO. Good job guys.
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